select.go

     1  // Copyright 2023 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  // Package toolchain implements dynamic switching of Go toolchains.
     6  package toolchain
     7  
     8  import (
     9  	"bytes"
    10  	"context"
    11  	"errors"
    12  	"flag"
    13  	"fmt"
    14  	"go/build"
    15  	"internal/godebug"
    16  	"io"
    17  	"io/fs"
    18  	"log"
    19  	"os"
    20  	"path/filepath"
    21  	"runtime"
    22  	"strconv"
    23  	"strings"
    24  
    25  	"cmd/go/internal/base"
    26  	"cmd/go/internal/cfg"
    27  	"cmd/go/internal/gover"
    28  	"cmd/go/internal/modfetch"
    29  	"cmd/go/internal/modload"
    30  	"cmd/go/internal/run"
    31  	"cmd/go/internal/work"
    32  	"cmd/internal/pathcache"
    33  	"cmd/internal/telemetry/counter"
    34  
    35  	"golang.org/x/mod/module"
    36  )
    37  
    38  const (
    39  	// We download golang.org/toolchain version v0.0.1-<gotoolchain>.<goos>-<goarch>.
    40  	// If the 0.0.1 indicates anything at all, its the version of the toolchain packaging:
    41  	// if for some reason we needed to change the way toolchains are packaged into
    42  	// module zip files in a future version of Go, we could switch to v0.0.2 and then
    43  	// older versions expecting the old format could use v0.0.1 and newer versions
    44  	// would use v0.0.2. Of course, then we'd also have to publish two of each
    45  	// module zip file. It's not likely we'll ever need to change this.
    46  	gotoolchainModule  = "golang.org/toolchain"
    47  	gotoolchainVersion = "v0.0.1"
    48  
    49  	// targetEnv is a special environment variable set to the expected
    50  	// toolchain version during the toolchain switch by the parent
    51  	// process and cleared in the child process. When set, that indicates
    52  	// to the child to confirm that it provides the expected toolchain version.
    53  	targetEnv = "GOTOOLCHAIN_INTERNAL_SWITCH_VERSION"
    54  
    55  	// countEnv is a special environment variable
    56  	// that is incremented during each toolchain switch, to detect loops.
    57  	// It is cleared before invoking programs in 'go run', 'go test', 'go generate', and 'go tool'
    58  	// by invoking them in an environment filtered with FilterEnv,
    59  	// so user programs should not see this in their environment.
    60  	countEnv = "GOTOOLCHAIN_INTERNAL_SWITCH_COUNT"
    61  
    62  	// maxSwitch is the maximum toolchain switching depth.
    63  	// Most uses should never see more than three.
    64  	// (Perhaps one for the initial GOTOOLCHAIN dispatch,
    65  	// a second for go get doing an upgrade, and a third if
    66  	// for some reason the chosen upgrade version is too small
    67  	// by a little.)
    68  	// When the count reaches maxSwitch - 10, we start logging
    69  	// the switched versions for debugging before crashing with
    70  	// a fatal error upon reaching maxSwitch.
    71  	// That should be enough to see the repetition.
    72  	maxSwitch = 100
    73  )
    74  
    75  // FilterEnv returