Source file src/net/http/server.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // HTTP server. See RFC 7230 through 7235.
     6  
     7  package http
     8  
     9  import (
    10  	"bufio"
    11  	"bytes"
    12  	"context"
    13  	"crypto/tls"
    14  	"errors"
    15  	"fmt"
    16  	"internal/godebug"
    17  	"io"
    18  	"log"
    19  	"maps"
    20  	"math/rand"
    21  	"net"
    22  	"net/textproto"
    23  	"net/url"
    24  	urlpkg "net/url"
    25  	"path"
    26  	"runtime"
    27  	"slices"
    28  	"strconv"
    29  	"strings"
    30  	"sync"
    31  	"sync/atomic"
    32  	"time"
    33  	_ "unsafe" // for linkname
    34  
    35  	"golang.org/x/net/http/httpguts"
    36  )
    37  
    38  // Errors used by the HTTP server.
    39  var (
    40  	// ErrBodyNotAllowed is returned by ResponseWriter.Write calls
    41  	// when the HTTP method or response code does not permit a
    42  	// body.
    43  	ErrBodyNotAllowed = errors.New("http: request method or response status code does not allow body")
    44  
    45  	// ErrHijacked is returned by ResponseWriter.Write calls when
    46  	// the underlying connection has been hijacked using the
    47  	// Hijacker interface. A zero-byte write on a hijacked
    48  	// connection will return ErrHijacked without any other side
    49  	// effects.
    50  	ErrHijacked = errors.New("http: connection has been hijacked")
    51  
    52  	// ErrContentLength is returned by ResponseWriter.Write calls
    53  	// when a Handler set a Content-Length response header with a
    54  	// declared size and then attempted to write more bytes than
    55  	// declared.
    56  	ErrContentLength = errors.New("http: wrote more than the declared Content-Length")
    57  
    58  	// Deprecated: ErrWriteAfterFlush is no longer returned by
    59  	// anything in the net/http package. Callers should not
    60  	// compare errors against this variable.
    61  	ErrWriteAfterFlush = errors.New("unused")
    62  )
    63  
    64  // A Handler responds to an HTTP request.
    65  //
    66  // [Handler.ServeHTTP] should write reply headers and data to the [ResponseWriter]
    67  // and then return. Returning signals that the request is finished; it
    68  // is not valid to use the [ResponseWriter] or read from the
    69  // [Request.Body] after or concurrently with the completion of the
    70  // ServeHTTP call.
    71  //
    72  // Depending on the HTTP client software, HTTP protocol version, and
    73  // any intermediaries between the client and the Go server, it may not
    74  // be possible to read from the [Request.Body] after writing to the
    75  // [ResponseWriter]. Cautious handlers should read the [Request.Body]
    76  // first, and then reply.
    77  //
    78  // Except for reading the body, handlers should not modify the
    79  // provided Request.
    80  //
    81  // If ServeHTTP panics, the server (the caller of ServeHTTP) assumes
    82  // that the effect of the panic was isolated to the active request.
    83  // It recovers the panic, logs a stack trace to the server error log,
    84  // and either closes the network connection or sends an HTTP/2
    85  // RST_STREAM, depending on the HTTP protocol. To abort a handler so
    86  // the client sees an interrupted response but the server doesn't log
    87  // an error, panic with the value [ErrAbortHandler].
    88  type Handler interface {
    89  	ServeHTTP(ResponseWriter, *Request)
    90  }
    91  
    92  // A ResponseWriter interface is used by an HTTP handler to
    93  // construct an HTTP response.
    94  //
    95  // A ResponseWriter may not be used after [Handler.ServeHTTP] has returned.
    96  type ResponseWriter interface {
    97  	// Header returns the header map that will be sent by
    98  	// [ResponseWriter.WriteHeader]. The [Header] map also is the mechanism with which
    99  	// [Handler] implementations can set HTTP trailers.
   100  	//
   101  	// Changing the header map after a call to [ResponseWriter.WriteHeader] (or
   102  	// [ResponseWriter.Write]) has no effect unless the HTTP status code was of the
   103  	// 1xx class or the modified headers are trailers.
   104  	//
   105  	// There are two ways to set Trailers. The preferred way is to
   106  	// predeclare in the headers which trailers you will later
   107  	// send by setting the "Trailer" header to the names of the
   108  	// trailer keys which will come later. In this case, those
   109  	// keys of the Header map are treated as if they were
   110  	// trailers. See the example. The second way, for trailer
   111  	// keys not known to the [Handler] until after the first [ResponseWriter.Write],
   112  	// is to prefix the [Header] map keys with the [TrailerPrefix]
   113  	// constant value.
   114  	//
   115  	// To suppress automatic response headers (such as "Date"), set
   116  	// their value to nil.
   117  	Header() Header
   118  
   119  	// Write writes the data to the connection as part of an HTTP reply.
   120  	//
   121  	// If [ResponseWriter.WriteHeader] has not yet been called, Write calls
   122  	// WriteHeader(http.StatusOK) before writing the data. If the Header
   123  	// does not contain a Content-Type line, Write adds a Content-Type set
   124  	// to the result of passing the initial 512 bytes of written data to
   125  	// [DetectContentType]. Additionally, if the total size of all written
   126  	// data is under a few KB and there are no Flush calls, the
   127  	// Content-Length header is added automatically.
   128  	//
   129  	// Depending on the HTTP protocol version and the client, calling
   130  	// Write or WriteHeader may prevent future reads on the
   131  	// Request.Body. For HTTP/1.x requests, handlers should read any
   132  	// needed request body data before writing the response. Once the
   133  	// headers have been flushed (due to either an explicit Flusher.Flush
   134  	// call or writing enough data to trigger a flush), the request body
   135  	// may be unavailable. For HTTP/2 requests, the Go HTTP server permits
   136  	// handlers to continue to read the request body while concurrently
   137  	// writing the response. However, such behavior may not be supported
   138  	// by all HTTP/2 clients. Handlers should read before writing if
   139  	// possible to maximize compatibility.
   140  	Write([]byte) (int, error)
   141  
   142  	// WriteHeader sends an HTTP response header with the provided
   143  	// status code.
   144  	//
   145  	// If WriteHeader is not called explicitly, the first call to Write
   146  	// will trigger an implicit WriteHeader(http.StatusOK).
   147  	// Thus explicit calls to WriteHeader are mainly used to
   148  	// send error codes or 1xx informational responses.
   149  	//
   150  	// The provided code must be a valid HTTP 1xx-5xx status code.
   151  	// Any number of 1xx headers may be written, followed by at most
   152  	// one 2xx-5xx header. 1xx headers are sent immediately, but 2xx-5xx
   153  	// headers may be buffered. Use the Flusher interface to send
   154  	// buffered data. The header map is cleared when 2xx-5xx headers are
   155  	// sent, but not with 1xx headers.
   156  	//
   157  	// The server will automatically send a 100 (Continue) header
   158  	// on the first read from the request body if the request has
   159  	// an "Expect: 100-continue" header.
   160  	WriteHeader(statusCode int)
   161  }
   162  
   163  // The Flusher interface is implemented by ResponseWriters that allow
   164  // an HTTP handler to flush buffered data to the client.
   165  //
   166  // The default HTTP/1.x and HTTP/2 [ResponseWriter] implementations
   167  // support [Flusher], but ResponseWriter wrappers may not. Handlers
   168  // should always test for this ability at runtime.
   169  //
   170  // Note that even for ResponseWriters that support Flush,
   171  // if the client is connected through an HTTP proxy,
   172  // the buffered data may not reach the client until the response
   173  // completes.
   174  type Flusher interface {
   175  	// Flush sends any buffered data to the client.
   176  	Flush()
   177  }
   178  
   179  // The Hijacker interface is implemented by ResponseWriters that allow
   180  // an HTTP handler to take over the connection.
   181  //
   182  // The default [ResponseWriter] for HTTP/1.x connections supports
   183  // Hijacker, but HTTP/2 connections intentionally do not.
   184  // ResponseWriter wrappers may also not support Hijacker. Handlers
   185  // should always test for this ability at runtime.
   186  type Hijacker interface {
   187  	// Hijack lets the caller take over the connection.
   188  	// After a call to Hijack the HTTP server library
   189  	// will not do anything else with the connection.
   190  	//
   191  	// It becomes the caller's responsibility to manage
   192  	// and close the connection.
   193  	//
   194  	// The returned net.Conn may have read or write deadlines
   195  	// already set, depending on the configuration of the
   196  	// Server. It is the caller's responsibility to set
   197  	// or clear those deadlines as needed.
   198  	//
   199  	// The returned bufio.Reader may contain unprocessed buffered
   200  	// data from the client.
   201  	//
   202  	// After a call to Hijack, the original Request.Body must not
   203  	// be used. The original Request's Context remains valid and
   204  	// is not canceled until the Request's ServeHTTP method
   205  	// returns.
   206  	Hijack() (net.Conn, *bufio.ReadWriter, error)
   207  }
   208  
   209  // The CloseNotifier interface is implemented by ResponseWriters which
   210  // allow detecting when the underlying connection has gone away.
   211  //
   212  // This mechanism can be used to cancel long operations on the server
   213  // if the client has disconnected before the response is ready.
   214  //
   215  // Deprecated: the CloseNotifier interface predates Go's context package.
   216  // New code should use [Request.Context] instead.
   217  type CloseNotifier interface {
   218  	// CloseNotify returns a channel that receives at most a
   219  	// single value (true) when the client connection has gone
   220  	// away.
   221  	//
   222  	// CloseNotify may wait to notify until Request.Body has been
   223  	// fully read.
   224  	//
   225  	// After the Handler has returned, there is no guarantee
   226  	// that the channel receives a value.
   227  	//
   228  	// If the protocol is HTTP/1.1 and CloseNotify is called while
   229  	// processing an idempotent request (such as GET) while
   230  	// HTTP/1.1 pipelining is in use, the arrival of a subsequent
   231  	// pipelined request may cause a value to be sent on the
   232  	// returned channel. In practice HTTP/1.1 pipelining is not
   233  	// enabled in browsers and not seen often in the wild. If this
   234  	// is a problem, use HTTP/2 or only use CloseNotify on methods
   235  	// such as POST.
   236  	CloseNotify() <-chan bool
   237  }
   238  
   239  var (
   240  	// ServerContextKey is a context key. It can be used in HTTP
   241  	// handlers with Context.Value to access the server that
   242  	// started the handler. The associated value will be of
   243  	// type *Server.
   244  	ServerContextKey = &contextKey{"http-server"}
   245  
   246  	// LocalAddrContextKey is a context key. It can be used in
   247  	// HTTP handlers with Context.Value to access the local
   248  	// address the connection arrived on.
   249  	// The associated value will be of type net.Addr.
   250  	LocalAddrContextKey = &contextKey{"local-addr"}
   251  )
   252  
   253  // A conn represents the server side of an HTTP connection.
   254  type conn struct {
   255  	// server is the server on which the connection arrived.
   256  	// Immutable; never nil.
   257  	server *Server
   258  
   259  	// cancelCtx cancels the connection-level context.
   260  	cancelCtx context.CancelFunc
   261  
   262  	// rwc is the underlying network connection.
   263  	// This is never wrapped by other types and is the value given out
   264  	// to [Hijacker] callers. It is usually of type *net.TCPConn or
   265  	// *tls.Conn.
   266  	rwc net.Conn
   267  
   268  	// remoteAddr is rwc.RemoteAddr().String(). It is not populated synchronously
   269  	// inside the Listener's Accept goroutine, as some implementations block.
   270  	// It is populated immediately inside the (*conn).serve goroutine.
   271  	// This is the value of a Handler's (*Request).RemoteAddr.
   272  	remoteAddr string
   273  
   274  	// tlsState is the TLS connection state when using TLS.
   275  	// nil means not TLS.
   276  	tlsState *tls.ConnectionState
   277  
   278  	// werr is set to the first write error to rwc.
   279  	// It is set via checkConnErrorWriter{w}, where bufw writes.
   280  	werr error
   281  
   282  	// r is bufr's read source. It's a wrapper around rwc that provides
   283  	// io.LimitedReader-style limiting (while reading request headers)
   284  	// and functionality to support CloseNotifier. See *connReader docs.
   285  	r *connReader
   286  
   287  	// bufr reads from r.
   288  	bufr *bufio.Reader
   289  
   290  	// bufw writes to checkConnErrorWriter{c}, which populates werr on error.
   291  	bufw *bufio.Writer
   292  
   293  	// lastMethod is the method of the most recent request
   294  	// on this connection, if any.
   295  	lastMethod string
   296  
   297  	curReq atomic.Pointer[response] // (which has a Request in it)
   298  
   299  	curState atomic.Uint64 // packed (unixtime<<8|uint8(ConnState))
   300  
   301  	// mu guards hijackedv
   302  	mu sync.Mutex
   303  
   304  	// hijackedv is whether this connection has been hijacked
   305  	// by a Handler with the Hijacker interface.
   306  	// It is guarded by mu.
   307  	hijackedv bool
   308  }
   309  
   310  func (c *conn) hijacked() bool {
   311  	c.mu.Lock()
   312  	defer c.mu.Unlock()
   313  	return c.hijackedv
   314  }
   315  
   316  // c.mu must be held.
   317  func (c *conn) hijackLocked() (rwc net.Conn, buf *bufio.ReadWriter, err error) {
   318  	if c.hijackedv {
   319  		return nil, nil, ErrHijacked
   320  	}
   321  	c.r.abortPendingRead()
   322  
   323  	c.hijackedv = true
   324  	rwc = c.rwc
   325  	rwc.SetDeadline(time.Time{})
   326  
   327  	if c.r.hasByte {
   328  		if _, err := c.bufr.Peek(c.bufr.Buffered() + 1); err != nil {
   329  			return nil, nil, fmt.Errorf("unexpected Peek failure reading buffered byte: %v", err)
   330  		}
   331  	}
   332  	c.bufw.Reset(rwc)
   333  	buf = bufio.NewReadWriter(c.bufr, c.bufw)
   334  
   335  	c.setState(rwc, StateHijacked, runHooks)
   336  	return
   337  }
   338  
   339  // This should be >= 512 bytes for DetectContentType,
   340  // but otherwise it's somewhat arbitrary.
   341  const bufferBeforeChunkingSize = 2048
   342  
   343  // chunkWriter writes to a response's conn buffer, and is the writer
   344  // wrapped by the response.w buffered writer.
   345  //
   346  // chunkWriter also is responsible for finalizing the Header, including
   347  // conditionally setting the Content-Type and setting a Content-Length
   348  // in cases where the handler's final output is smaller than the buffer
   349  // size. It also conditionally adds chunk headers, when in chunking mode.
   350  //
   351  // See the comment above (*response).Write for the entire write flow.
   352  type chunkWriter struct {
   353  	res *response
   354  
   355  	// header is either nil or a deep clone of res.handlerHeader
   356  	// at the time of res.writeHeader, if res.writeHeader is
   357  	// called and extra buffering is being done to calculate
   358  	// Content-Type and/or Content-Length.
   359  	header Header
   360  
   361  	// wroteHeader tells whether the header's been written to "the
   362  	// wire" (or rather: w.conn.buf). this is unlike
   363  	// (*response).wroteHeader, which tells only whether it was
   364  	// logically written.
   365  	wroteHeader bool
   366  
   367  	// set by the writeHeader method:
   368  	chunking bool // using chunked transfer encoding for reply body
   369  }
   370  
   371  var (
   372  	crlf       = []byte("\r\n")
   373  	colonSpace = []byte(": ")
   374  )
   375  
   376  func (cw *chunkWriter) Write(p []byte) (n int, err error) {
   377  	if !cw.wroteHeader {
   378  		cw.writeHeader(p)
   379  	}
   380  	if cw.res.req.Method == "HEAD" {
   381  		// Eat writes.
   382  		return len(p), nil
   383  	}
   384  	if cw.chunking {
   385  		_, err = fmt.Fprintf(cw.res.conn.bufw, "%x\r\n", len(p))
   386  		if err != nil {
   387  			cw.res.conn.rwc.Close()
   388  			return
   389  		}
   390  	}
   391  	n, err = cw.res.conn.bufw.Write(p)
   392  	if cw.chunking && err == nil {
   393  		_, err = cw.res.conn.bufw.Write(crlf)
   394  	}
   395  	if err != nil {
   396  		cw.res.conn.rwc.Close()
   397  	}
   398  	return
   399  }
   400  
   401  func (cw *chunkWriter) flush() error {
   402  	if !cw.wroteHeader {
   403  		cw.writeHeader(nil)
   404  	}
   405  	return cw.res.conn.bufw.Flush()
   406  }
   407  
   408  func (cw *chunkWriter) close() {
   409  	if !cw.wroteHeader {
   410  		cw.writeHeader(nil)
   411  	}
   412  	if cw.chunking {
   413  		bw := cw.res.conn.bufw // conn's bufio writer
   414  		// zero chunk to mark EOF
   415  		bw.WriteString("0\r\n")
   416  		if trailers := cw.res.finalTrailers(); trailers != nil {
   417  			trailers.Write(bw) // the writer handles noting errors
   418  		}
   419  		// final blank line after the trailers (whether
   420  		// present or not)
   421  		bw.WriteString("\r\n")
   422  	}
   423  }
   424  
   425  // A response represents the server side of an HTTP response.
   426  type response struct {
   427  	conn             *conn
   428  	req              *Request // request for this response
   429  	reqBody          io.ReadCloser
   430  	cancelCtx        context.CancelFunc // when ServeHTTP exits
   431  	wroteHeader      bool               // a non-1xx header has been (logically) written
   432  	wants10KeepAlive bool               // HTTP/1.0 w/ Connection "keep-alive"
   433  	wantsClose       bool               // HTTP request has Connection "close"
   434  
   435  	// canWriteContinue is an atomic boolean that says whether or
   436  	// not a 100 Continue header can be written to the
   437  	// connection.
   438  	// writeContinueMu must be held while writing the header.
   439  	// These two fields together synchronize the body reader (the
   440  	// expectContinueReader, which wants to write 100 Continue)
   441  	// against the main writer.
   442  	writeContinueMu  sync.Mutex
   443  	canWriteContinue atomic.Bool
   444  
   445  	w  *bufio.Writer // buffers output in chunks to chunkWriter
   446  	cw chunkWriter
   447  
   448  	// handlerHeader is the Header that Handlers get access to,
   449  	// which may be retained and mutated even after WriteHeader.
   450  	// handlerHeader is copied into cw.header at WriteHeader
   451  	// time, and privately mutated thereafter.
   452  	handlerHeader Header
   453  	calledHeader  bool // handler accessed handlerHeader via Header
   454  
   455  	written       int64 // number of bytes written in body
   456  	contentLength int64 // explicitly-declared Content-Length; or -1
   457  	status        int   // status code passed to WriteHeader
   458  
   459  	// close connection after this reply.  set on request and
   460  	// updated after response from handler if there's a
   461  	// "Connection: keep-alive" response header and a
   462  	// Content-Length.
   463  	closeAfterReply bool
   464  
   465  	// When fullDuplex is false (the default), we consume any remaining
   466  	// request body before starting to write a response.
   467  	fullDuplex bool
   468  
   469  	// requestBodyLimitHit is set by requestTooLarge when
   470  	// maxBytesReader hits its max size. It is checked in
   471  	// WriteHeader, to make sure we don't consume the
   472  	// remaining request body to try to advance to the next HTTP
   473  	// request. Instead, when this is set, we stop reading
   474  	// subsequent requests on this connection and stop reading
   475  	// input from it.
   476  	requestBodyLimitHit bool
   477  
   478  	// trailers are the headers to be sent after the handler
   479  	// finishes writing the body. This field is initialized from
   480  	// the Trailer response header when the response header is
   481  	// written.
   482  	trailers []string
   483  
   484  	handlerDone atomic.Bool // set true when the handler exits
   485  
   486  	// Buffers for Date, Content-Length, and status code
   487  	dateBuf   [len(TimeFormat)]byte
   488  	clenBuf   [10]byte
   489  	statusBuf [3]byte
   490  
   491  	// lazyCloseNotifyMu protects closeNotifyCh and closeNotifyTriggered.
   492  	lazyCloseNotifyMu sync.Mutex
   493  	// closeNotifyCh is the channel returned by CloseNotify.
   494  	closeNotifyCh chan bool
   495  	// closeNotifyTriggered tracks prior closeNotify calls.
   496  	closeNotifyTriggered bool
   497  }
   498  
   499  func (c *response) SetReadDeadline(deadline time.Time) error {
   500  	return c.conn.rwc.SetReadDeadline(deadline)
   501  }
   502  
   503  func (c *response) SetWriteDeadline(deadline time.Time) error {
   504  	return c.conn.rwc.SetWriteDeadline(deadline)
   505  }
   506  
   507  func (c *response) EnableFullDuplex() error {
   508  	c.fullDuplex = true
   509  	return nil
   510  }
   511  
   512  // TrailerPrefix is a magic prefix for [ResponseWriter.Header] map keys
   513  // that, if present, signals that the map entry is actually for
   514  // the response trailers, and not the response headers. The prefix
   515  // is stripped after the ServeHTTP call finishes and the values are
   516  // sent in the trailers.
   517  //
   518  // This mechanism is intended only for trailers that are not known
   519  // prior to the headers being written. If the set of trailers is fixed
   520  // or known before the header is written, the normal Go trailers mechanism
   521  // is preferred:
   522  //
   523  //	https://pkg.go.dev/net/http#ResponseWriter
   524  //	https://pkg.go.dev/net/http#example-ResponseWriter-Trailers
   525  const TrailerPrefix = "Trailer:"
   526  
   527  // finalTrailers is called after the Handler exits and returns a non-nil
   528  // value if the Handler set any trailers.
   529  func (w *response) finalTrailers() Header {
   530  	var t Header
   531  	for k, vv := range w.handlerHeader {
   532  		if kk, found := strings.CutPrefix(k, TrailerPrefix); found {
   533  			if t == nil {
   534  				t = make(Header)
   535  			}
   536  			t[kk] = vv
   537  		}
   538  	}
   539  	for _, k := range w.trailers {
   540  		if t == nil {
   541  			t = make(Header)
   542  		}
   543  		for _, v := range w.handlerHeader[k] {
   544  			t.Add(k, v)
   545  		}
   546  	}
   547  	return t
   548  }
   549  
   550  // declareTrailer is called for each Trailer header when the
   551  // response header is written. It notes that a header will need to be
   552  // written in the trailers at the end of the response.
   553  func (w *response) declareTrailer(k string) {
   554  	k = CanonicalHeaderKey(k)
   555  	if !httpguts.ValidTrailerHeader(k) {
   556  		// Forbidden by RFC 7230, section 4.1.2
   557  		return
   558  	}
   559  	w.trailers = append(w.trailers, k)
   560  }
   561  
   562  // requestTooLarge is called by maxBytesReader when too much input has
   563  // been read from the client.
   564  func (w *response) requestTooLarge() {
   565  	w.closeAfterReply = true
   566  	w.requestBodyLimitHit = true
   567  	if !w.wroteHeader {
   568  		w.Header().Set("Connection", "close")
   569  	}
   570  }
   571  
   572  // disableWriteContinue stops Request.Body.Read from sending an automatic 100-Continue.
   573  // If a 100-Continue is being written, it waits for it to complete before continuing.
   574  func (w *response) disableWriteContinue() {
   575  	w.writeContinueMu.Lock()
   576  	w.canWriteContinue.Store(false)
   577  	w.writeContinueMu.Unlock()
   578  }
   579  
   580  // writerOnly hides an io.Writer value's optional ReadFrom method
   581  // from io.Copy.
   582  type writerOnly struct {
   583  	io.Writer
   584  }
   585  
   586  // ReadFrom is here to optimize copying from an [*os.File] regular file
   587  // to a [*net.TCPConn] with sendfile, or from a supported src type such
   588  // as a *net.TCPConn on Linux with splice.
   589  func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
   590  	buf := getCopyBuf()
   591  	defer putCopyBuf(buf)
   592  
   593  	// Our underlying w.conn.rwc is usually a *TCPConn (with its
   594  	// own ReadFrom method). If not, just fall back to the normal
   595  	// copy method.
   596  	rf, ok := w.conn.rwc.(io.ReaderFrom)
   597  	if !ok {
   598  		return io.CopyBuffer(writerOnly{w}, src, buf)
   599  	}
   600  
   601  	// Copy the first sniffLen bytes before switching to ReadFrom.
   602  	// This ensures we don't start writing the response before the
   603  	// source is available (see golang.org/issue/5660) and provides
   604  	// enough bytes to perform Content-Type sniffing when required.
   605  	if !w.cw.wroteHeader {
   606  		n0, err := io.CopyBuffer(writerOnly{w}, io.LimitReader(src, sniffLen), buf)
   607  		n += n0
   608  		if err != nil || n0 < sniffLen {
   609  			return n, err
   610  		}
   611  	}
   612  
   613  	w.w.Flush()  // get rid of any previous writes
   614  	w.cw.flush() // make sure Header is written; flush data to rwc
   615  
   616  	// Now that cw has been flushed, its chunking field is guaranteed initialized.
   617  	if !w.cw.chunking && w.bodyAllowed() && w.req.Method != "HEAD" {
   618  		n0, err := rf.ReadFrom(src)
   619  		n += n0
   620  		w.written += n0
   621  		return n, err
   622  	}
   623  
   624  	n0, err := io.CopyBuffer(writerOnly{w}, src, buf)
   625  	n += n0
   626  	return n, err
   627  }
   628  
   629  // debugServerConnections controls whether all server connections are wrapped
   630  // with a verbose logging wrapper.
   631  const debugServerConnections = false
   632  
   633  // Create new connection from rwc.
   634  func (s *Server) newConn(rwc net.Conn) *conn {
   635  	c := &conn{
   636  		server: s,
   637  		rwc:    rwc,
   638  	}
   639  	if debugServerConnections {
   640  		c.rwc = newLoggingConn("server", c.rwc)
   641  	}
   642  	return c
   643  }
   644  
   645  type readResult struct {
   646  	_   incomparable
   647  	n   int
   648  	err error
   649  	b   byte // byte read, if n == 1
   650  }
   651  
   652  // connReader is the io.Reader wrapper used by *conn. It combines a
   653  // selectively-activated io.LimitedReader (to bound request header
   654  // read sizes) with support for selectively keeping an io.Reader.Read
   655  // call blocked in a background goroutine to wait for activity and
   656  // trigger a CloseNotifier channel.
   657  // After a Handler has hijacked the conn and exited, connReader behaves like a
   658  // proxy for the net.Conn and the aforementioned behavior is bypassed.
   659  type connReader struct {
   660  	rwc net.Conn // rwc is the underlying network connection.
   661  
   662  	mu      sync.Mutex // guards following
   663  	conn    *conn      // conn is nil after handler exit.
   664  	hasByte bool
   665  	byteBuf [1]byte
   666  	cond    *sync.Cond
   667  	inRead  bool
   668  	aborted bool  // set true before conn.rwc deadline is set to past
   669  	remain  int64 // bytes remaining
   670  }
   671  
   672  func (cr *connReader) lock() {
   673  	cr.mu.Lock()
   674  	if cr.cond == nil {
   675  		cr.cond = sync.NewCond(&cr.mu)
   676  	}
   677  }
   678  
   679  func (cr *connReader) unlock() { cr.mu.Unlock() }
   680  
   681  func (cr *connReader) releaseConn() {
   682  	cr.lock()
   683  	defer cr.unlock()
   684  	cr.conn = nil
   685  }
   686  
   687  func (cr *connReader) startBackgroundRead() {
   688  	cr.lock()
   689  	defer cr.unlock()
   690  	if cr.inRead {
   691  		panic("invalid concurrent Body.Read call")
   692  	}
   693  	if cr.hasByte {
   694  		return
   695  	}
   696  	cr.inRead = true
   697  	cr.rwc.SetReadDeadline(time.Time{})
   698  	go cr.backgroundRead()
   699  }
   700  
   701  func (cr *connReader) backgroundRead() {
   702  	n, err := cr.rwc.Read(cr.byteBuf[:])
   703  	cr.lock()
   704  	if n == 1 {
   705  		cr.hasByte = true
   706  		// We were past the end of the previous request's body already
   707  		// (since we wouldn't be in a background read otherwise), so
   708  		// this is a pipelined HTTP request. Prior to Go 1.11 we used to
   709  		// send on the CloseNotify channel and cancel the context here,
   710  		// but the behavior was documented as only "may", and we only
   711  		// did that because that's how CloseNotify accidentally behaved
   712  		// in very early Go releases prior to context support. Once we
   713  		// added context support, people used a Handler's
   714  		// Request.Context() and passed it along. Having that context
   715  		// cancel on pipelined HTTP requests caused problems.
   716  		// Fortunately, almost nothing uses HTTP/1.x pipelining.
   717  		// Unfortunately, apt-get does, or sometimes does.
   718  		// New Go 1.11 behavior: don't fire CloseNotify or cancel
   719  		// contexts on pipelined requests. Shouldn't affect people, but
   720  		// fixes cases like Issue 23921. This does mean that a client
   721  		// closing their TCP connection after sending a pipelined
   722  		// request won't cancel the context, but we'll catch that on any
   723  		// write failure (in checkConnErrorWriter.Write).
   724  		// If the server never writes, yes, there are still contrived
   725  		// server & client behaviors where this fails to ever cancel the
   726  		// context, but that's kinda why HTTP/1.x pipelining died
   727  		// anyway.
   728  	}
   729  	if ne, ok := err.(net.Error); ok && cr.aborted && ne.Timeout() {
   730  		// Ignore this error. It's the expected error from
   731  		// another goroutine calling abortPendingRead.
   732  	} else if err != nil {
   733  		cr.handleReadErrorLocked(err)
   734  	}
   735  	cr.aborted = false
   736  	cr.inRead = false
   737  	cr.unlock()
   738  	cr.cond.Broadcast()
   739  }
   740  
   741  func (cr *connReader) abortPendingRead() {
   742  	cr.lock()
   743  	defer cr.unlock()
   744  	if !cr.inRead {
   745  		return
   746  	}
   747  	cr.aborted = true
   748  	cr.rwc.SetReadDeadline(aLongTimeAgo)
   749  	for cr.inRead {
   750  		cr.cond.Wait()
   751  	}
   752  	cr.rwc.SetReadDeadline(time.Time{})
   753  }
   754  
   755  func (cr *connReader) setReadLimit(remain int64) { cr.remain = remain }
   756  func (cr *connReader) setInfiniteReadLimit()     { cr.remain = maxInt64 }
   757  func (cr *connReader) hitReadLimit() bool        { return cr.remain <= 0 }
   758  
   759  // handleReadErrorLocked is called whenever a Read from the client returns a
   760  // non-nil error.
   761  //
   762  // The provided non-nil err is almost always io.EOF or a "use of
   763  // closed network connection". In any case, the error is not
   764  // particularly interesting, except perhaps for debugging during
   765  // development. Any error means the connection is dead and we should
   766  // down its context.
   767  //
   768  // The caller must hold connReader.mu.
   769  func (cr *connReader) handleReadErrorLocked(_ error) {
   770  	if cr.conn == nil {
   771  		return
   772  	}
   773  	cr.conn.cancelCtx()
   774  	if res := cr.conn.curReq.Load(); res != nil {
   775  		res.closeNotify()
   776  	}
   777  }
   778  
   779  func (cr *connReader) Read(p []byte) (n int, err error) {
   780  	cr.lock()
   781  	if cr.conn == nil {
   782  		cr.unlock()
   783  		return cr.rwc.Read(p)
   784  	}
   785  	if cr.inRead {
   786  		hijacked := cr.conn.hijacked()
   787  		cr.unlock()
   788  		if hijacked {
   789  			panic("invalid Body.Read call. After hijacked, the original Request must not be used")
   790  		}
   791  		panic("invalid concurrent Body.Read call")
   792  	}
   793  	if cr.hitReadLimit() {
   794  		cr.unlock()
   795  		return 0, io.EOF
   796  	}
   797  	if len(p) == 0 {
   798  		cr.unlock()
   799  		return 0, nil
   800  	}
   801  	if int64(len(p)) > cr.remain {
   802  		p = p[:cr.remain]
   803  	}
   804  	if cr.hasByte {
   805  		p[0] = cr.byteBuf[0]
   806  		cr.hasByte = false
   807  		cr.unlock()
   808  		return 1, nil
   809  	}
   810  	cr.inRead = true
   811  	cr.unlock()
   812  	n, err = cr.rwc.Read(p)
   813  
   814  	cr.lock()
   815  	cr.inRead = false
   816  	if err != nil {
   817  		cr.handleReadErrorLocked(err)
   818  	}
   819  	cr.remain -= int64(n)
   820  	cr.unlock()
   821  
   822  	cr.cond.Broadcast()
   823  	return n, err
   824  }
   825  
   826  var (
   827  	bufioReaderPool   sync.Pool
   828  	bufioWriter2kPool sync.Pool
   829  	bufioWriter4kPool sync.Pool
   830  )
   831  
   832  const copyBufPoolSize = 32 * 1024
   833  
   834  var copyBufPool = sync.Pool{New: func() any { return new([copyBufPoolSize]byte) }}
   835  
   836  func getCopyBuf() []byte {
   837  	return copyBufPool.Get().(*[copyBufPoolSize]byte)[:]
   838  }
   839  
   840  func putCopyBuf(b []byte) {
   841  	if len(b) != copyBufPoolSize {
   842  		panic("trying to put back buffer of the wrong size in the copyBufPool")
   843  	}
   844  	copyBufPool.Put((*[copyBufPoolSize]byte)(b))
   845  }
   846  
   847  func bufioWriterPool(size int) *sync.Pool {
   848  	switch size {
   849  	case 2 << 10:
   850  		return &bufioWriter2kPool
   851  	case 4 << 10:
   852  		return &bufioWriter4kPool
   853  	}
   854  	return nil
   855  }
   856  
   857  func newBufioReader(r io.Reader) *bufio.Reader {
   858  	if v := bufioReaderPool.Get(); v != nil {
   859  		br := v.(*bufio.Reader)
   860  		br.Reset(r)
   861  		return br
   862  	}
   863  	// Note: if this reader size is ever changed, update
   864  	// TestHandlerBodyClose's assumptions.
   865  	return bufio.NewReader(r)
   866  }
   867  
   868  func putBufioReader(br *bufio.Reader) {
   869  	br.Reset(nil)
   870  	bufioReaderPool.Put(br)
   871  }
   872  
   873  func newBufioWriterSize(w io.Writer, size int) *bufio.Writer {
   874  	pool := bufioWriterPool(size)
   875  	if pool != nil {
   876  		if v := pool.Get(); v != nil {
   877  			bw := v.(*bufio.Writer)
   878  			bw.Reset(w)
   879  			return bw
   880  		}
   881  	}
   882  	return bufio.NewWriterSize(w, size)
   883  }
   884  
   885  func putBufioWriter(bw *bufio.Writer) {
   886  	bw.Reset(nil)
   887  	if pool := bufioWriterPool(bw.Available()); pool != nil {
   888  		pool.Put(bw)
   889  	}
   890  }
   891  
   892  // DefaultMaxHeaderBytes is the maximum permitted size of the headers
   893  // in an HTTP request.
   894  // This can be overridden by setting [Server.MaxHeaderBytes].
   895  const DefaultMaxHeaderBytes = 1 << 20 // 1 MB
   896  
   897  func (s *Server) maxHeaderBytes() int {
   898  	if s.MaxHeaderBytes > 0 {
   899  		return s.MaxHeaderBytes
   900  	}
   901  	return DefaultMaxHeaderBytes
   902  }
   903  
   904  func (s *Server) initialReadLimitSize() int64 {
   905  	return int64(s.maxHeaderBytes()) + 4096 // bufio slop
   906  }
   907  
   908  // tlsHandshakeTimeout returns the time limit permitted for the TLS
   909  // handshake, or zero for unlimited.
   910  //
   911  // It returns the minimum of any positive ReadHeaderTimeout,
   912  // ReadTimeout, or WriteTimeout.
   913  func (s *Server) tlsHandshakeTimeout() time.Duration {
   914  	var ret time.Duration
   915  	for _, v := range [...]time.Duration{
   916  		s.ReadHeaderTimeout,
   917  		s.ReadTimeout,
   918  		s.WriteTimeout,
   919  	} {
   920  		if v <= 0 {
   921  			continue
   922  		}
   923  		if ret == 0 || v < ret {
   924  			ret = v
   925  		}
   926  	}
   927  	return ret
   928  }
   929  
   930  // wrapper around io.ReadCloser which on first read, sends an
   931  // HTTP/1.1 100 Continue header
   932  type expectContinueReader struct {
   933  	resp       *response
   934  	readCloser io.ReadCloser
   935  	closed     atomic.Bool
   936  	sawEOF     atomic.Bool
   937  }
   938  
   939  func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
   940  	if ecr.closed.Load() {
   941  		return 0, ErrBodyReadAfterClose
   942  	}
   943  	w := ecr.resp
   944  	if w.canWriteContinue.Load() {
   945  		w.writeContinueMu.Lock()
   946  		if w.canWriteContinue.Load() {
   947  			w.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
   948  			w.conn.bufw.Flush()
   949  			w.canWriteContinue.Store(false)
   950  		}
   951  		w.writeContinueMu.Unlock()
   952  	}
   953  	n, err = ecr.readCloser.Read(p)
   954  	if err == io.EOF {
   955  		ecr.sawEOF.Store(true)
   956  	}
   957  	return
   958  }
   959  
   960  func (ecr *expectContinueReader) Close() error {
   961  	ecr.closed.Store(true)
   962  	return ecr.readCloser.Close()
   963  }
   964  
   965  // TimeFormat is the time format to use when generating times in HTTP
   966  // headers. It is like [time.RFC1123] but hard-codes GMT as the time
   967  // zone. The time being formatted must be in UTC for Format to
   968  // generate the correct format.
   969  //
   970  // For parsing this time format, see [ParseTime].
   971  const TimeFormat = "Mon, 02 Jan 2006 15:04:05 GMT"
   972  
   973  var errTooLarge = errors.New("http: request too large")
   974  
   975  // Read next request from connection.
   976  func (c *conn) readRequest(ctx context.Context) (w *response, err error) {
   977  	if c.hijacked() {
   978  		return nil, ErrHijacked
   979  	}
   980  
   981  	var (
   982  		wholeReqDeadline time.Time // or zero if none
   983  		hdrDeadline      time.Time // or zero if none
   984  	)
   985  	t0 := time.Now()
   986  	if d := c.server.readHeaderTimeout(); d > 0 {
   987  		hdrDeadline = t0.Add(d)
   988  	}
   989  	if d := c.server.ReadTimeout; d > 0 {
   990  		wholeReqDeadline = t0.Add(d)
   991  	}
   992  	c.rwc.SetReadDeadline(hdrDeadline)
   993  	if d := c.server.WriteTimeout; d > 0 {
   994  		defer func() {
   995  			c.rwc.SetWriteDeadline(time.Now().Add(d))
   996  		}()
   997  	}
   998  
   999  	c.r.setReadLimit(c.server.initialReadLimitSize())
  1000  	if c.lastMethod == "POST" {
  1001  		// RFC 7230 section 3 tolerance for old buggy clients.
  1002  		peek, _ := c.bufr.Peek(4) // ReadRequest will get err below
  1003  		c.bufr.Discard(numLeadingCRorLF(peek))
  1004  	}
  1005  	req, err := readRequest(c.bufr)
  1006  	if err != nil {
  1007  		if c.r.hitReadLimit() {
  1008  			return nil, errTooLarge
  1009  		}
  1010  		return nil, err
  1011  	}
  1012  
  1013  	if !http1ServerSupportsRequest(req) {
  1014  		return nil, statusError{StatusHTTPVersionNotSupported, "unsupported protocol version"}
  1015  	}
  1016  
  1017  	c.lastMethod = req.Method
  1018  	c.r.setInfiniteReadLimit()
  1019  
  1020  	hosts, haveHost := req.Header["Host"]
  1021  	isH2Upgrade := req.isH2Upgrade()
  1022  	if req.ProtoAtLeast(1, 1) && (!haveHost || len(hosts) == 0) && !isH2Upgrade && req.Method != "CONNECT" {
  1023  		return nil, badRequestError("missing required Host header")
  1024  	}
  1025  	if len(hosts) == 1 && !httpguts.ValidHostHeader(hosts[0]) {
  1026  		return nil, badRequestError("malformed Host header")
  1027  	}
  1028  	for k, vv := range req.Header {
  1029  		if !httpguts.ValidHeaderFieldName(k) {
  1030  			return nil, badRequestError("invalid header name")
  1031  		}
  1032  		for _, v := range vv {
  1033  			if !httpguts.ValidHeaderFieldValue(v) {
  1034  				return nil, badRequestError("invalid header value")
  1035  			}
  1036  		}
  1037  	}
  1038  	delete(req.Header, "Host")
  1039  
  1040  	ctx, cancelCtx := context.WithCancel(ctx)
  1041  	req.ctx = ctx
  1042  	req.RemoteAddr = c.remoteAddr
  1043  	req.TLS = c.tlsState
  1044  	if body, ok := req.Body.(*body); ok {
  1045  		body.doEarlyClose = true
  1046  	}
  1047  
  1048  	// Adjust the read deadline if necessary.
  1049  	if !hdrDeadline.Equal(wholeReqDeadline) {
  1050  		c.rwc.SetReadDeadline(wholeReqDeadline)
  1051  	}
  1052  
  1053  	w = &response{
  1054  		conn:          c,
  1055  		cancelCtx:     cancelCtx,
  1056  		req:           req,
  1057  		reqBody:       req.Body,
  1058  		handlerHeader: make(Header),
  1059  		contentLength: -1,
  1060  
  1061  		// We populate these ahead of time so we're not
  1062  		// reading from req.Header after their Handler starts
  1063  		// and maybe mutates it (Issue 14940)
  1064  		wants10KeepAlive: req.wantsHttp10KeepAlive(),
  1065  		wantsClose:       req.wantsClose(),
  1066  	}
  1067  	if isH2Upgrade {
  1068  		w.closeAfterReply = true
  1069  	}
  1070  	w.cw.res = w
  1071  	w.w = newBufioWriterSize(&w.cw, bufferBeforeChunkingSize)
  1072  	return w, nil
  1073  }
  1074  
  1075  // http1ServerSupportsRequest reports whether Go's HTTP/1.x server
  1076  // supports the given request.
  1077  func http1ServerSupportsRequest(req *Request) bool {
  1078  	if req.ProtoMajor == 1 {
  1079  		return true
  1080  	}
  1081  	// Accept "PRI * HTTP/2.0" upgrade requests, so Handlers can
  1082  	// wire up their own HTTP/2 upgrades.
  1083  	if req.ProtoMajor == 2 && req.ProtoMinor == 0 &&
  1084  		req.Method == "PRI" && req.RequestURI == "*" {
  1085  		return true
  1086  	}
  1087  	// Reject HTTP/0.x, and all other HTTP/2+ requests (which
  1088  	// aren't encoded in ASCII anyway).
  1089  	return false
  1090  }
  1091  
  1092  func (w *response) Header() Header {
  1093  	if w.cw.header == nil && w.wroteHeader && !w.cw.wroteHeader {
  1094  		// Accessing the header between logically writing it
  1095  		// and physically writing it means we need to allocate
  1096  		// a clone to snapshot the logically written state.
  1097  		w.cw.header = w.handlerHeader.Clone()
  1098  	}
  1099  	w.calledHeader = true
  1100  	return w.handlerHeader
  1101  }
  1102  
  1103  // maxPostHandlerReadBytes is the max number of Request.Body bytes not
  1104  // consumed by a handler that the server will read from the client
  1105  // in order to keep a connection alive. If there are more bytes
  1106  // than this, the server, to be paranoid, instead sends a
  1107  // "Connection close" response.
  1108  //
  1109  // This number is approximately what a typical machine's TCP buffer
  1110  // size is anyway.  (if we have the bytes on the machine, we might as
  1111  // well read them)
  1112  const maxPostHandlerReadBytes = 256 << 10
  1113  
  1114  func checkWriteHeaderCode(code int) {
  1115  	// Issue 22880: require valid WriteHeader status codes.
  1116  	// For now we only enforce that it's three digits.
  1117  	// In the future we might block things over 599 (600 and above aren't defined
  1118  	// at https://httpwg.org/specs/rfc7231.html#status.codes).
  1119  	// But for now any three digits.
  1120  	//
  1121  	// We used to send "HTTP/1.1 000 0" on the wire in responses but there's
  1122  	// no equivalent bogus thing we can realistically send in HTTP/2,
  1123  	// so we'll consistently panic instead and help people find their bugs
  1124  	// early. (We can't return an error from WriteHeader even if we wanted to.)
  1125  	if code < 100 || code > 999 {
  1126  		panic(fmt.Sprintf("invalid WriteHeader code %v", code))
  1127  	}
  1128  }
  1129  
  1130  // relevantCaller searches the call stack for the first function outside of net/http.
  1131  // The purpose of this function is to provide more helpful error messages.
  1132  func relevantCaller() runtime.Frame {
  1133  	pc := make([]uintptr, 16)
  1134  	n := runtime.Callers(1, pc)
  1135  	frames := runtime.CallersFrames(pc[:n])
  1136  	var frame runtime.Frame
  1137  	for {
  1138  		frame, more := frames.Next()
  1139  		if !strings.HasPrefix(frame.Function, "net/http.") {
  1140  			return frame
  1141  		}
  1142  		if !more {
  1143  			break
  1144  		}
  1145  	}
  1146  	return frame
  1147  }
  1148  
  1149  func (w *response) WriteHeader(code int) {
  1150  	if w.conn.hijacked() {
  1151  		caller := relevantCaller()
  1152  		w.conn.server.logf("http: response.WriteHeader on hijacked connection from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
  1153  		return
  1154  	}
  1155  	if w.wroteHeader {
  1156  		caller := relevantCaller()
  1157  		w.conn.server.logf("http: superfluous response.WriteHeader call from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
  1158  		return
  1159  	}
  1160  	checkWriteHeaderCode(code)
  1161  
  1162  	if code < 101 || code > 199 {
  1163  		// Sending a 100 Continue or any non-1xx header disables the
  1164  		// automatically-sent 100 Continue from Request.Body.Read.
  1165  		w.disableWriteContinue()
  1166  	}
  1167  
  1168  	// Handle informational headers.
  1169  	//
  1170  	// We shouldn't send any further headers after 101 Switching Protocols,
  1171  	// so it takes the non-informational path.
  1172  	if code >= 100 && code <= 199 && code != StatusSwitchingProtocols {
  1173  		writeStatusLine(w.conn.bufw, w.req.ProtoAtLeast(1, 1), code, w.statusBuf[:])
  1174  
  1175  		// Per RFC 8297 we must not clear the current header map
  1176  		w.handlerHeader.WriteSubset(w.conn.bufw, excludedHeadersNoBody)
  1177  		w.conn.bufw.Write(crlf)
  1178  		w.conn.bufw.Flush()
  1179  
  1180  		return
  1181  	}
  1182  
  1183  	w.wroteHeader = true
  1184  	w.status = code
  1185  
  1186  	if w.calledHeader && w.cw.header == nil {
  1187  		w.cw.header = w.handlerHeader.Clone()
  1188  	}
  1189  
  1190  	if cl := w.handlerHeader.get("Content-Length"); cl != "" {
  1191  		v, err := strconv.ParseInt(cl, 10, 64)
  1192  		if err == nil && v >= 0 {
  1193  			w.contentLength = v
  1194  		} else {
  1195  			w.conn.server.logf("http: invalid Content-Length of %q", cl)
  1196  			w.handlerHeader.Del("Content-Length")
  1197  		}
  1198  	}
  1199  }
  1200  
  1201  // extraHeader is the set of headers sometimes added by chunkWriter.writeHeader.
  1202  // This type is used to avoid extra allocations from cloning and/or populating
  1203  // the response Header map and all its 1-element slices.
  1204  type extraHeader struct {
  1205  	contentType      string
  1206  	connection       string
  1207  	transferEncoding string
  1208  	date             []byte // written if not nil
  1209  	contentLength    []byte // written if not nil
  1210  }
  1211  
  1212  // Sorted the same as extraHeader.Write's loop.
  1213  var extraHeaderKeys = [][]byte{
  1214  	[]byte("Content-Type"),
  1215  	[]byte("Connection"),
  1216  	[]byte("Transfer-Encoding"),
  1217  }
  1218  
  1219  var (
  1220  	headerContentLength = []byte("Content-Length: ")
  1221  	headerDate          = []byte("Date: ")
  1222  )
  1223  
  1224  // Write writes the headers described in h to w.
  1225  //
  1226  // This method has a value receiver, despite the somewhat large size
  1227  // of h, because it prevents an allocation. The escape analysis isn't
  1228  // smart enough to realize this function doesn't mutate h.
  1229  func (h extraHeader) Write(w *bufio.Writer) {
  1230  	if h.date != nil {
  1231  		w.Write(headerDate)
  1232  		w.Write(h.date)
  1233  		w.Write(crlf)
  1234  	}
  1235  	if h.contentLength != nil {
  1236  		w.Write(headerContentLength)
  1237  		w.Write(h.contentLength)
  1238  		w.Write(crlf)
  1239  	}
  1240  	for i, v := range []string{h.contentType, h.connection, h.transferEncoding} {
  1241  		if v != "" {
  1242  			w.Write(extraHeaderKeys[i])
  1243  			w.Write(colonSpace)
  1244  			w.WriteString(v)
  1245  			w.Write(crlf)
  1246  		}
  1247  	}
  1248  }
  1249  
  1250  // writeHeader finalizes the header sent to the client and writes it
  1251  // to cw.res.conn.bufw.
  1252  //
  1253  // p is not written by writeHeader, but is the first chunk of the body
  1254  // that will be written. It is sniffed for a Content-Type if none is
  1255  // set explicitly. It's also used to set the Content-Length, if the
  1256  // total body size was small and the handler has already finished
  1257  // running.
  1258  func (cw *chunkWriter) writeHeader(p []byte) {
  1259  	if cw.wroteHeader {
  1260  		return
  1261  	}
  1262  	cw.wroteHeader = true
  1263  
  1264  	w := cw.res
  1265  	keepAlivesEnabled := w.conn.server.doKeepAlives()
  1266  	isHEAD := w.req.Method == "HEAD"
  1267  
  1268  	// header is written out to w.conn.buf below. Depending on the
  1269  	// state of the handler, we either own the map or not. If we
  1270  	// don't own it, the exclude map is created lazily for
  1271  	// WriteSubset to remove headers. The setHeader struct holds
  1272  	// headers we need to add.
  1273  	header := cw.header
  1274  	owned := header != nil
  1275  	if !owned {
  1276  		header = w.handlerHeader
  1277  	}
  1278  	var excludeHeader map[string]bool
  1279  	delHeader := func(key string) {
  1280  		if owned {
  1281  			header.Del(key)
  1282  			return
  1283  		}
  1284  		if _, ok := header[key]; !ok {
  1285  			return
  1286  		}
  1287  		if excludeHeader == nil {
  1288  			excludeHeader = make(map[string]bool)
  1289  		}
  1290  		excludeHeader[key] = true
  1291  	}
  1292  	var setHeader extraHeader
  1293  
  1294  	// Don't write out the fake "Trailer:foo" keys. See TrailerPrefix.
  1295  	trailers := false
  1296  	for k := range cw.header {
  1297  		if strings.HasPrefix(k, TrailerPrefix) {
  1298  			if excludeHeader == nil {
  1299  				excludeHeader = make(map[string]bool)
  1300  			}
  1301  			excludeHeader[k] = true
  1302  			trailers = true
  1303  		}
  1304  	}
  1305  	for _, v := range cw.header["Trailer"] {
  1306  		trailers = true
  1307  		foreachHeaderElement(v, cw.res.declareTrailer)
  1308  	}
  1309  
  1310  	te := header.get("Transfer-Encoding")
  1311  	hasTE := te != ""
  1312  
  1313  	// If the handler is done but never sent a Content-Length
  1314  	// response header and this is our first (and last) write, set
  1315  	// it, even to zero. This helps HTTP/1.0 clients keep their
  1316  	// "keep-alive" connections alive.
  1317  	// Exceptions: 304/204/1xx responses never get Content-Length, and if
  1318  	// it was a HEAD request, we don't know the difference between
  1319  	// 0 actual bytes and 0 bytes because the handler noticed it
  1320  	// was a HEAD request and chose not to write anything. So for
  1321  	// HEAD, the handler should either write the Content-Length or
  1322  	// write non-zero bytes. If it's actually 0 bytes and the
  1323  	// handler never looked at the Request.Method, we just don't
  1324  	// send a Content-Length header.
  1325  	// Further, we don't send an automatic Content-Length if they
  1326  	// set a Transfer-Encoding, because they're generally incompatible.
  1327  	if w.handlerDone.Load() && !trailers && !hasTE && bodyAllowedForStatus(w.status) && !header.has("Content-Length") && (!isHEAD || len(p) > 0) {
  1328  		w.contentLength = int64(len(p))
  1329  		setHeader.contentLength = strconv.AppendInt(cw.res.clenBuf[:0], int64(len(p)), 10)
  1330  	}
  1331  
  1332  	// If this was an HTTP/1.0 request with keep-alive and we sent a
  1333  	// Content-Length back, we can make this a keep-alive response ...
  1334  	if w.wants10KeepAlive && keepAlivesEnabled {
  1335  		sentLength := header.get("Content-Length") != ""
  1336  		if sentLength && header.get("Connection") == "keep-alive" {
  1337  			w.closeAfterReply = false
  1338  		}
  1339  	}
  1340  
  1341  	// Check for an explicit (and valid) Content-Length header.
  1342  	hasCL := w.contentLength != -1
  1343  
  1344  	if w.wants10KeepAlive && (isHEAD || hasCL || !bodyAllowedForStatus(w.status)) {
  1345  		_, connectionHeaderSet := header["Connection"]
  1346  		if !connectionHeaderSet {
  1347  			setHeader.connection = "keep-alive"
  1348  		}
  1349  	} else if !w.req.ProtoAtLeast(1, 1) || w.wantsClose {
  1350  		w.closeAfterReply = true
  1351  	}
  1352  
  1353  	if header.get("Connection") == "close" || !keepAlivesEnabled {
  1354  		w.closeAfterReply = true
  1355  	}
  1356  
  1357  	// If the client wanted a 100-continue but we never sent it to
  1358  	// them (or, more strictly: we never finished reading their
  1359  	// request body), don't reuse this connection.
  1360  	//
  1361  	// This behavior was first added on the theory that we don't know
  1362  	// if the next bytes on the wire are going to be the remainder of
  1363  	// the request body or the subsequent request (see issue 11549),
  1364  	// but that's not correct: If we keep using the connection,
  1365  	// the client is required to send the request body whether we
  1366  	// asked for it or not.
  1367  	//
  1368  	// We probably do want to skip reusing the connection in most cases,
  1369  	// however. If the client is offering a large request body that we
  1370  	// don't intend to use, then it's better to close the connection
  1371  	// than to read the body. For now, assume that if we're sending
  1372  	// headers, the handler is done reading the body and we should
  1373  	// drop the connection if we haven't seen EOF.
  1374  	if ecr, ok := w.req.Body.(*expectContinueReader); ok && !ecr.sawEOF.Load() {
  1375  		w.closeAfterReply = true
  1376  	}
  1377  
  1378  	// We do this by default because there are a number of clients that
  1379  	// send a full request before starting to read the response, and they
  1380  	// can deadlock if we start writing the response with unconsumed body
  1381  	// remaining. See Issue 15527 for some history.
  1382  	//
  1383  	// If full duplex mode has been enabled with ResponseController.EnableFullDuplex,
  1384  	// then leave the request body alone.
  1385  	//
  1386  	// We don't take this path when w.closeAfterReply is set.
  1387  	// We may not need to consume the request to get ready for the next one
  1388  	// (since we're closing the conn), but a client which sends a full request
  1389  	// before reading a response may deadlock in this case.
  1390  	// This behavior has been present since CL 5268043 (2011), however,
  1391  	// so it doesn't seem to be causing problems.
  1392  	if w.req.ContentLength != 0 && !w.closeAfterReply && !w.fullDuplex {
  1393  		var discard, tooBig bool
  1394  
  1395  		switch bdy := w.req.Body.(type) {
  1396  		case *expectContinueReader:
  1397  			// We only get here if we have already fully consumed the request body
  1398  			// (see above).
  1399  		case *body:
  1400  			bdy.mu.Lock()
  1401  			switch {
  1402  			case bdy.closed:
  1403  				if !bdy.sawEOF {
  1404  					// Body was closed in handler with non-EOF error.
  1405  					w.closeAfterReply = true
  1406  				}
  1407  			case bdy.unreadDataSizeLocked() >= maxPostHandlerReadBytes:
  1408  				tooBig = true
  1409  			default:
  1410  				discard = true
  1411  			}
  1412  			bdy.mu.Unlock()
  1413  		default:
  1414  			discard = true
  1415  		}
  1416  
  1417  		if discard {
  1418  			_, err := io.CopyN(io.Discard, w.reqBody, maxPostHandlerReadBytes+1)
  1419  			switch err {
  1420  			case nil:
  1421  				// There must be even more data left over.
  1422  				tooBig = true
  1423  			case ErrBodyReadAfterClose:
  1424  				// Body was already consumed and closed.
  1425  			case io.EOF:
  1426  				// The remaining body was just consumed, close it.
  1427  				err = w.reqBody.Close()
  1428  				if err != nil {
  1429  					w.closeAfterReply = true
  1430  				}
  1431  			default:
  1432  				// Some other kind of error occurred, like a read timeout, or
  1433  				// corrupt chunked encoding. In any case, whatever remains
  1434  				// on the wire must not be parsed as another HTTP request.
  1435  				w.closeAfterReply = true
  1436  			}
  1437  		}
  1438  
  1439  		if tooBig {
  1440  			w.requestTooLarge()
  1441  			delHeader("Connection")
  1442  			setHeader.connection = "close"
  1443  		}
  1444  	}
  1445  
  1446  	code := w.status
  1447  	if bodyAllowedForStatus(code) {
  1448  		// If no content type, apply sniffing algorithm to body.
  1449  		_, haveType := header["Content-Type"]
  1450  
  1451  		// If the Content-Encoding was set and is non-blank,
  1452  		// we shouldn't sniff the body. See Issue 31753.
  1453  		ce := header.Get("Content-Encoding")
  1454  		hasCE := len(ce) > 0
  1455  		if !hasCE && !haveType && !hasTE && len(p) > 0 {
  1456  			setHeader.contentType = DetectContentType(p)
  1457  		}
  1458  	} else {
  1459  		for _, k := range suppressedHeaders(code) {
  1460  			delHeader(k)
  1461  		}
  1462  	}
  1463  
  1464  	if !header.has("Date") {
  1465  		setHeader.date = time.Now().UTC().AppendFormat(cw.res.dateBuf[:0], TimeFormat)
  1466  	}
  1467  
  1468  	if hasCL && hasTE && te != "identity" {
  1469  		// TODO: return an error if WriteHeader gets a return parameter
  1470  		// For now just ignore the Content-Length.
  1471  		w.conn.server.logf("http: WriteHeader called with both Transfer-Encoding of %q and a Content-Length of %d",
  1472  			te, w.contentLength)
  1473  		delHeader("Content-Length")
  1474  		hasCL = false
  1475  	}
  1476  
  1477  	if w.req.Method == "HEAD" || !bodyAllowedForStatus(code) || code == StatusNoContent {
  1478  		// Response has no body.
  1479  		delHeader("Transfer-Encoding")
  1480  	} else if hasCL {
  1481  		// Content-Length has been provided, so no chunking is to be done.
  1482  		delHeader("Transfer-Encoding")
  1483  	} else if w.req.ProtoAtLeast(1, 1) {
  1484  		// HTTP/1.1 or greater: Transfer-Encoding has been set to identity, and no
  1485  		// content-length has been provided. The connection must be closed after the
  1486  		// reply is written, and no chunking is to be done. This is the setup
  1487  		// recommended in the Server-Sent Events candidate recommendation 11,
  1488  		// section 8.
  1489  		if hasTE && te == "identity" {
  1490  			cw.chunking = false
  1491  			w.closeAfterReply = true
  1492  			delHeader("Transfer-Encoding")
  1493  		} else {
  1494  			// HTTP/1.1 or greater: use chunked transfer encoding
  1495  			// to avoid closing the connection at EOF.
  1496  			cw.chunking = true
  1497  			setHeader.transferEncoding = "chunked"
  1498  			if hasTE && te == "chunked" {
  1499  				// We will send the chunked Transfer-Encoding header later.
  1500  				delHeader("Transfer-Encoding")
  1501  			}
  1502  		}
  1503  	} else {
  1504  		// HTTP version < 1.1: cannot do chunked transfer
  1505  		// encoding and we don't know the Content-Length so
  1506  		// signal EOF by closing connection.
  1507  		w.closeAfterReply = true
  1508  		delHeader("Transfer-Encoding") // in case already set
  1509  	}
  1510  
  1511  	// Cannot use Content-Length with non-identity Transfer-Encoding.
  1512  	if cw.chunking {
  1513  		delHeader("Content-Length")
  1514  	}
  1515  	if !w.req.ProtoAtLeast(1, 0) {
  1516  		return
  1517  	}
  1518  
  1519  	// Only override the Connection header if it is not a successful
  1520  	// protocol switch response and if KeepAlives are not enabled.
  1521  	// See https://golang.org/issue/36381.
  1522  	delConnectionHeader := w.closeAfterReply &&
  1523  		(!keepAlivesEnabled || !hasToken(cw.header.get("Connection"), "close")) &&
  1524  		!isProtocolSwitchResponse(w.status, header)
  1525  	if delConnectionHeader {
  1526  		delHeader("Connection")
  1527  		if w.req.ProtoAtLeast(1, 1) {
  1528  			setHeader.connection = "close"
  1529  		}
  1530  	}
  1531  
  1532  	writeStatusLine(w.conn.bufw, w.req.ProtoAtLeast(1, 1), code, w.statusBuf[:])
  1533  	cw.header.WriteSubset(w.conn.bufw, excludeHeader)
  1534  	setHeader.Write(w.conn.bufw)
  1535  	w.conn.bufw.Write(crlf)
  1536  }
  1537  
  1538  // foreachHeaderElement splits v according to the "#rule" construction
  1539  // in RFC 7230 section 7 and calls fn for each non-empty element.
  1540  func foreachHeaderElement(v string, fn func(string)) {
  1541  	v = textproto.TrimString(v)
  1542  	if v == "" {
  1543  		return
  1544  	}
  1545  	if !strings.Contains(v, ",") {
  1546  		fn(v)
  1547  		return
  1548  	}
  1549  	for f := range strings.SplitSeq(v, ",") {
  1550  		if f = textproto.TrimString(f); f != "" {
  1551  			fn(f)
  1552  		}
  1553  	}
  1554  }
  1555  
  1556  // writeStatusLine writes an HTTP/1.x Status-Line (RFC 7230 Section 3.1.2)
  1557  // to bw. is11 is whether the HTTP request is HTTP/1.1. false means HTTP/1.0.
  1558  // code is the response status code.
  1559  // scratch is an optional scratch buffer. If it has at least capacity 3, it's used.
  1560  func writeStatusLine(bw *bufio.Writer, is11 bool, code int, scratch []byte) {
  1561  	if is11 {
  1562  		bw.WriteString("HTTP/1.1 ")
  1563  	} else {
  1564  		bw.WriteString("HTTP/1.0 ")
  1565  	}
  1566  	if text := StatusText(code); text != "" {
  1567  		bw.Write(strconv.AppendInt(scratch[:0], int64(code), 10))
  1568  		bw.WriteByte(' ')
  1569  		bw.WriteString(text)
  1570  		bw.WriteString("\r\n")
  1571  	} else {
  1572  		// don't worry about performance
  1573  		fmt.Fprintf(bw, "%03d status code %d\r\n", code, code)
  1574  	}
  1575  }
  1576  
  1577  // bodyAllowed reports whether a Write is allowed for this response type.
  1578  // It's illegal to call this before the header has been flushed.
  1579  func (w *response) bodyAllowed() bool {
  1580  	if !w.wroteHeader {
  1581  		panic("net/http: bodyAllowed called before the header was written")
  1582  	}
  1583  	return bodyAllowedForStatus(w.status)
  1584  }
  1585  
  1586  // The Life Of A Write is like this:
  1587  //
  1588  // Handler starts. No header has been sent. The handler can either
  1589  // write a header, or just start writing. Writing before sending a header
  1590  // sends an implicitly empty 200 OK header.
  1591  //
  1592  // If the handler didn't declare a Content-Length up front, we either
  1593  // go into chunking mode or, if the handler finishes running before
  1594  // the chunking buffer size, we compute a Content-Length and send that
  1595  // in the header instead.
  1596  //
  1597  // Likewise, if the handler didn't set a Content-Type, we sniff that
  1598  // from the initial chunk of output.
  1599  //
  1600  // The Writers are wired together like:
  1601  //
  1602  //  1. *response (the ResponseWriter) ->
  1603  //  2. (*response).w, a [*bufio.Writer] of bufferBeforeChunkingSize bytes ->
  1604  //  3. chunkWriter.Writer (whose writeHeader finalizes Content-Length/Type)
  1605  //     and which writes the chunk headers, if needed ->
  1606  //  4. conn.bufw, a *bufio.Writer of default (4kB) bytes, writing to ->
  1607  //  5. checkConnErrorWriter{c}, which notes any non-nil error on Write
  1608  //     and populates c.werr with it if so, but otherwise writes to ->
  1609  //  6. the rwc, the [net.Conn].
  1610  //
  1611  // TODO(bradfitz): short-circuit some of the buffering when the
  1612  // initial header contains both a Content-Type and Content-Length.
  1613  // Also short-circuit in (1) when the header's been sent and not in
  1614  // chunking mode, writing directly to (4) instead, if (2) has no
  1615  // buffered data. More generally, we could short-circuit from (1) to
  1616  // (3) even in chunking mode if the write size from (1) is over some
  1617  // threshold and nothing is in (2).  The answer might be mostly making
  1618  // bufferBeforeChunkingSize smaller and having bufio's fast-paths deal
  1619  // with this instead.
  1620  func (w *response) Write(data []byte) (n int, err error) {
  1621  	return w.write(len(data), data, "")
  1622  }
  1623  
  1624  func (w *response) WriteString(data string) (n int, err error) {
  1625  	return w.write(len(data), nil, data)
  1626  }
  1627  
  1628  // either dataB or dataS is non-zero.
  1629  func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err error) {
  1630  	if w.conn.hijacked() {
  1631  		if lenData > 0 {
  1632  			caller := relevantCaller()
  1633  			w.conn.server.logf("http: response.Write on hijacked connection from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
  1634  		}
  1635  		return 0, ErrHijacked
  1636  	}
  1637  
  1638  	if w.canWriteContinue.Load() {
  1639  		// Body reader wants to write 100 Continue but hasn't yet. Tell it not to.
  1640  		w.disableWriteContinue()
  1641  	}
  1642  
  1643  	if !w.wroteHeader {
  1644  		w.WriteHeader(StatusOK)
  1645  	}
  1646  	if lenData == 0 {
  1647  		return 0, nil
  1648  	}
  1649  	if !w.bodyAllowed() {
  1650  		return 0, ErrBodyNotAllowed
  1651  	}
  1652  
  1653  	w.written += int64(lenData) // ignoring errors, for errorKludge
  1654  	if w.contentLength != -1 && w.written > w.contentLength {
  1655  		return 0, ErrContentLength
  1656  	}
  1657  	if dataB != nil {
  1658  		return w.w.Write(dataB)
  1659  	} else {
  1660  		return w.w.WriteString(dataS)
  1661  	}
  1662  }
  1663  
  1664  func (w *response) finishRequest() {
  1665  	w.handlerDone.Store(true)
  1666  
  1667  	if !w.wroteHeader {
  1668  		w.WriteHeader(StatusOK)
  1669  	}
  1670  
  1671  	w.w.Flush()
  1672  	putBufioWriter(w.w)
  1673  	w.cw.close()
  1674  	w.conn.bufw.Flush()
  1675  
  1676  	w.conn.r.abortPendingRead()
  1677  
  1678  	// Close the body (regardless of w.closeAfterReply) so we can
  1679  	// re-use its bufio.Reader later safely.
  1680  	w.reqBody.Close()
  1681  
  1682  	if w.req.MultipartForm != nil {
  1683  		w.req.MultipartForm.RemoveAll()
  1684  	}
  1685  }
  1686  
  1687  // shouldReuseConnection reports whether the underlying TCP connection can be reused.
  1688  // It must only be called after the handler is done executing.
  1689  func (w *response) shouldReuseConnection() bool {
  1690  	if w.closeAfterReply {
  1691  		// The request or something set while executing the
  1692  		// handler indicated we shouldn't reuse this
  1693  		// connection.
  1694  		return false
  1695  	}
  1696  
  1697  	if w.req.Method != "HEAD" && w.contentLength != -1 && w.bodyAllowed() && w.contentLength != w.written {
  1698  		// Did not write enough. Avoid getting out of sync.
  1699  		return false
  1700  	}
  1701  
  1702  	// There was some error writing to the underlying connection
  1703  	// during the request, so don't re-use this conn.
  1704  	if w.conn.werr != nil {
  1705  		return false
  1706  	}
  1707  
  1708  	if w.closedRequestBodyEarly() {
  1709  		return false
  1710  	}
  1711  
  1712  	return true
  1713  }
  1714  
  1715  func (w *response) closedRequestBodyEarly() bool {
  1716  	body, ok := w.req.Body.(*body)
  1717  	return ok && body.didEarlyClose()
  1718  }
  1719  
  1720  func (w *response) Flush() {
  1721  	w.FlushError()
  1722  }
  1723  
  1724  func (w *response) FlushError() error {
  1725  	if !w.wroteHeader {
  1726  		w.WriteHeader(StatusOK)
  1727  	}
  1728  	err := w.w.Flush()
  1729  	e2 := w.cw.flush()
  1730  	if err == nil {
  1731  		err = e2
  1732  	}
  1733  	return err
  1734  }
  1735  
  1736  func (c *conn) finalFlush() {
  1737  	if c.bufr != nil {
  1738  		// Steal the bufio.Reader (~4KB worth of memory) and its associated
  1739  		// reader for a future connection.
  1740  		putBufioReader(c.bufr)
  1741  		c.bufr = nil
  1742  	}
  1743  
  1744  	if c.bufw != nil {
  1745  		c.bufw.Flush()
  1746  		// Steal the bufio.Writer (~4KB worth of memory) and its associated
  1747  		// writer for a future connection.
  1748  		putBufioWriter(c.bufw)
  1749  		c.bufw = nil
  1750  	}
  1751  }
  1752  
  1753  // Close the connection.
  1754  func (c *conn) close() {
  1755  	c.finalFlush()
  1756  	c.rwc.Close()
  1757  }
  1758  
  1759  // rstAvoidanceDelay is the amount of time we sleep after closing the
  1760  // write side of a TCP connection before closing the entire socket.
  1761  // By sleeping, we increase the chances that the client sees our FIN
  1762  // and processes its final data before they process the subsequent RST
  1763  // from closing a connection with known unread data.
  1764  // This RST seems to occur mostly on BSD systems. (And Windows?)
  1765  // This timeout is somewhat arbitrary (~latency around the planet),
  1766  // and may be modified by tests.
  1767  //
  1768  // TODO(bcmills): This should arguably be a server configuration parameter,
  1769  // not a hard-coded value.
  1770  var rstAvoidanceDelay = 500 * time.Millisecond
  1771  
  1772  type closeWriter interface {
  1773  	CloseWrite() error
  1774  }
  1775  
  1776  var _ closeWriter = (*net.TCPConn)(nil)
  1777  
  1778  // closeWriteAndWait flushes any outstanding data and sends a FIN packet (if
  1779  // client is connected via TCP), signaling that we're done. We then
  1780  // pause for a bit, hoping the client processes it before any
  1781  // subsequent RST.
  1782  //
  1783  // See https://golang.org/issue/3595
  1784  func (c *conn) closeWriteAndWait() {
  1785  	c.finalFlush()
  1786  	if tcp, ok := c.rwc.(closeWriter); ok {
  1787  		tcp.CloseWrite()
  1788  	}
  1789  
  1790  	// When we return from closeWriteAndWait, the caller will fully close the
  1791  	// connection. If client is still writing to the connection, this will cause
  1792  	// the write to fail with ECONNRESET or similar. Unfortunately, many TCP
  1793  	// implementations will also drop unread packets from the client's read buffer
  1794  	// when a write fails, causing our final response to be truncated away too.
  1795  	//
  1796  	// As a result, https://www.rfc-editor.org/rfc/rfc7230#section-6.6 recommends
  1797  	// that “[t]he server … continues to read from the connection until it
  1798  	// receives a corresponding close by the client, or until the server is
  1799  	// reasonably certain that its own TCP stack has received the client's
  1800  	// acknowledgement of the packet(s) containing the server's last response.”
  1801  	//
  1802  	// Unfortunately, we have no straightforward way to be “reasonably certain”
  1803  	// that we have received the client's ACK, and at any rate we don't want to
  1804  	// allow a misbehaving client to soak up server connections indefinitely by
  1805  	// withholding an ACK, nor do we want to go through the complexity or overhead
  1806  	// of using low-level APIs to figure out when a TCP round-trip has completed.
  1807  	//
  1808  	// Instead, we declare that we are “reasonably certain” that we received the
  1809  	// ACK if maxRSTAvoidanceDelay has elapsed.
  1810  	time.Sleep(rstAvoidanceDelay)
  1811  }
  1812  
  1813  // validNextProto reports whether the proto is a valid ALPN protocol name.
  1814  // Everything is valid except the empty string and built-in protocol types,
  1815  // so that those can't be overridden with alternate implementations.
  1816  func validNextProto(proto string) bool {
  1817  	switch proto {
  1818  	case "", "http/1.1", "http/1.0":
  1819  		return false
  1820  	}
  1821  	return true
  1822  }
  1823  
  1824  const (
  1825  	runHooks  = true
  1826  	skipHooks = false
  1827  )
  1828  
  1829  func (c *conn) setState(nc net.Conn, state ConnState, runHook bool) {
  1830  	srv := c.server
  1831  	switch state {
  1832  	case StateNew:
  1833  		srv.trackConn(c, true)
  1834  	case StateHijacked, StateClosed:
  1835  		srv.trackConn(c, false)
  1836  	}
  1837  	if state > 0xff || state < 0 {
  1838  		panic("internal error")
  1839  	}
  1840  	packedState := uint64(time.Now().Unix()<<8) | uint64(state)
  1841  	c.curState.Store(packedState)
  1842  	if !runHook {
  1843  		return
  1844  	}
  1845  	if hook := srv.ConnState; hook != nil {
  1846  		hook(nc, state)
  1847  	}
  1848  }
  1849  
  1850  func (c *conn) getState() (state ConnState, unixSec int64) {
  1851  	packedState := c.curState.Load()
  1852  	return ConnState(packedState & 0xff), int64(packedState >> 8)
  1853  }
  1854  
  1855  // badRequestError is a literal string (used by in the server in HTML,
  1856  // unescaped) to tell the user why their request was bad. It should
  1857  // be plain text without user info or other embedded errors.
  1858  func badRequestError(e string) error { return statusError{StatusBadRequest, e} }
  1859  
  1860  // statusError is an error used to respond to a request with an HTTP status.
  1861  // The text should be plain text without user info or other embedded errors.
  1862  type statusError struct {
  1863  	code int
  1864  	text string
  1865  }
  1866  
  1867  func (e statusError) Error() string { return StatusText(e.code) + ": " + e.text }
  1868  
  1869  // ErrAbortHandler is a sentinel panic value to abort a handler.
  1870  // While any panic from ServeHTTP aborts the response to the client,
  1871  // panicking with ErrAbortHandler also suppresses logging of a stack
  1872  // trace to the server's error log.
  1873  var ErrAbortHandler = errors.New("net/http: abort Handler")
  1874  
  1875  // isCommonNetReadError reports whether err is a common error
  1876  // encountered during reading a request off the network when the
  1877  // client has gone away or had its read fail somehow. This is used to
  1878  // determine which logs are interesting enough to log about.
  1879  func isCommonNetReadError(err error) bool {
  1880  	if err == io.EOF {
  1881  		return true
  1882  	}
  1883  	if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
  1884  		return true
  1885  	}
  1886  	if oe, ok := err.(*net.OpError); ok && oe.Op == "read" {
  1887  		return true
  1888  	}
  1889  	return false
  1890  }
  1891  
  1892  type connectionStater interface {
  1893  	ConnectionState() tls.ConnectionState
  1894  }
  1895  
  1896  // Serve a new connection.
  1897  func (c *conn) serve(ctx context.Context) {
  1898  	if ra := c.rwc.RemoteAddr(); ra != nil {
  1899  		c.remoteAddr = ra.String()
  1900  	}
  1901  	ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
  1902  	var inFlightResponse *response
  1903  	defer func() {
  1904  		if err := recover(); err != nil && err != ErrAbortHandler {
  1905  			const size = 64 << 10
  1906  			buf := make([]byte, size)
  1907  			buf = buf[:runtime.Stack(buf, false)]
  1908  			c.server.logf("http: panic serving %v: %v\n%s", c.remoteAddr, err, buf)
  1909  		}
  1910  		if inFlightResponse != nil {
  1911  			inFlightResponse.cancelCtx()
  1912  			inFlightResponse.disableWriteContinue()
  1913  		}
  1914  		if !c.hijacked() {
  1915  			if inFlightResponse != nil {
  1916  				inFlightResponse.conn.r.abortPendingRead()
  1917  				inFlightResponse.reqBody.Close()
  1918  			}
  1919  			c.close()
  1920  			c.setState(c.rwc, StateClosed, runHooks)
  1921  		}
  1922  	}()
  1923  
  1924  	if tlsConn, ok := c.rwc.(*tls.Conn); ok {
  1925  		tlsTO := c.server.tlsHandshakeTimeout()
  1926  		if tlsTO > 0 {
  1927  			dl := time.Now().Add(tlsTO)
  1928  			c.rwc.SetReadDeadline(dl)
  1929  			c.rwc.SetWriteDeadline(dl)
  1930  		}
  1931  		if err := tlsConn.HandshakeContext(ctx); err != nil {
  1932  			// If the handshake failed due to the client not speaking
  1933  			// TLS, assume they're speaking plaintext HTTP and write a
  1934  			// 400 response on the TLS conn's underlying net.Conn.
  1935  			var reason string
  1936  			if re, ok := err.(tls.RecordHeaderError); ok && re.Conn != nil && tlsRecordHeaderLooksLikeHTTP(re.RecordHeader) {
  1937  				io.WriteString(re.Conn, "HTTP/1.0 400 Bad Request\r\n\r\nClient sent an HTTP request to an HTTPS server.\n")
  1938  				re.Conn.Close()
  1939  				reason = "client sent an HTTP request to an HTTPS server"
  1940  			} else {
  1941  				reason = err.Error()
  1942  			}
  1943  			c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), reason)
  1944  			return
  1945  		}
  1946  		// Restore Conn-level deadlines.
  1947  		if tlsTO > 0 {
  1948  			c.rwc.SetReadDeadline(time.Time{})
  1949  			c.rwc.SetWriteDeadline(time.Time{})
  1950  		}
  1951  		c.tlsState = new(tls.ConnectionState)
  1952  		*c.tlsState = tlsConn.ConnectionState()
  1953  		if proto := c.tlsState.NegotiatedProtocol; validNextProto(proto) {
  1954  			if fn := c.server.TLSNextProto[proto]; fn != nil {
  1955  				h := initALPNRequest{ctx, tlsConn, serverHandler{c.server}}
  1956  				// Mark freshly created HTTP/2 as active and prevent any server state hooks
  1957  				// from being run on these connections. This prevents closeIdleConns from
  1958  				// closing such connections. See issue https://golang.org/issue/39776.
  1959  				c.setState(c.rwc, StateActive, skipHooks)
  1960  				fn(c.server, tlsConn, h)
  1961  			}
  1962  			return
  1963  		}
  1964  	}
  1965  
  1966  	// HTTP/1.x from here on.
  1967  
  1968  	// Set Request.TLS if the conn is not a *tls.Conn, but implements ConnectionState.
  1969  	if c.tlsState == nil {
  1970  		if tc, ok := c.rwc.(connectionStater); ok {
  1971  			c.tlsState = new(tls.ConnectionState)
  1972  			*c.tlsState = tc.ConnectionState()
  1973  		}
  1974  	}
  1975  
  1976  	ctx, cancelCtx := context.WithCancel(ctx)
  1977  	c.cancelCtx = cancelCtx
  1978  	defer cancelCtx()
  1979  
  1980  	c.r = &connReader{conn: c, rwc: c.rwc}
  1981  	c.bufr = newBufioReader(c.r)
  1982  	c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
  1983  
  1984  	protos := c.server.protocols()
  1985  	if c.tlsState == nil && protos.UnencryptedHTTP2() {
  1986  		if c.maybeServeUnencryptedHTTP2(ctx) {
  1987  			return
  1988  		}
  1989  	}
  1990  	if !protos.HTTP1() {
  1991  		return
  1992  	}
  1993  
  1994  	for {
  1995  		w, err := c.readRequest(ctx)
  1996  		if c.r.remain != c.server.initialReadLimitSize() {
  1997  			// If we read any bytes off the wire, we're active.
  1998  			c.setState(c.rwc, StateActive, runHooks)
  1999  		}
  2000  		if c.server.shuttingDown() {
  2001  			return
  2002  		}
  2003  		if err != nil {
  2004  			const errorHeaders = "\r\nContent-Type: text/plain; charset=utf-8\r\nConnection: close\r\n\r\n"
  2005  
  2006  			switch {
  2007  			case err == errTooLarge:
  2008  				// Their HTTP client may or may not be
  2009  				// able to read this if we're
  2010  				// responding to them and hanging up
  2011  				// while they're still writing their
  2012  				// request. Undefined behavior.
  2013  				const publicErr = "431 Request Header Fields Too Large"
  2014  				fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
  2015  				c.closeWriteAndWait()
  2016  				return
  2017  
  2018  			case isUnsupportedTEError(err):
  2019  				// Respond as per RFC 7230 Section 3.3.1 which says,
  2020  				//      A server that receives a request message with a
  2021  				//      transfer coding it does not understand SHOULD
  2022  				//      respond with 501 (Unimplemented).
  2023  				code := StatusNotImplemented
  2024  
  2025  				// We purposefully aren't echoing back the transfer-encoding's value,
  2026  				// so as to mitigate the risk of cross side scripting by an attacker.
  2027  				fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s%sUnsupported transfer encoding", code, StatusText(code), errorHeaders)
  2028  				return
  2029  
  2030  			case isCommonNetReadError(err):
  2031  				return // don't reply
  2032  
  2033  			default:
  2034  				if v, ok := err.(statusError); ok {
  2035  					fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s: %s%s%d %s: %s", v.code, StatusText(v.code), v.text, errorHeaders, v.code, StatusText(v.code), v.text)
  2036  					return
  2037  				}
  2038  				const publicErr = "400 Bad Request"
  2039  				fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
  2040  				return
  2041  			}
  2042  		}
  2043  
  2044  		// Expect 100 Continue support
  2045  		req := w.req
  2046  		if req.expectsContinue() {
  2047  			if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
  2048  				// Wrap the Body reader with one that replies on the connection
  2049  				req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
  2050  				w.canWriteContinue.Store(true)
  2051  			}
  2052  		} else if req.Header.get("Expect") != "" {
  2053  			w.sendExpectationFailed()
  2054  			return
  2055  		}
  2056  
  2057  		c.curReq.Store(w)
  2058  
  2059  		if requestBodyRemains(req.Body) {
  2060  			registerOnHitEOF(req.Body, w.conn.r.startBackgroundRead)
  2061  		} else {
  2062  			w.conn.r.startBackgroundRead()
  2063  		}
  2064  
  2065  		// HTTP cannot have multiple simultaneous active requests.[*]
  2066  		// Until the server replies to this request, it can't read another,
  2067  		// so we might as well run the handler in this goroutine.
  2068  		// [*] Not strictly true: HTTP pipelining. We could let them all process
  2069  		// in parallel even if their responses need to be serialized.
  2070  		// But we're not going to implement HTTP pipelining because it
  2071  		// was never deployed in the wild and the answer is HTTP/2.
  2072  		inFlightResponse = w
  2073  		serverHandler{c.server}.ServeHTTP(w, w.req)
  2074  		inFlightResponse = nil
  2075  		w.cancelCtx()
  2076  		if c.hijacked() {
  2077  			c.r.releaseConn()
  2078  			return
  2079  		}
  2080  		w.finishRequest()
  2081  		c.rwc.SetWriteDeadline(time.Time{})
  2082  		if !w.shouldReuseConnection() {
  2083  			if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
  2084  				c.closeWriteAndWait()
  2085  			}
  2086  			return
  2087  		}
  2088  		c.setState(c.rwc, StateIdle, runHooks)
  2089  		c.curReq.Store(nil)
  2090  
  2091  		if !w.conn.server.doKeepAlives() {
  2092  			// We're in shutdown mode. We might've replied
  2093  			// to the user without "Connection: close" and
  2094  			// they might think they can send another
  2095  			// request, but such is life with HTTP/1.1.
  2096  			return
  2097  		}
  2098  
  2099  		if d := c.server.idleTimeout(); d > 0 {
  2100  			c.rwc.SetReadDeadline(time.Now().Add(d))
  2101  		} else {
  2102  			c.rwc.SetReadDeadline(time.Time{})
  2103  		}
  2104  
  2105  		// Wait for the connection to become readable again before trying to
  2106  		// read the next request. This prevents a ReadHeaderTimeout or
  2107  		// ReadTimeout from starting until the first bytes of the next request
  2108  		// have been received.
  2109  		if _, err := c.bufr.Peek(4); err != nil {
  2110  			return
  2111  		}
  2112  
  2113  		c.rwc.SetReadDeadline(time.Time{})
  2114  	}
  2115  }
  2116  
  2117  // unencryptedHTTP2Request is an HTTP handler that initializes
  2118  // certain uninitialized fields in its *Request.
  2119  //
  2120  // It's the unencrypted version of initALPNRequest.
  2121  type unencryptedHTTP2Request struct {
  2122  	ctx context.Context
  2123  	c   net.Conn
  2124  	h   serverHandler
  2125  }
  2126  
  2127  func (h unencryptedHTTP2Request) BaseContext() context.Context { return h.ctx }
  2128  
  2129  func (h unencryptedHTTP2Request) ServeHTTP(rw ResponseWriter, req *Request) {
  2130  	if req.Body == nil {
  2131  		req.Body = NoBody
  2132  	}
  2133  	if req.RemoteAddr == "" {
  2134  		req.RemoteAddr = h.c.RemoteAddr().String()
  2135  	}
  2136  	h.h.ServeHTTP(rw, req)
  2137  }
  2138  
  2139  // unencryptedNetConnInTLSConn is used to pass an unencrypted net.Conn to
  2140  // functions that only accept a *tls.Conn.
  2141  type unencryptedNetConnInTLSConn struct {
  2142  	net.Conn // panic on all net.Conn methods
  2143  	conn     net.Conn
  2144  }
  2145  
  2146  func (c unencryptedNetConnInTLSConn) UnencryptedNetConn() net.Conn {
  2147  	return c.conn
  2148  }
  2149  
  2150  func unencryptedTLSConn(c net.Conn) *tls.Conn {
  2151  	return tls.Client(unencryptedNetConnInTLSConn{conn: c}, nil)
  2152  }
  2153  
  2154  // TLSNextProto key to use for unencrypted HTTP/2 connections.
  2155  // Not actually a TLS-negotiated protocol.
  2156  const nextProtoUnencryptedHTTP2 = "unencrypted_http2"
  2157  
  2158  func (c *conn) maybeServeUnencryptedHTTP2(ctx context.Context) bool {
  2159  	fn, ok := c.server.TLSNextProto[nextProtoUnencryptedHTTP2]
  2160  	if !ok {
  2161  		return false
  2162  	}
  2163  	hasPreface := func(c *conn, preface []byte) bool {
  2164  		c.r.setReadLimit(int64(len(preface)) - int64(c.bufr.Buffered()))
  2165  		got, err := c.bufr.Peek(len(preface))
  2166  		c.r.setInfiniteReadLimit()
  2167  		return err == nil && bytes.Equal(got, preface)
  2168  	}
  2169  	if !hasPreface(c, []byte("PRI * HTTP/2.0")) {
  2170  		return false
  2171  	}
  2172  	if !hasPreface(c, []byte("PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n")) {
  2173  		return false
  2174  	}
  2175  	c.setState(c.rwc, StateActive, skipHooks)
  2176  	h := unencryptedHTTP2Request{ctx, c.rwc, serverHandler{c.server}}
  2177  	fn(c.server, unencryptedTLSConn(c.rwc), h)
  2178  	return true
  2179  }
  2180  
  2181  func (w *response) sendExpectationFailed() {
  2182  	// TODO(bradfitz): let ServeHTTP handlers handle
  2183  	// requests with non-standard expectation[s]? Seems
  2184  	// theoretical at best, and doesn't fit into the
  2185  	// current ServeHTTP model anyway. We'd need to
  2186  	// make the ResponseWriter an optional
  2187  	// "ExpectReplier" interface or something.
  2188  	//
  2189  	// For now we'll just obey RFC 7231 5.1.1 which says
  2190  	// "A server that receives an Expect field-value other
  2191  	// than 100-continue MAY respond with a 417 (Expectation
  2192  	// Failed) status code to indicate that the unexpected
  2193  	// expectation cannot be met."
  2194  	w.Header().Set("Connection", "close")
  2195  	w.WriteHeader(StatusExpectationFailed)
  2196  	w.finishRequest()
  2197  }
  2198  
  2199  // Hijack implements the [Hijacker.Hijack] method. Our response is both a [ResponseWriter]
  2200  // and a [Hijacker].
  2201  func (w *response) Hijack() (rwc net.Conn, buf *bufio.ReadWriter, err error) {
  2202  	if w.handlerDone.Load() {
  2203  		panic("net/http: Hijack called after ServeHTTP finished")
  2204  	}
  2205  	w.disableWriteContinue()
  2206  	if w.wroteHeader {
  2207  		w.cw.flush()
  2208  	}
  2209  
  2210  	c := w.conn
  2211  	c.mu.Lock()
  2212  	defer c.mu.Unlock()
  2213  
  2214  	// Release the bufioWriter that writes to the chunk writer, it is not
  2215  	// used after a connection has been hijacked.
  2216  	rwc, buf, err = c.hijackLocked()
  2217  	if err == nil {
  2218  		putBufioWriter(w.w)
  2219  		w.w = nil
  2220  	}
  2221  	return rwc, buf, err
  2222  }
  2223  
  2224  func (w *response) CloseNotify() <-chan bool {
  2225  	w.lazyCloseNotifyMu.Lock()
  2226  	defer w.lazyCloseNotifyMu.Unlock()
  2227  	if w.handlerDone.Load() {
  2228  		panic("net/http: CloseNotify called after ServeHTTP finished")
  2229  	}
  2230  	if w.closeNotifyCh == nil {
  2231  		w.closeNotifyCh = make(chan bool, 1)
  2232  		if w.closeNotifyTriggered {
  2233  			w.closeNotifyCh <- true // action prior closeNotify call
  2234  		}
  2235  	}
  2236  	return w.closeNotifyCh
  2237  }
  2238  
  2239  func (w *response) closeNotify() {
  2240  	w.lazyCloseNotifyMu.Lock()
  2241  	defer w.lazyCloseNotifyMu.Unlock()
  2242  	if w.closeNotifyTriggered {
  2243  		return // already triggered
  2244  	}
  2245  	w.closeNotifyTriggered = true
  2246  	if w.closeNotifyCh != nil {
  2247  		w.closeNotifyCh <- true
  2248  	}
  2249  }
  2250  
  2251  func registerOnHitEOF(rc io.ReadCloser, fn func()) {
  2252  	switch v := rc.(type) {
  2253  	case *expectContinueReader:
  2254  		registerOnHitEOF(v.readCloser, fn)
  2255  	case *body:
  2256  		v.registerOnHitEOF(fn)
  2257  	default:
  2258  		panic("unexpected type " + fmt.Sprintf("%T", rc))
  2259  	}
  2260  }
  2261  
  2262  // requestBodyRemains reports whether future calls to Read
  2263  // on rc might yield more data.
  2264  func requestBodyRemains(rc io.ReadCloser) bool {
  2265  	if rc == NoBody {
  2266  		return false
  2267  	}
  2268  	switch v := rc.(type) {
  2269  	case *expectContinueReader:
  2270  		return requestBodyRemains(v.readCloser)
  2271  	case *body:
  2272  		return v.bodyRemains()
  2273  	default:
  2274  		panic("unexpected type " + fmt.Sprintf("%T", rc))
  2275  	}
  2276  }
  2277  
  2278  // The HandlerFunc type is an adapter to allow the use of
  2279  // ordinary functions as HTTP handlers. If f is a function
  2280  // with the appropriate signature, HandlerFunc(f) is a
  2281  // [Handler] that calls f.
  2282  type HandlerFunc func(ResponseWriter, *Request)
  2283  
  2284  // ServeHTTP calls f(w, r).
  2285  func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
  2286  	f(w, r)
  2287  }
  2288  
  2289  // Helper handlers
  2290  
  2291  // Error replies to the request with the specified error message and HTTP code.
  2292  // It does not otherwise end the request; the caller should ensure no further
  2293  // writes are done to w.
  2294  // The error message should be plain text.
  2295  //
  2296  // Error deletes the Content-Length header,
  2297  // sets Content-Type to “text/plain; charset=utf-8”,
  2298  // and sets X-Content-Type-Options to “nosniff”.
  2299  // This configures the header properly for the error message,
  2300  // in case the caller had set it up expecting a successful output.
  2301  func Error(w ResponseWriter, error string, code int) {
  2302  	h := w.Header()
  2303  
  2304  	// Delete the Content-Length header, which might be for some other content.
  2305  	// Assuming the error string fits in the writer's buffer, we'll figure
  2306  	// out the correct Content-Length for it later.
  2307  	//
  2308  	// We don't delete Content-Encoding, because some middleware sets
  2309  	// Content-Encoding: gzip and wraps the ResponseWriter to compress on-the-fly.
  2310  	// See https://go.dev/issue/66343.
  2311  	h.Del("Content-Length")
  2312  
  2313  	// There might be content type already set, but we reset it to
  2314  	// text/plain for the error message.
  2315  	h.Set("Content-Type", "text/plain; charset=utf-8")
  2316  	h.Set("X-Content-Type-Options", "nosniff")
  2317  	w.WriteHeader(code)
  2318  	fmt.Fprintln(w, error)
  2319  }
  2320  
  2321  // NotFound replies to the request with an HTTP 404 not found error.
  2322  func NotFound(w ResponseWriter, r *Request) { Error(w, "404 page not found", StatusNotFound) }
  2323  
  2324  // NotFoundHandler returns a simple request handler
  2325  // that replies to each request with a “404 page not found” reply.
  2326  func NotFoundHandler() Handler { return HandlerFunc(NotFound) }
  2327  
  2328  // StripPrefix returns a handler that serves HTTP requests by removing the
  2329  // given prefix from the request URL's Path (and RawPath if set) and invoking
  2330  // the handler h. StripPrefix handles a request for a path that doesn't begin
  2331  // with prefix by replying with an HTTP 404 not found error. The prefix must
  2332  // match exactly: if the prefix in the request contains escaped characters
  2333  // the reply is also an HTTP 404 not found error.
  2334  func StripPrefix(prefix string, h Handler) Handler {
  2335  	if prefix == "" {
  2336  		return h
  2337  	}
  2338  	return HandlerFunc(func(w ResponseWriter, r *Request) {
  2339  		p := strings.TrimPrefix(r.URL.Path, prefix)
  2340  		rp := strings.TrimPrefix(r.URL.RawPath, prefix)
  2341  		if len(p) < len(r.URL.Path) && (r.URL.RawPath == "" || len(rp) < len(r.URL.RawPath)) {
  2342  			r2 := new(Request)
  2343  			*r2 = *r
  2344  			r2.URL = new(url.URL)
  2345  			*r2.URL = *r.URL
  2346  			r2.URL.Path = p
  2347  			r2.URL.RawPath = rp
  2348  			h.ServeHTTP(w, r2)
  2349  		} else {
  2350  			NotFound(w, r)
  2351  		}
  2352  	})
  2353  }
  2354  
  2355  // Redirect replies to the request with a redirect to url,
  2356  // which may be a path relative to the request path.
  2357  // Any non-ASCII characters in url will be percent-encoded,
  2358  // but existing percent encodings will not be changed.
  2359  //
  2360  // The provided code should be in the 3xx range and is usually
  2361  // [StatusMovedPermanently], [StatusFound] or [StatusSeeOther].
  2362  //
  2363  // If the Content-Type header has not been set, [Redirect] sets it
  2364  // to "text/html; charset=utf-8" and writes a small HTML body.
  2365  // Setting the Content-Type header to any value, including nil,
  2366  // disables that behavior.
  2367  func Redirect(w ResponseWriter, r *Request, url string, code int) {
  2368  	if u, err := urlpkg.Parse(url); err == nil {
  2369  		// If url was relative, make its path absolute by
  2370  		// combining with request path.
  2371  		// The client would probably do this for us,
  2372  		// but doing it ourselves is more reliable.
  2373  		// See RFC 7231, section 7.1.2
  2374  		if u.Scheme == "" && u.Host == "" {
  2375  			oldpath := r.URL.EscapedPath()
  2376  			if oldpath == "" { // should not happen, but avoid a crash if it does
  2377  				oldpath = "/"
  2378  			}
  2379  
  2380  			// no leading http://server
  2381  			if url == "" || url[0] != '/' {
  2382  				// make relative path absolute
  2383  				olddir, _ := path.Split(oldpath)
  2384  				url = olddir + url
  2385  			}
  2386  
  2387  			var query string
  2388  			if i := strings.Index(url, "?"); i != -1 {
  2389  				url, query = url[:i], url[i:]
  2390  			}
  2391  
  2392  			// clean up but preserve trailing slash
  2393  			trailing := strings.HasSuffix(url, "/")
  2394  			url = path.Clean(url)
  2395  			if trailing && !strings.HasSuffix(url, "/") {
  2396  				url += "/"
  2397  			}
  2398  			url += query
  2399  		}
  2400  	}
  2401  
  2402  	h := w.Header()
  2403  
  2404  	// RFC 7231 notes that a short HTML body is usually included in
  2405  	// the response because older user agents may not understand 301/307.
  2406  	// Do it only if the request didn't already have a Content-Type header.
  2407  	_, hadCT := h["Content-Type"]
  2408  
  2409  	h.Set("Location", hexEscapeNonASCII(url))
  2410  	if !hadCT && (r.Method == "GET" || r.Method == "HEAD") {
  2411  		h.Set("Content-Type", "text/html; charset=utf-8")
  2412  	}
  2413  	w.WriteHeader(code)
  2414  
  2415  	// Shouldn't send the body for POST or HEAD; that leaves GET.
  2416  	if !hadCT && r.Method == "GET" {
  2417  		body := "<a href=\"" + htmlEscape(url) + "\">" + StatusText(code) + "</a>.\n"
  2418  		fmt.Fprintln(w, body)
  2419  	}
  2420  }
  2421  
  2422  var htmlReplacer = strings.NewReplacer(
  2423  	"&", "&amp;",
  2424  	"<", "&lt;",
  2425  	">", "&gt;",
  2426  	// "&#34;" is shorter than "&quot;".
  2427  	`"`, "&#34;",
  2428  	// "&#39;" is shorter than "&apos;" and apos was not in HTML until HTML5.
  2429  	"'", "&#39;",
  2430  )
  2431  
  2432  func htmlEscape(s string) string {
  2433  	return htmlReplacer.Replace(s)
  2434  }
  2435  
  2436  // Redirect to a fixed URL
  2437  type redirectHandler struct {
  2438  	url  string
  2439  	code int
  2440  }
  2441  
  2442  func (rh *redirectHandler) ServeHTTP(w ResponseWriter, r *Request) {
  2443  	Redirect(w, r, rh.url, rh.code)
  2444  }
  2445  
  2446  // RedirectHandler returns a request handler that redirects
  2447  // each request it receives to the given url using the given
  2448  // status code.
  2449  //
  2450  // The provided code should be in the 3xx range and is usually
  2451  // [StatusMovedPermanently], [StatusFound] or [StatusSeeOther].
  2452  func RedirectHandler(url string, code int) Handler {
  2453  	return &redirectHandler{url, code}
  2454  }
  2455  
  2456  // ServeMux is an HTTP request multiplexer.
  2457  // It matches the URL of each incoming request against a list of registered
  2458  // patterns and calls the handler for the pattern that
  2459  // most closely matches the URL.
  2460  //
  2461  // # Patterns
  2462  //
  2463  // Patterns can match the method, host and path of a request.
  2464  // Some examples:
  2465  //
  2466  //   - "/index.html" matches the path "/index.html" for any host and method.
  2467  //   - "GET /static/" matches a GET request whose path begins with "/static/".
  2468  //   - "example.com/" matches any request to the host "example.com".
  2469  //   - "example.com/{$}" matches requests with host "example.com" and path "/".
  2470  //   - "/b/{bucket}/o/{objectname...}" matches paths whose first segment is "b"
  2471  //     and whose third segment is "o". The name "bucket" denotes the second
  2472  //     segment and "objectname" denotes the remainder of the path.
  2473  //
  2474  // In general, a pattern looks like
  2475  //
  2476  //	[METHOD ][HOST]/[PATH]
  2477  //
  2478  // All three parts are optional; "/" is a valid pattern.
  2479  // If METHOD is present, it must be followed by at least one space or tab.
  2480  //
  2481  // Literal (that is, non-wildcard) parts of a pattern match
  2482  // the corresponding parts of a request case-sensitively.
  2483  //
  2484  // A pattern with no method matches every method. A pattern
  2485  // with the method GET matches both GET and HEAD requests.
  2486  // Otherwise, the method must match exactly.
  2487  //
  2488  // A pattern with no host matches every host.
  2489  // A pattern with a host matches URLs on that host only.
  2490  //
  2491  // A path can include wildcard segments of the form {NAME} or {NAME...}.
  2492  // For example, "/b/{bucket}/o/{objectname...}".
  2493  // The wildcard name must be a valid Go identifier.
  2494  // Wildcards must be full path segments: they must be preceded by a slash and followed by
  2495  // either a slash or the end of the string.
  2496  // For example, "/b_{bucket}" is not a valid pattern.
  2497  //
  2498  // Normally a wildcard matches only a single path segment,
  2499  // ending at the next literal slash (not %2F) in the request URL.
  2500  // But if the "..." is present, then the wildcard matches the remainder of the URL path, including slashes.
  2501  // (Therefore it is invalid for a "..." wildcard to appear anywhere but at the end of a pattern.)
  2502  // The match for a wildcard can be obtained by calling [Request.PathValue] with the wildcard's name.
  2503  // A trailing slash in a path acts as an anonymous "..." wildcard.
  2504  //
  2505  // The special wildcard {$} matches only the end of the URL.
  2506  // For example, the pattern "/{$}" matches only the path "/",
  2507  // whereas the pattern "/" matches every path.
  2508  //
  2509  // For matching, both pattern paths and incoming request paths are unescaped segment by segment.
  2510  // So, for example, the path "/a%2Fb/100%25" is treated as having two segments, "a/b" and "100%".
  2511  // The pattern "/a%2fb/" matches it, but the pattern "/a/b/" does not.
  2512  //
  2513  // # Precedence
  2514  //
  2515  // If two or more patterns match a request, then the most specific pattern takes precedence.
  2516  // A pattern P1 is more specific than P2 if P1 matches a strict subset of P2’s requests;
  2517  // that is, if P2 matches all the requests of P1 and more.
  2518  // If neither is more specific, then the patterns conflict.
  2519  // There is one exception to this rule, for backwards compatibility:
  2520  // if two patterns would otherwise conflict and one has a host while the other does not,
  2521  // then the pattern with the host takes precedence.
  2522  // If a pattern passed to [ServeMux.Handle] or [ServeMux.HandleFunc] conflicts with
  2523  // another pattern that is already registered, those functions panic.
  2524  //
  2525  // As an example of the general rule, "/images/thumbnails/" is more specific than "/images/",
  2526  // so both can be registered.
  2527  // The former matches paths beginning with "/images/thumbnails/"
  2528  // and the latter will match any other path in the "/images/" subtree.
  2529  //
  2530  // As another example, consider the patterns "GET /" and "/index.html":
  2531  // both match a GET request for "/index.html", but the former pattern
  2532  // matches all other GET and HEAD requests, while the latter matches any
  2533  // request for "/index.html" that uses a different method.
  2534  // The patterns conflict.
  2535  //
  2536  // # Trailing-slash redirection
  2537  //
  2538  // Consider a [ServeMux] with a handler for a subtree, registered using a trailing slash or "..." wildcard.
  2539  // If the ServeMux receives a request for the subtree root without a trailing slash,
  2540  // it redirects the request by adding the trailing slash.
  2541  // This behavior can be overridden with a separate registration for the path without
  2542  // the trailing slash or "..." wildcard. For example, registering "/images/" causes ServeMux
  2543  // to redirect a request for "/images" to "/images/", unless "/images" has
  2544  // been registered separately.
  2545  //
  2546  // # Request sanitizing
  2547  //
  2548  // ServeMux also takes care of sanitizing the URL request path and the Host
  2549  // header, stripping the port number and redirecting any request containing . or
  2550  // .. segments or repeated slashes to an equivalent, cleaner URL.
  2551  // Escaped path elements such as "%2e" for "." and "%2f" for "/" are preserved
  2552  // and aren't considered separators for request routing.
  2553  //
  2554  // # Compatibility
  2555  //
  2556  // The pattern syntax and matching behavior of ServeMux changed significantly
  2557  // in Go 1.22. To restore the old behavior, set the GODEBUG environment variable
  2558  // to "httpmuxgo121=1". This setting is read once, at program startup; changes
  2559  // during execution will be ignored.
  2560  //
  2561  // The backwards-incompatible changes include:
  2562  //   - Wildcards are just ordinary literal path segments in 1.21.
  2563  //     For example, the pattern "/{x}" will match only that path in 1.21,
  2564  //     but will match any one-segment path in 1.22.
  2565  //   - In 1.21, no pattern was rejected, unless it was empty or conflicted with an existing pattern.
  2566  //     In 1.22, syntactically invalid patterns will cause [ServeMux.Handle] and [ServeMux.HandleFunc] to panic.
  2567  //     For example, in 1.21, the patterns "/{"  and "/a{x}" match themselves,
  2568  //     but in 1.22 they are invalid and will cause a panic when registered.
  2569  //   - In 1.22, each segment of a pattern is unescaped; this was not done in 1.21.
  2570  //     For example, in 1.22 the pattern "/%61" matches the path "/a" ("%61" being the URL escape sequence for "a"),
  2571  //     but in 1.21 it would match only the path "/%2561" (where "%25" is the escape for the percent sign).
  2572  //   - When matching patterns to paths, in 1.22 each segment of the path is unescaped; in 1.21, the entire path is unescaped.
  2573  //     This change mostly affects how paths with %2F escapes adjacent to slashes are treated.
  2574  //     See https://go.dev/issue/21955 for details.
  2575  type ServeMux struct {
  2576  	mu     sync.RWMutex
  2577  	tree   routingNode
  2578  	index  routingIndex
  2579  	mux121 serveMux121 // used only when GODEBUG=httpmuxgo121=1
  2580  }
  2581  
  2582  // NewServeMux allocates and returns a new [ServeMux].
  2583  func NewServeMux() *ServeMux {
  2584  	return &ServeMux{}
  2585  }
  2586  
  2587  // DefaultServeMux is the default [ServeMux] used by [Serve].
  2588  var DefaultServeMux = &defaultServeMux
  2589  
  2590  var defaultServeMux ServeMux
  2591  
  2592  // cleanPath returns the canonical path for p, eliminating . and .. elements.
  2593  func cleanPath(p string) string {
  2594  	if p == "" {
  2595  		return "/"
  2596  	}
  2597  	if p[0] != '/' {
  2598  		p = "/" + p
  2599  	}
  2600  	np := path.Clean(p)
  2601  	// path.Clean removes trailing slash except for root;
  2602  	// put the trailing slash back if necessary.
  2603  	if p[len(p)-1] == '/' && np != "/" {
  2604  		// Fast path for common case of p being the string we want:
  2605  		if len(p) == len(np)+1 && strings.HasPrefix(p, np) {
  2606  			np = p
  2607  		} else {
  2608  			np += "/"
  2609  		}
  2610  	}
  2611  	return np
  2612  }
  2613  
  2614  // stripHostPort returns h without any trailing ":<port>".
  2615  func stripHostPort(h string) string {
  2616  	// If no port on host, return unchanged
  2617  	if !strings.Contains(h, ":") {
  2618  		return h
  2619  	}
  2620  	host, _, err := net.SplitHostPort(h)
  2621  	if err != nil {
  2622  		return h // on error, return unchanged
  2623  	}
  2624  	return host
  2625  }
  2626  
  2627  // Handler returns the handler to use for the given request,
  2628  // consulting r.Method, r.Host, and r.URL.Path. It always returns
  2629  // a non-nil handler. If the path is not in its canonical form, the
  2630  // handler will be an internally-generated handler that redirects
  2631  // to the canonical path. If the host contains a port, it is ignored
  2632  // when matching handlers.
  2633  //
  2634  // The path and host are used unchanged for CONNECT requests.
  2635  //
  2636  // Handler also returns the registered pattern that matches the
  2637  // request or, in the case of internally-generated redirects,
  2638  // the path that will match after following the redirect.
  2639  //
  2640  // If there is no registered handler that applies to the request,
  2641  // Handler returns a “page not found” or “method not supported”
  2642  // handler and an empty pattern.
  2643  //
  2644  // Handler does not modify its argument. In particular, it does not
  2645  // populate named path wildcards, so r.PathValue will always return
  2646  // the empty string.
  2647  func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {
  2648  	if use121 {
  2649  		return mux.mux121.findHandler(r)
  2650  	}
  2651  	h, p, _, _ := mux.findHandler(r)
  2652  	return h, p
  2653  }
  2654  
  2655  // findHandler finds a handler for a request.
  2656  // If there is a matching handler, it returns it and the pattern that matched.
  2657  // Otherwise it returns a Redirect or NotFound handler with the path that would match
  2658  // after the redirect.
  2659  func (mux *ServeMux) findHandler(r *Request) (h Handler, patStr string, _ *pattern, matches []string) {
  2660  	var n *routingNode
  2661  	host := r.URL.Host
  2662  	escapedPath := r.URL.EscapedPath()
  2663  	path := escapedPath
  2664  	// CONNECT requests are not canonicalized.
  2665  	if r.Method == "CONNECT" {
  2666  		// If r.URL.Path is /tree and its handler is not registered,
  2667  		// the /tree -> /tree/ redirect applies to CONNECT requests
  2668  		// but the path canonicalization does not.
  2669  		_, _, u := mux.matchOrRedirect(host, r.Method, path, r.URL)
  2670  		if u != nil {
  2671  			return RedirectHandler(u.String(), StatusTemporaryRedirect), u.Path, nil, nil
  2672  		}
  2673  		// Redo the match, this time with r.Host instead of r.URL.Host.
  2674  		// Pass a nil URL to skip the trailing-slash redirect logic.
  2675  		n, matches, _ = mux.matchOrRedirect(r.Host, r.Method, path, nil)
  2676  	} else {
  2677  		// All other requests have any port stripped and path cleaned
  2678  		// before passing to mux.handler.
  2679  		host = stripHostPort(r.Host)
  2680  		path = cleanPath(path)
  2681  
  2682  		// If the given path is /tree and its handler is not registered,
  2683  		// redirect for /tree/.
  2684  		var u *url.URL
  2685  		n, matches, u = mux.matchOrRedirect(host, r.Method, path, r.URL)
  2686  		if u != nil {
  2687  			return RedirectHandler(u.String(), StatusTemporaryRedirect), n.pattern.String(), nil, nil
  2688  		}
  2689  		if path != escapedPath {
  2690  			// Redirect to cleaned path.
  2691  			patStr := ""
  2692  			if n != nil {
  2693  				patStr = n.pattern.String()
  2694  			}
  2695  			u := &url.URL{Path: path, RawQuery: r.URL.RawQuery}
  2696  			return RedirectHandler(u.String(), StatusTemporaryRedirect), patStr, nil, nil
  2697  		}
  2698  	}
  2699  	if n == nil {
  2700  		// We didn't find a match with the request method. To distinguish between
  2701  		// Not Found and Method Not Allowed, see if there is another pattern that
  2702  		// matches except for the method.
  2703  		allowedMethods := mux.matchingMethods(host, path)
  2704  		if len(allowedMethods) > 0 {
  2705  			return HandlerFunc(func(w ResponseWriter, r *Request) {
  2706  				w.Header().Set("Allow", strings.Join(allowedMethods, ", "))
  2707  				Error(w, StatusText(StatusMethodNotAllowed), StatusMethodNotAllowed)
  2708  			}), "", nil, nil
  2709  		}
  2710  		return NotFoundHandler(), "", nil, nil
  2711  	}
  2712  	return n.handler, n.pattern.String(), n.pattern, matches
  2713  }
  2714  
  2715  // matchOrRedirect looks up a node in the tree that matches the host, method and path.
  2716  //
  2717  // If the url argument is non-nil, handler also deals with trailing-slash
  2718  // redirection: when a path doesn't match exactly, the match is tried again
  2719  // after appending "/" to the path. If that second match succeeds, the last
  2720  // return value is the URL to redirect to.
  2721  func (mux *ServeMux) matchOrRedirect(host, method, path string, u *url.URL) (_ *routingNode, matches []string, redirectTo *url.URL) {
  2722  	mux.mu.RLock()
  2723  	defer mux.mu.RUnlock()
  2724  
  2725  	n, matches := mux.tree.match(host, method, path)
  2726  	// We can terminate here if any of the following is true:
  2727  	// - We have an exact match already.
  2728  	// - We were asked not to try trailing slash redirection.
  2729  	// - The URL already has a trailing slash.
  2730  	// - The URL is an empty string.
  2731  	if !exactMatch(n, path) && u != nil && !strings.HasSuffix(path, "/") && path != "" {
  2732  		// If there is an exact match with a trailing slash, then redirect.
  2733  		path += "/"
  2734  		n2, _ := mux.tree.match(host, method, path)
  2735  		if exactMatch(n2, path) {
  2736  			// It is safe to return n2 here: it is used only in the second RedirectHandler case
  2737  			// of findHandler, and that method returns before it does the "n == nil" check where
  2738  			// the first return value matters. We return it here only to make the pattern available
  2739  			// to findHandler.
  2740  			return n2, nil, &url.URL{Path: cleanPath(u.Path) + "/", RawQuery: u.RawQuery}
  2741  		}
  2742  	}
  2743  	return n, matches, nil
  2744  }
  2745  
  2746  // exactMatch reports whether the node's pattern exactly matches the path.
  2747  // As a special case, if the node is nil, exactMatch return false.
  2748  //
  2749  // Before wildcards were introduced, it was clear that an exact match meant
  2750  // that the pattern and path were the same string. The only other possibility
  2751  // was that a trailing-slash pattern, like "/", matched a path longer than
  2752  // it, like "/a".
  2753  //
  2754  // With wildcards, we define an inexact match as any one where a multi wildcard
  2755  // matches a non-empty string. All other matches are exact.
  2756  // For example, these are all exact matches:
  2757  //
  2758  //	pattern   path
  2759  //	/a        /a
  2760  //	/{x}      /a
  2761  //	/a/{$}    /a/
  2762  //	/a/       /a/
  2763  //
  2764  // The last case has a multi wildcard (implicitly), but the match is exact because
  2765  // the wildcard matches the empty string.
  2766  //
  2767  // Examples of matches that are not exact:
  2768  //
  2769  //	pattern   path
  2770  //	/         /a
  2771  //	/a/{x...} /a/b
  2772  func exactMatch(n *routingNode, path string) bool {
  2773  	if n == nil {
  2774  		return false
  2775  	}
  2776  	// We can't directly implement the definition (empty match for multi
  2777  	// wildcard) because we don't record a match for anonymous multis.
  2778  
  2779  	// If there is no multi, the match is exact.
  2780  	if !n.pattern.lastSegment().multi {
  2781  		return true
  2782  	}
  2783  
  2784  	// If the path doesn't end in a trailing slash, then the multi match
  2785  	// is non-empty.
  2786  	if len(path) > 0 && path[len(path)-1] != '/' {
  2787  		return false
  2788  	}
  2789  	// Only patterns ending in {$} or a multi wildcard can
  2790  	// match a path with a trailing slash.
  2791  	// For the match to be exact, the number of pattern
  2792  	// segments should be the same as the number of slashes in the path.
  2793  	// E.g. "/a/b/{$}" and "/a/b/{...}" exactly match "/a/b/", but "/a/" does not.
  2794  	return len(n.pattern.segments) == strings.Count(path, "/")
  2795  }
  2796  
  2797  // matchingMethods return a sorted list of all methods that would match with the given host and path.
  2798  func (mux *ServeMux) matchingMethods(host, path string) []string {
  2799  	// Hold the read lock for the entire method so that the two matches are done
  2800  	// on the same set of registered patterns.
  2801  	mux.mu.RLock()
  2802  	defer mux.mu.RUnlock()
  2803  	ms := map[string]bool{}
  2804  	mux.tree.matchingMethods(host, path, ms)
  2805  	// matchOrRedirect will try appending a trailing slash if there is no match.
  2806  	if !strings.HasSuffix(path, "/") {
  2807  		mux.tree.matchingMethods(host, path+"/", ms)
  2808  	}
  2809  	return slices.Sorted(maps.Keys(ms))
  2810  }
  2811  
  2812  // ServeHTTP dispatches the request to the handler whose
  2813  // pattern most closely matches the request URL.
  2814  func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
  2815  	if r.RequestURI == "*" {
  2816  		if r.ProtoAtLeast(1, 1) {
  2817  			w.Header().Set("Connection", "close")
  2818  		}
  2819  		w.WriteHeader(StatusBadRequest)
  2820  		return
  2821  	}
  2822  	var h Handler
  2823  	if use121 {
  2824  		h, _ = mux.mux121.findHandler(r)
  2825  	} else {
  2826  		h, r.Pattern, r.pat, r.matches = mux.findHandler(r)
  2827  	}
  2828  	h.ServeHTTP(w, r)
  2829  }
  2830  
  2831  // The four functions below all call ServeMux.register so that callerLocation
  2832  // always refers to user code.
  2833  
  2834  // Handle registers the handler for the given pattern.
  2835  // If the given pattern conflicts with one that is already registered
  2836  // or if the pattern is invalid, Handle panics.
  2837  //
  2838  // See [ServeMux] for details on valid patterns and conflict rules.
  2839  func (mux *ServeMux) Handle(pattern string, handler Handler) {
  2840  	if use121 {
  2841  		mux.mux121.handle(pattern, handler)
  2842  	} else {
  2843  		mux.register(pattern, handler)
  2844  	}
  2845  }
  2846  
  2847  // HandleFunc registers the handler function for the given pattern.
  2848  // If the given pattern conflicts with one that is already registered
  2849  // or if the pattern is invalid, HandleFunc panics.
  2850  //
  2851  // See [ServeMux] for details on valid patterns and conflict rules.
  2852  func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
  2853  	if use121 {
  2854  		mux.mux121.handleFunc(pattern, handler)
  2855  	} else {
  2856  		mux.register(pattern, HandlerFunc(handler))
  2857  	}
  2858  }
  2859  
  2860  // Handle registers the handler for the given pattern in [DefaultServeMux].
  2861  // The documentation for [ServeMux] explains how patterns are matched.
  2862  func Handle(pattern string, handler Handler) {
  2863  	if use121 {
  2864  		DefaultServeMux.mux121.handle(pattern, handler)
  2865  	} else {
  2866  		DefaultServeMux.register(pattern, handler)
  2867  	}
  2868  }
  2869  
  2870  // HandleFunc registers the handler function for the given pattern in [DefaultServeMux].
  2871  // The documentation for [ServeMux] explains how patterns are matched.
  2872  func HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
  2873  	if use121 {
  2874  		DefaultServeMux.mux121.handleFunc(pattern, handler)
  2875  	} else {
  2876  		DefaultServeMux.register(pattern, HandlerFunc(handler))
  2877  	}
  2878  }
  2879  
  2880  func (mux *ServeMux) register(pattern string, handler Handler) {
  2881  	if err := mux.registerErr(pattern, handler); err != nil {
  2882  		panic(err)
  2883  	}
  2884  }
  2885  
  2886  func (mux *ServeMux) registerErr(patstr string, handler Handler) error {
  2887  	if patstr == "" {
  2888  		return errors.New("http: invalid pattern")
  2889  	}
  2890  	if handler == nil {
  2891  		return errors.New("http: nil handler")
  2892  	}
  2893  	if f, ok := handler.(HandlerFunc); ok && f == nil {
  2894  		return errors.New("http: nil handler")
  2895  	}
  2896  
  2897  	pat, err := parsePattern(patstr)
  2898  	if err != nil {
  2899  		return fmt.Errorf("parsing %q: %w", patstr, err)
  2900  	}
  2901  
  2902  	// Get the caller's location, for better conflict error messages.
  2903  	// Skip register and whatever calls it.
  2904  	_, file, line, ok := runtime.Caller(3)
  2905  	if !ok {
  2906  		pat.loc = "unknown location"
  2907  	} else {
  2908  		pat.loc = fmt.Sprintf("%s:%d", file, line)
  2909  	}
  2910  
  2911  	mux.mu.Lock()
  2912  	defer mux.mu.Unlock()
  2913  	// Check for conflict.
  2914  	if err := mux.index.possiblyConflictingPatterns(pat, func(pat2 *pattern) error {
  2915  		if pat.conflictsWith(pat2) {
  2916  			d := describeConflict(pat, pat2)
  2917  			return fmt.Errorf("pattern %q (registered at %s) conflicts with pattern %q (registered at %s):\n%s",
  2918  				pat, pat.loc, pat2, pat2.loc, d)
  2919  		}
  2920  		return nil
  2921  	}); err != nil {
  2922  		return err
  2923  	}
  2924  	mux.tree.addPattern(pat, handler)
  2925  	mux.index.addPattern(pat)
  2926  	return nil
  2927  }
  2928  
  2929  // Serve accepts incoming HTTP connections on the listener l,
  2930  // creating a new service goroutine for each. The service goroutines
  2931  // read requests and then call handler to reply to them.
  2932  //
  2933  // The handler is typically nil, in which case [DefaultServeMux] is used.
  2934  //
  2935  // HTTP/2 support is only enabled if the Listener returns [*tls.Conn]
  2936  // connections and they were configured with "h2" in the TLS
  2937  // Config.NextProtos.
  2938  //
  2939  // Serve always returns a non-nil error.
  2940  func Serve(l net.Listener, handler Handler) error {
  2941  	srv := &Server{Handler: handler}
  2942  	return srv.Serve(l)
  2943  }
  2944  
  2945  // ServeTLS accepts incoming HTTPS connections on the listener l,
  2946  // creating a new service goroutine for each. The service goroutines
  2947  // read requests and then call handler to reply to them.
  2948  //
  2949  // The handler is typically nil, in which case [DefaultServeMux] is used.
  2950  //
  2951  // Additionally, files containing a certificate and matching private key
  2952  // for the server must be provided. If the certificate is signed by a
  2953  // certificate authority, the certFile should be the concatenation
  2954  // of the server's certificate, any intermediates, and the CA's certificate.
  2955  //
  2956  // ServeTLS always returns a non-nil error.
  2957  func ServeTLS(l net.Listener, handler Handler, certFile, keyFile string) error {
  2958  	srv := &Server{Handler: handler}
  2959  	return srv.ServeTLS(l, certFile, keyFile)
  2960  }
  2961  
  2962  // A Server defines parameters for running an HTTP server.
  2963  // The zero value for Server is a valid configuration.
  2964  type Server struct {
  2965  	// Addr optionally specifies the TCP address for the server to listen on,
  2966  	// in the form "host:port". If empty, ":http" (port 80) is used.
  2967  	// The service names are defined in RFC 6335 and assigned by IANA.
  2968  	// See net.Dial for details of the address format.
  2969  	Addr string
  2970  
  2971  	Handler Handler // handler to invoke, http.DefaultServeMux if nil
  2972  
  2973  	// DisableGeneralOptionsHandler, if true, passes "OPTIONS *" requests to the Handler,
  2974  	// otherwise responds with 200 OK and Content-Length: 0.
  2975  	DisableGeneralOptionsHandler bool
  2976  
  2977  	// TLSConfig optionally provides a TLS configuration for use
  2978  	// by ServeTLS and ListenAndServeTLS. Note that this value is
  2979  	// cloned by ServeTLS and ListenAndServeTLS, so it's not
  2980  	// possible to modify the configuration with methods like
  2981  	// tls.Config.SetSessionTicketKeys. To use
  2982  	// SetSessionTicketKeys, use Server.Serve with a TLS Listener
  2983  	// instead.
  2984  	TLSConfig *tls.Config
  2985  
  2986  	// ReadTimeout is the maximum duration for reading the entire
  2987  	// request, including the body. A zero or negative value means
  2988  	// there will be no timeout.
  2989  	//
  2990  	// Because ReadTimeout does not let Handlers make per-request
  2991  	// decisions on each request body's acceptable deadline or
  2992  	// upload rate, most users will prefer to use
  2993  	// ReadHeaderTimeout. It is valid to use them both.
  2994  	ReadTimeout time.Duration
  2995  
  2996  	// ReadHeaderTimeout is the amount of time allowed to read
  2997  	// request headers. The connection's read deadline is reset
  2998  	// after reading the headers and the Handler can decide what
  2999  	// is considered too slow for the body. If zero, the value of
  3000  	// ReadTimeout is used. If negative, or if zero and ReadTimeout
  3001  	// is zero or negative, there is no timeout.
  3002  	ReadHeaderTimeout time.Duration
  3003  
  3004  	// WriteTimeout is the maximum duration before timing out
  3005  	// writes of the response. It is reset whenever a new
  3006  	// request's header is read. Like ReadTimeout, it does not
  3007  	// let Handlers make decisions on a per-request basis.
  3008  	// A zero or negative value means there will be no timeout.
  3009  	WriteTimeout time.Duration
  3010  
  3011  	// IdleTimeout is the maximum amount of time to wait for the
  3012  	// next request when keep-alives are enabled. If zero, the value
  3013  	// of ReadTimeout is used. If negative, or if zero and ReadTimeout
  3014  	// is zero or negative, there is no timeout.
  3015  	IdleTimeout time.Duration
  3016  
  3017  	// MaxHeaderBytes controls the maximum number of bytes the
  3018  	// server will read parsing the request header's keys and
  3019  	// values, including the request line. It does not limit the
  3020  	// size of the request body.
  3021  	// If zero, DefaultMaxHeaderBytes is used.
  3022  	MaxHeaderBytes int
  3023  
  3024  	// TLSNextProto optionally specifies a function to take over
  3025  	// ownership of the provided TLS connection when an ALPN
  3026  	// protocol upgrade has occurred. The map key is the protocol
  3027  	// name negotiated. The Handler argument should be used to
  3028  	// handle HTTP requests and will initialize the Request's TLS
  3029  	// and RemoteAddr if not already set. The connection is
  3030  	// automatically closed when the function returns.
  3031  	// If TLSNextProto is not nil, HTTP/2 support is not enabled
  3032  	// automatically.
  3033  	//
  3034  	// Historically, TLSNextProto was used to disable HTTP/2 support.
  3035  	// The Server.Protocols field now provides a simpler way to do this.
  3036  	TLSNextProto map[string]func(*Server, *tls.Conn, Handler)
  3037  
  3038  	// ConnState specifies an optional callback function that is
  3039  	// called when a client connection changes state. See the
  3040  	// ConnState type and associated constants for details.
  3041  	ConnState func(net.Conn, ConnState)
  3042  
  3043  	// ErrorLog specifies an optional logger for errors accepting
  3044  	// connections, unexpected behavior from handlers, and
  3045  	// underlying FileSystem errors.
  3046  	// If nil, logging is done via the log package's standard logger.
  3047  	ErrorLog *log.Logger
  3048  
  3049  	// BaseContext optionally specifies a function that returns
  3050  	// the base context for incoming requests on this server.
  3051  	// The provided Listener is the specific Listener that's
  3052  	// about to start accepting requests.
  3053  	// If BaseContext is nil, the default is context.Background().
  3054  	// If non-nil, it must return a non-nil context.
  3055  	BaseContext func(net.Listener) context.Context
  3056  
  3057  	// ConnContext optionally specifies a function that modifies
  3058  	// the context used for a new connection c. The provided ctx
  3059  	// is derived from the base context and has a ServerContextKey
  3060  	// value.
  3061  	ConnContext func(ctx context.Context, c net.Conn) context.Context
  3062  
  3063  	// HTTP2 configures HTTP/2 connections.
  3064  	HTTP2 *HTTP2Config
  3065  
  3066  	// Protocols is the set of protocols accepted by the server.
  3067  	//
  3068  	// If Protocols includes UnencryptedHTTP2, the server will accept
  3069  	// unencrypted HTTP/2 connections. The server can serve both
  3070  	// HTTP/1 and unencrypted HTTP/2 on the same address and port.
  3071  	//
  3072  	// If Protocols is nil, the default is usually HTTP/1 and HTTP/2.
  3073  	// If TLSNextProto is non-nil and does not contain an "h2" entry,
  3074  	// the default is HTTP/1 only.
  3075  	Protocols *Protocols
  3076  
  3077  	inShutdown atomic.Bool // true when server is in shutdown
  3078  
  3079  	disableKeepAlives atomic.Bool
  3080  	nextProtoOnce     sync.Once // guards setupHTTP2_* init
  3081  	nextProtoErr      error     // result of http2.ConfigureServer if used
  3082  
  3083  	mu         sync.Mutex
  3084  	listeners  map[*net.Listener]struct{}
  3085  	activeConn map[*conn]struct{}
  3086  	onShutdown []func()
  3087  
  3088  	listenerGroup sync.WaitGroup
  3089  }
  3090  
  3091  // Close immediately closes all active net.Listeners and any
  3092  // connections in state [StateNew], [StateActive], or [StateIdle]. For a
  3093  // graceful shutdown, use [Server.Shutdown].
  3094  //
  3095  // Close does not attempt to close (and does not even know about)
  3096  // any hijacked connections, such as WebSockets.
  3097  //
  3098  // Close returns any error returned from closing the [Server]'s
  3099  // underlying Listener(s).
  3100  func (s *Server) Close() error {
  3101  	s.inShutdown.Store(true)
  3102  	s.mu.Lock()
  3103  	defer s.mu.Unlock()
  3104  	err := s.closeListenersLocked()
  3105  
  3106  	// Unlock s.mu while waiting for listenerGroup.
  3107  	// The group Add and Done calls are made with s.mu held,
  3108  	// to avoid adding a new listener in the window between
  3109  	// us setting inShutdown above and waiting here.
  3110  	s.mu.Unlock()
  3111  	s.listenerGroup.Wait()
  3112  	s.mu.Lock()
  3113  
  3114  	for c := range s.activeConn {
  3115  		c.rwc.Close()
  3116  		delete(s.activeConn, c)
  3117  	}
  3118  	return err
  3119  }
  3120  
  3121  // shutdownPollIntervalMax is the max polling interval when checking
  3122  // quiescence during Server.Shutdown. Polling starts with a small
  3123  // interval and backs off to the max.
  3124  // Ideally we could find a solution that doesn't involve polling,
  3125  // but which also doesn't have a high runtime cost (and doesn't
  3126  // involve any contentious mutexes), but that is left as an
  3127  // exercise for the reader.
  3128  const shutdownPollIntervalMax = 500 * time.Millisecond
  3129  
  3130  // Shutdown gracefully shuts down the server without interrupting any
  3131  // active connections. Shutdown works by first closing all open
  3132  // listeners, then closing all idle connections, and then waiting
  3133  // indefinitely for connections to return to idle and then shut down.
  3134  // If the provided context expires before the shutdown is complete,
  3135  // Shutdown returns the context's error, otherwise it returns any
  3136  // error returned from closing the [Server]'s underlying Listener(s).
  3137  //
  3138  // When Shutdown is called, [Serve], [ServeTLS], [ListenAndServe], and
  3139  // [ListenAndServeTLS] immediately return [ErrServerClosed]. Make sure the
  3140  // program doesn't exit and waits instead for Shutdown to return.
  3141  //
  3142  // Shutdown does not attempt to close nor wait for hijacked
  3143  // connections such as WebSockets. The caller of Shutdown should
  3144  // separately notify such long-lived connections of shutdown and wait
  3145  // for them to close, if desired. See [Server.RegisterOnShutdown] for a way to
  3146  // register shutdown notification functions.
  3147  //
  3148  // Once Shutdown has been called on a server, it may not be reused;
  3149  // future calls to methods such as Serve will return ErrServerClosed.
  3150  func (s *Server) Shutdown(ctx context.Context) error {
  3151  	s.inShutdown.Store(true)
  3152  
  3153  	s.mu.Lock()
  3154  	lnerr := s.closeListenersLocked()
  3155  	for _, f := range s.onShutdown {
  3156  		go f()
  3157  	}
  3158  	s.mu.Unlock()
  3159  	s.listenerGroup.Wait()
  3160  
  3161  	pollIntervalBase := time.Millisecond
  3162  	nextPollInterval := func() time.Duration {
  3163  		// Add 10% jitter.
  3164  		interval := pollIntervalBase + time.Duration(rand.Intn(int(pollIntervalBase/10)))
  3165  		// Double and clamp for next time.
  3166  		pollIntervalBase *= 2
  3167  		if pollIntervalBase > shutdownPollIntervalMax {
  3168  			pollIntervalBase = shutdownPollIntervalMax
  3169  		}
  3170  		return interval
  3171  	}
  3172  
  3173  	timer := time.NewTimer(nextPollInterval())
  3174  	defer timer.Stop()
  3175  	for {
  3176  		if s.closeIdleConns() {
  3177  			return lnerr
  3178  		}
  3179  		select {
  3180  		case <-ctx.Done():
  3181  			return ctx.Err()
  3182  		case <-timer.C:
  3183  			timer.Reset(nextPollInterval())
  3184  		}
  3185  	}
  3186  }
  3187  
  3188  // RegisterOnShutdown registers a function to call on [Server.Shutdown].
  3189  // This can be used to gracefully shutdown connections that have
  3190  // undergone ALPN protocol upgrade or that have been hijacked.
  3191  // This function should start protocol-specific graceful shutdown,
  3192  // but should not wait for shutdown to complete.
  3193  func (s *Server) RegisterOnShutdown(f func()) {
  3194  	s.mu.Lock()
  3195  	s.onShutdown = append(s.onShutdown, f)
  3196  	s.mu.Unlock()
  3197  }
  3198  
  3199  // closeIdleConns closes all idle connections and reports whether the
  3200  // server is quiescent.
  3201  func (s *Server) closeIdleConns() bool {
  3202  	s.mu.Lock()
  3203  	defer s.mu.Unlock()
  3204  	quiescent := true
  3205  	for c := range s.activeConn {
  3206  		st, unixSec := c.getState()
  3207  		// Issue 22682: treat StateNew connections as if
  3208  		// they're idle if we haven't read the first request's
  3209  		// header in over 5 seconds.
  3210  		if st == StateNew && unixSec < time.Now().Unix()-5 {
  3211  			st = StateIdle
  3212  		}
  3213  		if st != StateIdle || unixSec == 0 {
  3214  			// Assume unixSec == 0 means it's a very new
  3215  			// connection, without state set yet.
  3216  			quiescent = false
  3217  			continue
  3218  		}
  3219  		c.rwc.Close()
  3220  		delete(s.activeConn, c)
  3221  	}
  3222  	return quiescent
  3223  }
  3224  
  3225  func (s *Server) closeListenersLocked() error {
  3226  	var err error
  3227  	for ln := range s.listeners {
  3228  		if cerr := (*ln).Close(); cerr != nil && err == nil {
  3229  			err = cerr
  3230  		}
  3231  	}
  3232  	return err
  3233  }
  3234  
  3235  // A ConnState represents the state of a client connection to a server.
  3236  // It's used by the optional [Server.ConnState] hook.
  3237  type ConnState int
  3238  
  3239  const (
  3240  	// StateNew represents a new connection that is expected to
  3241  	// send a request immediately. Connections begin at this
  3242  	// state and then transition to either StateActive or
  3243  	// StateClosed.
  3244  	StateNew ConnState = iota
  3245  
  3246  	// StateActive represents a connection that has read 1 or more
  3247  	// bytes of a request. The Server.ConnState hook for
  3248  	// StateActive fires before the request has entered a handler
  3249  	// and doesn't fire again until the request has been
  3250  	// handled. After the request is handled, the state
  3251  	// transitions to StateClosed, StateHijacked, or StateIdle.
  3252  	// For HTTP/2, StateActive fires on the transition from zero
  3253  	// to one active request, and only transitions away once all
  3254  	// active requests are complete. That means that ConnState
  3255  	// cannot be used to do per-request work; ConnState only notes
  3256  	// the overall state of the connection.
  3257  	StateActive
  3258  
  3259  	// StateIdle represents a connection that has finished
  3260  	// handling a request and is in the keep-alive state, waiting
  3261  	// for a new request. Connections transition from StateIdle
  3262  	// to either StateActive or StateClosed.
  3263  	StateIdle
  3264  
  3265  	// StateHijacked represents a hijacked connection.
  3266  	// This is a terminal state. It does not transition to StateClosed.
  3267  	StateHijacked
  3268  
  3269  	// StateClosed represents a closed connection.
  3270  	// This is a terminal state. Hijacked connections do not
  3271  	// transition to StateClosed.
  3272  	StateClosed
  3273  )
  3274  
  3275  var stateName = map[ConnState]string{
  3276  	StateNew:      "new",
  3277  	StateActive:   "active",
  3278  	StateIdle:     "idle",
  3279  	StateHijacked: "hijacked",
  3280  	StateClosed:   "closed",
  3281  }
  3282  
  3283  func (c ConnState) String() string {
  3284  	return stateName[c]
  3285  }
  3286  
  3287  // serverHandler delegates to either the server's Handler or
  3288  // DefaultServeMux and also handles "OPTIONS *" requests.
  3289  type serverHandler struct {
  3290  	srv *Server
  3291  }
  3292  
  3293  // ServeHTTP should be an internal detail,
  3294  // but widely used packages access it using linkname.
  3295  // Notable members of the hall of shame include:
  3296  //   - github.com/erda-project/erda-infra
  3297  //
  3298  // Do not remove or change the type signature.
  3299  // See go.dev/issue/67401.
  3300  //
  3301  //go:linkname badServeHTTP net/http.serverHandler.ServeHTTP
  3302  func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
  3303  	handler := sh.srv.Handler
  3304  	if handler == nil {
  3305  		handler = DefaultServeMux
  3306  	}
  3307  	if !sh.srv.DisableGeneralOptionsHandler && req.RequestURI == "*" && req.Method == "OPTIONS" {
  3308  		handler = globalOptionsHandler{}
  3309  	}
  3310  
  3311  	handler.ServeHTTP(rw, req)
  3312  }
  3313  
  3314  func badServeHTTP(serverHandler, ResponseWriter, *Request)
  3315  
  3316  // AllowQuerySemicolons returns a handler that serves requests by converting any
  3317  // unescaped semicolons in the URL query to ampersands, and invoking the handler h.
  3318  //
  3319  // This restores the pre-Go 1.17 behavior of splitting query parameters on both
  3320  // semicolons and ampersands. (See golang.org/issue/25192). Note that this
  3321  // behavior doesn't match that of many proxies, and the mismatch can lead to
  3322  // security issues.
  3323  //
  3324  // AllowQuerySemicolons should be invoked before [Request.ParseForm] is called.
  3325  func AllowQuerySemicolons(h Handler) Handler {
  3326  	return HandlerFunc(func(w ResponseWriter, r *Request) {
  3327  		if strings.Contains(r.URL.RawQuery, ";") {
  3328  			r2 := new(Request)
  3329  			*r2 = *r
  3330  			r2.URL = new(url.URL)
  3331  			*r2.URL = *r.URL
  3332  			r2.URL.RawQuery = strings.ReplaceAll(r.URL.RawQuery, ";", "&")
  3333  			h.ServeHTTP(w, r2)
  3334  		} else {
  3335  			h.ServeHTTP(w, r)
  3336  		}
  3337  	})
  3338  }
  3339  
  3340  // ListenAndServe listens on the TCP network address s.Addr and then
  3341  // calls [Serve] to handle requests on incoming connections.
  3342  // Accepted connections are configured to enable TCP keep-alives.
  3343  //
  3344  // If s.Addr is blank, ":http" is used.
  3345  //
  3346  // ListenAndServe always returns a non-nil error. After [Server.Shutdown] or [Server.Close],
  3347  // the returned error is [ErrServerClosed].
  3348  func (s *Server) ListenAndServe() error {
  3349  	if s.shuttingDown() {
  3350  		return ErrServerClosed
  3351  	}
  3352  	addr := s.Addr
  3353  	if addr == "" {
  3354  		addr = ":http"
  3355  	}
  3356  	ln, err := net.Listen("tcp", addr)
  3357  	if err != nil {
  3358  		return err
  3359  	}
  3360  	return s.Serve(ln)
  3361  }
  3362  
  3363  var testHookServerServe func(*Server, net.Listener) // used if non-nil
  3364  
  3365  // shouldConfigureHTTP2ForServe reports whether Server.Serve should configure
  3366  // automatic HTTP/2. (which sets up the s.TLSNextProto map)
  3367  func (s *Server) shouldConfigureHTTP2ForServe() bool {
  3368  	if s.TLSConfig == nil {
  3369  		// Compatibility with Go 1.6:
  3370  		// If there's no TLSConfig, it's possible that the user just
  3371  		// didn't set it on the http.Server, but did pass it to
  3372  		// tls.NewListener and passed that listener to Serve.
  3373  		// So we should configure HTTP/2 (to set up s.TLSNextProto)
  3374  		// in case the listener returns an "h2" *tls.Conn.
  3375  		return true
  3376  	}
  3377  	if s.protocols().UnencryptedHTTP2() {
  3378  		return true
  3379  	}
  3380  	// The user specified a TLSConfig on their http.Server.
  3381  	// In this, case, only configure HTTP/2 if their tls.Config
  3382  	// explicitly mentions "h2". Otherwise http2.ConfigureServer
  3383  	// would modify the tls.Config to add it, but they probably already
  3384  	// passed this tls.Config to tls.NewListener. And if they did,
  3385  	// it's too late anyway to fix it. It would only be potentially racy.
  3386  	// See Issue 15908.
  3387  	return slices.Contains(s.TLSConfig.NextProtos, http2NextProtoTLS)
  3388  }
  3389  
  3390  // ErrServerClosed is returned by the [Server.Serve], [ServeTLS], [ListenAndServe],
  3391  // and [ListenAndServeTLS] methods after a call to [Server.Shutdown] or [Server.Close].
  3392  var ErrServerClosed = errors.New("http: Server closed")
  3393  
  3394  // Serve accepts incoming connections on the Listener l, creating a
  3395  // new service goroutine for each. The service goroutines read requests and
  3396  // then call s.Handler to reply to them.
  3397  //
  3398  // HTTP/2 support is only enabled if the Listener returns [*tls.Conn]
  3399  // connections and they were configured with "h2" in the TLS
  3400  // Config.NextProtos.
  3401  //
  3402  // Serve always returns a non-nil error and closes l.
  3403  // After [Server.Shutdown] or [Server.Close], the returned error is [ErrServerClosed].
  3404  func (s *Server) Serve(l net.Listener) error {
  3405  	if fn := testHookServerServe; fn != nil {
  3406  		fn(s, l) // call hook with unwrapped listener
  3407  	}
  3408  
  3409  	origListener := l
  3410  	l = &onceCloseListener{Listener: l}
  3411  	defer l.Close()
  3412  
  3413  	if err := s.setupHTTP2_Serve(); err != nil {
  3414  		return err
  3415  	}
  3416  
  3417  	if !s.trackListener(&l, true) {
  3418  		return ErrServerClosed
  3419  	}
  3420  	defer s.trackListener(&l, false)
  3421  
  3422  	baseCtx := context.Background()
  3423  	if s.BaseContext != nil {
  3424  		baseCtx = s.BaseContext(origListener)
  3425  		if baseCtx == nil {
  3426  			panic("BaseContext returned a nil context")
  3427  		}
  3428  	}
  3429  
  3430  	var tempDelay time.Duration // how long to sleep on accept failure
  3431  
  3432  	ctx := context.WithValue(baseCtx, ServerContextKey, s)
  3433  	for {
  3434  		rw, err := l.Accept()
  3435  		if err != nil {
  3436  			if s.shuttingDown() {
  3437  				return ErrServerClosed
  3438  			}
  3439  			if ne, ok := err.(net.Error); ok && ne.Temporary() {
  3440  				if tempDelay == 0 {
  3441  					tempDelay = 5 * time.Millisecond
  3442  				} else {
  3443  					tempDelay *= 2
  3444  				}
  3445  				if max := 1 * time.Second; tempDelay > max {
  3446  					tempDelay = max
  3447  				}
  3448  				s.logf("http: Accept error: %v; retrying in %v", err, tempDelay)
  3449  				time.Sleep(tempDelay)
  3450  				continue
  3451  			}
  3452  			return err
  3453  		}
  3454  		connCtx := ctx
  3455  		if cc := s.ConnContext; cc != nil {
  3456  			connCtx = cc(connCtx, rw)
  3457  			if connCtx == nil {
  3458  				panic("ConnContext returned nil")
  3459  			}
  3460  		}
  3461  		tempDelay = 0
  3462  		c := s.newConn(rw)
  3463  		c.setState(c.rwc, StateNew, runHooks) // before Serve can return
  3464  		go c.serve(connCtx)
  3465  	}
  3466  }
  3467  
  3468  // ServeTLS accepts incoming connections on the Listener l, creating a
  3469  // new service goroutine for each. The service goroutines perform TLS
  3470  // setup and then read requests, calling s.Handler to reply to them.
  3471  //
  3472  // Files containing a certificate and matching private key for the
  3473  // server must be provided if neither the [Server]'s
  3474  // TLSConfig.Certificates, TLSConfig.GetCertificate nor
  3475  // config.GetConfigForClient are populated.
  3476  // If the certificate is signed by a certificate authority, the
  3477  // certFile should be the concatenation of the server's certificate,
  3478  // any intermediates, and the CA's certificate.
  3479  //
  3480  // ServeTLS always returns a non-nil error. After [Server.Shutdown] or [Server.Close], the
  3481  // returned error is [ErrServerClosed].
  3482  func (s *Server) ServeTLS(l net.Listener, certFile, keyFile string) error {
  3483  	// Setup HTTP/2 before s.Serve, to initialize s.TLSConfig
  3484  	// before we clone it and create the TLS Listener.
  3485  	if err := s.setupHTTP2_ServeTLS(); err != nil {
  3486  		return err
  3487  	}
  3488  
  3489  	config := cloneTLSConfig(s.TLSConfig)
  3490  	config.NextProtos = adjustNextProtos(config.NextProtos, s.protocols())
  3491  
  3492  	configHasCert := len(config.Certificates) > 0 || config.GetCertificate != nil || config.GetConfigForClient != nil
  3493  	if !configHasCert || certFile != "" || keyFile != "" {
  3494  		var err error
  3495  		config.Certificates = make([]tls.Certificate, 1)
  3496  		config.Certificates[0], err = tls.LoadX509KeyPair(certFile, keyFile)
  3497  		if err != nil {
  3498  			return err
  3499  		}
  3500  	}
  3501  
  3502  	tlsListener := tls.NewListener(l, config)
  3503  	return s.Serve(tlsListener)
  3504  }
  3505  
  3506  func (s *Server) protocols() Protocols {
  3507  	if s.Protocols != nil {
  3508  		return *s.Protocols // user-configured set
  3509  	}
  3510  
  3511  	// The historic way of disabling HTTP/2 is to set TLSNextProto to
  3512  	// a non-nil map with no "h2" entry.
  3513  	_, hasH2 := s.TLSNextProto["h2"]
  3514  	http2Disabled := s.TLSNextProto != nil && !hasH2
  3515  
  3516  	// If GODEBUG=http2server=0, then HTTP/2 is disabled unless
  3517  	// the user has manually added an "h2" entry to TLSNextProto
  3518  	// (probably by using x/net/http2 directly).
  3519  	if http2server.Value() == "0" && !hasH2 {
  3520  		http2Disabled = true
  3521  	}
  3522  
  3523  	var p Protocols
  3524  	p.SetHTTP1(true) // default always includes HTTP/1
  3525  	if !http2Disabled {
  3526  		p.SetHTTP2(true)
  3527  	}
  3528  	return p
  3529  }
  3530  
  3531  // adjustNextProtos adds or removes "http/1.1" and "h2" entries from
  3532  // a tls.Config.NextProtos list, according to the set of protocols in protos.
  3533  func adjustNextProtos(nextProtos []string, protos Protocols) []string {
  3534  	// Make a copy of NextProtos since it might be shared with some other tls.Config.
  3535  	// (tls.Config.Clone doesn't do a deep copy.)
  3536  	//
  3537  	// We could avoid an allocation in the common case by checking to see if the slice
  3538  	// is already in order, but this is just one small allocation per connection.
  3539  	nextProtos = slices.Clone(nextProtos)
  3540  	var have Protocols
  3541  	nextProtos = slices.DeleteFunc(nextProtos, func(s string) bool {
  3542  		switch s {
  3543  		case "http/1.1":
  3544  			if !protos.HTTP1() {
  3545  				return true
  3546  			}
  3547  			have.SetHTTP1(true)
  3548  		case "h2":
  3549  			if !protos.HTTP2() {
  3550  				return true
  3551  			}
  3552  			have.SetHTTP2(true)
  3553  		}
  3554  		return false
  3555  	})
  3556  	if protos.HTTP2() && !have.HTTP2() {
  3557  		nextProtos = append(nextProtos, "h2")
  3558  	}
  3559  	if protos.HTTP1() && !have.HTTP1() {
  3560  		nextProtos = append(nextProtos, "http/1.1")
  3561  	}
  3562  	return nextProtos
  3563  }
  3564  
  3565  // trackListener adds or removes a net.Listener to the set of tracked
  3566  // listeners.
  3567  //
  3568  // We store a pointer to interface in the map set, in case the
  3569  // net.Listener is not comparable. This is safe because we only call
  3570  // trackListener via Serve and can track+defer untrack the same
  3571  // pointer to local variable there. We never need to compare a
  3572  // Listener from another caller.
  3573  //
  3574  // It reports whether the server is still up (not Shutdown or Closed).
  3575  func (s *Server) trackListener(ln *net.Listener, add bool) bool {
  3576  	s.mu.Lock()
  3577  	defer s.mu.Unlock()
  3578  	if s.listeners == nil {
  3579  		s.listeners = make(map[*net.Listener]struct{})
  3580  	}
  3581  	if add {
  3582  		if s.shuttingDown() {
  3583  			return false
  3584  		}
  3585  		s.listeners[ln] = struct{}{}
  3586  		s.listenerGroup.Add(1)
  3587  	} else {
  3588  		delete(s.listeners, ln)
  3589  		s.listenerGroup.Done()
  3590  	}
  3591  	return true
  3592  }
  3593  
  3594  func (s *Server) trackConn(c *conn, add bool) {
  3595  	s.mu.Lock()
  3596  	defer s.mu.Unlock()
  3597  	if s.activeConn == nil {
  3598  		s.activeConn = make(map[*conn]struct{})
  3599  	}
  3600  	if add {
  3601  		s.activeConn[c] = struct{}{}
  3602  	} else {
  3603  		delete(s.activeConn, c)
  3604  	}
  3605  }
  3606  
  3607  func (s *Server) idleTimeout() time.Duration {
  3608  	if s.IdleTimeout != 0 {
  3609  		return s.IdleTimeout
  3610  	}
  3611  	return s.ReadTimeout
  3612  }
  3613  
  3614  func (s *Server) readHeaderTimeout() time.Duration {
  3615  	if s.ReadHeaderTimeout != 0 {
  3616  		return s.ReadHeaderTimeout
  3617  	}
  3618  	return s.ReadTimeout
  3619  }
  3620  
  3621  func (s *Server) doKeepAlives() bool {
  3622  	return !s.disableKeepAlives.Load() && !s.shuttingDown()
  3623  }
  3624  
  3625  func (s *Server) shuttingDown() bool {
  3626  	return s.inShutdown.Load()
  3627  }
  3628  
  3629  // SetKeepAlivesEnabled controls whether HTTP keep-alives are enabled.
  3630  // By default, keep-alives are always enabled. Only very
  3631  // resource-constrained environments or servers in the process of
  3632  // shutting down should disable them.
  3633  func (s *Server) SetKeepAlivesEnabled(v bool) {
  3634  	if v {
  3635  		s.disableKeepAlives.Store(false)
  3636  		return
  3637  	}
  3638  	s.disableKeepAlives.Store(true)
  3639  
  3640  	// Close idle HTTP/1 conns:
  3641  	s.closeIdleConns()
  3642  
  3643  	// TODO: Issue 26303: close HTTP/2 conns as soon as they become idle.
  3644  }
  3645  
  3646  func (s *Server) logf(format string, args ...any) {
  3647  	if s.ErrorLog != nil {
  3648  		s.ErrorLog.Printf(format, args...)
  3649  	} else {
  3650  		log.Printf(format, args...)
  3651  	}
  3652  }
  3653  
  3654  // logf prints to the ErrorLog of the *Server associated with request r
  3655  // via ServerContextKey. If there's no associated server, or if ErrorLog
  3656  // is nil, logging is done via the log package's standard logger.
  3657  func logf(r *Request, format string, args ...any) {
  3658  	s, _ := r.Context().Value(ServerContextKey).(*Server)
  3659  	if s != nil && s.ErrorLog != nil {
  3660  		s.ErrorLog.Printf(format, args...)
  3661  	} else {
  3662  		log.Printf(format, args...)
  3663  	}
  3664  }
  3665  
  3666  // ListenAndServe listens on the TCP network address addr and then calls
  3667  // [Serve] with handler to handle requests on incoming connections.
  3668  // Accepted connections are configured to enable TCP keep-alives.
  3669  //
  3670  // The handler is typically nil, in which case [DefaultServeMux] is used.
  3671  //
  3672  // ListenAndServe always returns a non-nil error.
  3673  func ListenAndServe(addr string, handler Handler) error {
  3674  	server := &Server{Addr: addr, Handler: handler}
  3675  	return server.ListenAndServe()
  3676  }
  3677  
  3678  // ListenAndServeTLS acts identically to [ListenAndServe], except that it
  3679  // expects HTTPS connections. Additionally, files containing a certificate and
  3680  // matching private key for the server must be provided. If the certificate
  3681  // is signed by a certificate authority, the certFile should be the concatenation
  3682  // of the server's certificate, any intermediates, and the CA's certificate.
  3683  func ListenAndServeTLS(addr, certFile, keyFile string, handler Handler) error {
  3684  	server := &Server{Addr: addr, Handler: handler}
  3685  	return server.ListenAndServeTLS(certFile, keyFile)
  3686  }
  3687  
  3688  // ListenAndServeTLS listens on the TCP network address s.Addr and
  3689  // then calls [ServeTLS] to handle requests on incoming TLS connections.
  3690  // Accepted connections are configured to enable TCP keep-alives.
  3691  //
  3692  // Filenames containing a certificate and matching private key for the
  3693  // server must be provided if neither the [Server]'s TLSConfig.Certificates
  3694  // nor TLSConfig.GetCertificate are populated. If the certificate is
  3695  // signed by a certificate authority, the certFile should be the
  3696  // concatenation of the server's certificate, any intermediates, and
  3697  // the CA's certificate.
  3698  //
  3699  // If s.Addr is blank, ":https" is used.
  3700  //
  3701  // ListenAndServeTLS always returns a non-nil error. After [Server.Shutdown] or
  3702  // [Server.Close], the returned error is [ErrServerClosed].
  3703  func (s *Server) ListenAndServeTLS(certFile, keyFile string) error {
  3704  	if s.shuttingDown() {
  3705  		return ErrServerClosed
  3706  	}
  3707  	addr := s.Addr
  3708  	if addr == "" {
  3709  		addr = ":https"
  3710  	}
  3711  
  3712  	ln, err := net.Listen("tcp", addr)
  3713  	if err != nil {
  3714  		return err
  3715  	}
  3716  
  3717  	defer ln.Close()
  3718  
  3719  	return s.ServeTLS(ln, certFile, keyFile)
  3720  }
  3721  
  3722  // setupHTTP2_ServeTLS conditionally configures HTTP/2 on
  3723  // s and reports whether there was an error setting it up. If it is
  3724  // not configured for policy reasons, nil is returned.
  3725  func (s *Server) setupHTTP2_ServeTLS() error {
  3726  	s.nextProtoOnce.Do(s.onceSetNextProtoDefaults)
  3727  	return s.nextProtoErr
  3728  }
  3729  
  3730  // setupHTTP2_Serve is called from (*Server).Serve and conditionally
  3731  // configures HTTP/2 on s using a more conservative policy than
  3732  // setupHTTP2_ServeTLS because Serve is called after tls.Listen,
  3733  // and may be called concurrently. See shouldConfigureHTTP2ForServe.
  3734  //
  3735  // The tests named TestTransportAutomaticHTTP2* and
  3736  // TestConcurrentServerServe in server_test.go demonstrate some
  3737  // of the supported use cases and motivations.
  3738  func (s *Server) setupHTTP2_Serve() error {
  3739  	s.nextProtoOnce.Do(s.onceSetNextProtoDefaults_Serve)
  3740  	return s.nextProtoErr
  3741  }
  3742  
  3743  func (s *Server) onceSetNextProtoDefaults_Serve() {
  3744  	if s.shouldConfigureHTTP2ForServe() {
  3745  		s.onceSetNextProtoDefaults()
  3746  	}
  3747  }
  3748  
  3749  var http2server = godebug.New("http2server")
  3750  
  3751  // onceSetNextProtoDefaults configures HTTP/2, if the user hasn't
  3752  // configured otherwise. (by setting s.TLSNextProto non-nil)
  3753  // It must only be called via s.nextProtoOnce (use s.setupHTTP2_*).
  3754  func (s *Server) onceSetNextProtoDefaults() {
  3755  	if omitBundledHTTP2 {
  3756  		return
  3757  	}
  3758  	p := s.protocols()
  3759  	if !p.HTTP2() && !p.UnencryptedHTTP2() {
  3760  		return
  3761  	}
  3762  	if http2server.Value() == "0" {
  3763  		http2server.IncNonDefault()
  3764  		return
  3765  	}
  3766  	if _, ok := s.TLSNextProto["h2"]; ok {
  3767  		// TLSNextProto already contains an HTTP/2 implementation.
  3768  		// The user probably called golang.org/x/net/http2.ConfigureServer
  3769  		// to add it.
  3770  		return
  3771  	}
  3772  	conf := &http2Server{}
  3773  	s.nextProtoErr = http2ConfigureServer(s, conf)
  3774  }
  3775  
  3776  // TimeoutHandler returns a [Handler] that runs h with the given time limit.
  3777  //
  3778  // The new Handler calls h.ServeHTTP to handle each request, but if a
  3779  // call runs for longer than its time limit, the handler responds with
  3780  // a 503 Service Unavailable error and the given message in its body.
  3781  // (If msg is empty, a suitable default message will be sent.)
  3782  // After such a timeout, writes by h to its [ResponseWriter] will return
  3783  // [ErrHandlerTimeout].
  3784  //
  3785  // TimeoutHandler supports the [Pusher] interface but does not support
  3786  // the [Hijacker] or [Flusher] interfaces.
  3787  func TimeoutHandler(h Handler, dt time.Duration, msg string) Handler {
  3788  	return &timeoutHandler{
  3789  		handler: h,
  3790  		body:    msg,
  3791  		dt:      dt,
  3792  	}
  3793  }
  3794  
  3795  // ErrHandlerTimeout is returned on [ResponseWriter] Write calls
  3796  // in handlers which have timed out.
  3797  var ErrHandlerTimeout = errors.New("http: Handler timeout")
  3798  
  3799  type timeoutHandler struct {
  3800  	handler Handler
  3801  	body    string
  3802  	dt      time.Duration
  3803  
  3804  	// When set, no context will be created and this context will
  3805  	// be used instead.
  3806  	testContext context.Context
  3807  }
  3808  
  3809  func (h *timeoutHandler) errorBody() string {
  3810  	if h.body != "" {
  3811  		return h.body
  3812  	}
  3813  	return "<html><head><title>Timeout</title></head><body><h1>Timeout</h1></body></html>"
  3814  }
  3815  
  3816  func (h *timeoutHandler) ServeHTTP(w ResponseWriter, r *Request) {
  3817  	ctx := h.testContext
  3818  	if ctx == nil {
  3819  		var cancelCtx context.CancelFunc
  3820  		ctx, cancelCtx = context.WithTimeout(r.Context(), h.dt)
  3821  		defer cancelCtx()
  3822  	}
  3823  	r = r.WithContext(ctx)
  3824  	done := make(chan struct{})
  3825  	tw := &timeoutWriter{
  3826  		w:   w,
  3827  		h:   make(Header),
  3828  		req: r,
  3829  	}
  3830  	panicChan := make(chan any, 1)
  3831  	go func() {
  3832  		defer func() {
  3833  			if p := recover(); p != nil {
  3834  				panicChan <- p
  3835  			}
  3836  		}()
  3837  		h.handler.ServeHTTP(tw, r)
  3838  		close(done)
  3839  	}()
  3840  	select {
  3841  	case p := <-panicChan:
  3842  		panic(p)
  3843  	case <-done:
  3844  		tw.mu.Lock()
  3845  		defer tw.mu.Unlock()
  3846  		dst := w.Header()
  3847  		maps.Copy(dst, tw.h)
  3848  		if !tw.wroteHeader {
  3849  			tw.code = StatusOK
  3850  		}
  3851  		w.WriteHeader(tw.code)
  3852  		w.Write(tw.wbuf.Bytes())
  3853  	case <-ctx.Done():
  3854  		tw.mu.Lock()
  3855  		defer tw.mu.Unlock()
  3856  		switch err := ctx.Err(); err {
  3857  		case context.DeadlineExceeded:
  3858  			w.WriteHeader(StatusServiceUnavailable)
  3859  			io.WriteString(w, h.errorBody())
  3860  			tw.err = ErrHandlerTimeout
  3861  		default:
  3862  			w.WriteHeader(StatusServiceUnavailable)
  3863  			tw.err = err
  3864  		}
  3865  	}
  3866  }
  3867  
  3868  type timeoutWriter struct {
  3869  	w    ResponseWriter
  3870  	h    Header
  3871  	wbuf bytes.Buffer
  3872  	req  *Request
  3873  
  3874  	mu          sync.Mutex
  3875  	err         error
  3876  	wroteHeader bool
  3877  	code        int
  3878  }
  3879  
  3880  var _ Pusher = (*timeoutWriter)(nil)
  3881  
  3882  // Push implements the [Pusher] interface.
  3883  func (tw *timeoutWriter) Push(target string, opts *PushOptions) error {
  3884  	if pusher, ok := tw.w.(Pusher); ok {
  3885  		return pusher.Push(target, opts)
  3886  	}
  3887  	return ErrNotSupported
  3888  }
  3889  
  3890  func (tw *timeoutWriter) Header() Header { return tw.h }
  3891  
  3892  func (tw *timeoutWriter) Write(p []byte) (int, error) {
  3893  	tw.mu.Lock()
  3894  	defer tw.mu.Unlock()
  3895  	if tw.err != nil {
  3896  		return 0, tw.err
  3897  	}
  3898  	if !tw.wroteHeader {
  3899  		tw.writeHeaderLocked(StatusOK)
  3900  	}
  3901  	return tw.wbuf.Write(p)
  3902  }
  3903  
  3904  func (tw *timeoutWriter) writeHeaderLocked(code int) {
  3905  	checkWriteHeaderCode(code)
  3906  
  3907  	switch {
  3908  	case tw.err != nil:
  3909  		return
  3910  	case tw.wroteHeader:
  3911  		if tw.req != nil {
  3912  			caller := relevantCaller()
  3913  			logf(tw.req, "http: superfluous response.WriteHeader call from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
  3914  		}
  3915  	default:
  3916  		tw.wroteHeader = true
  3917  		tw.code = code
  3918  	}
  3919  }
  3920  
  3921  func (tw *timeoutWriter) WriteHeader(code int) {
  3922  	tw.mu.Lock()
  3923  	defer tw.mu.Unlock()
  3924  	tw.writeHeaderLocked(code)
  3925  }
  3926  
  3927  // onceCloseListener wraps a net.Listener, protecting it from
  3928  // multiple Close calls.
  3929  type onceCloseListener struct {
  3930  	net.Listener
  3931  	once     sync.Once
  3932  	closeErr error
  3933  }
  3934  
  3935  func (oc *onceCloseListener) Close() error {
  3936  	oc.once.Do(oc.close)
  3937  	return oc.closeErr
  3938  }
  3939  
  3940  func (oc *onceCloseListener) close() { oc.closeErr = oc.Listener.Close() }
  3941  
  3942  // globalOptionsHandler responds to "OPTIONS *" requests.
  3943  type globalOptionsHandler struct{}
  3944  
  3945  func (globalOptionsHandler) ServeHTTP(w ResponseWriter, r *Request) {
  3946  	w.Header().Set("Content-Length", "0")
  3947  	if r.ContentLength != 0 {
  3948  		// Read up to 4KB of OPTIONS body (as mentioned in the
  3949  		// spec as being reserved for future use), but anything
  3950  		// over that is considered a waste of server resources
  3951  		// (or an attack) and we abort and close the connection,
  3952  		// courtesy of MaxBytesReader's EOF behavior.
  3953  		mb := MaxBytesReader(w, r.Body, 4<<10)
  3954  		io.Copy(io.Discard, mb)
  3955  	}
  3956  }
  3957  
  3958  // initALPNRequest is an HTTP handler that initializes certain
  3959  // uninitialized fields in its *Request. Such partially-initialized
  3960  // Requests come from ALPN protocol handlers.
  3961  type initALPNRequest struct {
  3962  	ctx context.Context
  3963  	c   *tls.Conn
  3964  	h   serverHandler
  3965  }
  3966  
  3967  // BaseContext is an exported but unadvertised [http.Handler] method
  3968  // recognized by x/net/http2 to pass down a context; the TLSNextProto
  3969  // API predates context support so we shoehorn through the only
  3970  // interface we have available.
  3971  func (h initALPNRequest) BaseContext() context.Context { return h.ctx }
  3972  
  3973  func (h initALPNRequest) ServeHTTP(rw ResponseWriter, req *Request) {
  3974  	if req.TLS == nil {
  3975  		req.TLS = &tls.ConnectionState{}
  3976  		*req.TLS = h.c.ConnectionState()
  3977  	}
  3978  	if req.Body == nil {
  3979  		req.Body = NoBody
  3980  	}
  3981  	if req.RemoteAddr == "" {
  3982  		req.RemoteAddr = h.c.RemoteAddr().String()
  3983  	}
  3984  	h.h.ServeHTTP(rw, req)
  3985  }
  3986  
  3987  // loggingConn is used for debugging.
  3988  type loggingConn struct {
  3989  	name string
  3990  	net.Conn
  3991  }
  3992  
  3993  var (
  3994  	uniqNameMu   sync.Mutex
  3995  	uniqNameNext = make(map[string]int)
  3996  )
  3997  
  3998  func newLoggingConn(baseName string, c net.Conn) net.Conn {
  3999  	uniqNameMu.Lock()
  4000  	defer uniqNameMu.Unlock()
  4001  	uniqNameNext[baseName]++
  4002  	return &loggingConn{
  4003  		name: fmt.Sprintf("%s-%d", baseName, uniqNameNext[baseName]),
  4004  		Conn: c,
  4005  	}
  4006  }
  4007  
  4008  func (c *loggingConn) Write(p []byte) (n int, err error) {
  4009  	log.Printf("%s.Write(%d) = ....", c.name, len(p))
  4010  	n, err = c.Conn.Write(p)
  4011  	log.Printf("%s.Write(%d) = %d, %v", c.name, len(p), n, err)
  4012  	return
  4013  }
  4014  
  4015  func (c *loggingConn) Read(p []byte) (n int, err error) {
  4016  	log.Printf("%s.Read(%d) = ....", c.name, len(p))
  4017  	n, err = c.Conn.Read(p)
  4018  	log.Printf("%s.Read(%d) = %d, %v", c.name, len(p), n, err)
  4019  	return
  4020  }
  4021  
  4022  func (c *loggingConn) Close() (err error) {
  4023  	log.Printf("%s.Close() = ...", c.name)
  4024  	err = c.Conn.Close()
  4025  	log.Printf("%s.Close() = %v", c.name, err)
  4026  	return
  4027  }
  4028  
  4029  // checkConnErrorWriter writes to c.rwc and records any write errors to c.werr.
  4030  // It only contains one field (and a pointer field at that), so it
  4031  // fits in an interface value without an extra allocation.
  4032  type checkConnErrorWriter struct {
  4033  	c *conn
  4034  }
  4035  
  4036  func (w checkConnErrorWriter) Write(p []byte) (n int, err error) {
  4037  	n, err = w.c.rwc.Write(p)
  4038  	if err != nil && w.c.werr == nil {
  4039  		w.c.werr = err
  4040  		w.c.cancelCtx()
  4041  	}
  4042  	return
  4043  }
  4044  
  4045  func numLeadingCRorLF(v []byte) (n int) {
  4046  	for _, b := range v {
  4047  		if b == '\r' || b == '\n' {
  4048  			n++
  4049  			continue
  4050  		}
  4051  		break
  4052  	}
  4053  	return
  4054  }
  4055  
  4056  // tlsRecordHeaderLooksLikeHTTP reports whether a TLS record header
  4057  // looks like it might've been a misdirected plaintext HTTP request.
  4058  func tlsRecordHeaderLooksLikeHTTP(hdr [5]byte) bool {
  4059  	switch string(hdr[:]) {
  4060  	case "GET /", "HEAD ", "POST ", "PUT /", "OPTIO":
  4061  		return true
  4062  	}
  4063  	return false
  4064  }
  4065  
  4066  // MaxBytesHandler returns a [Handler] that runs h with its [ResponseWriter] and [Request.Body] wrapped by a MaxBytesReader.
  4067  func MaxBytesHandler(h Handler, n int64) Handler {
  4068  	return HandlerFunc(func(w ResponseWriter, r *Request) {
  4069  		r2 := *r
  4070  		r2.Body = MaxBytesReader(w, r.Body, n)
  4071  		h.ServeHTTP(w, &r2)
  4072  	})
  4073  }
  4074  

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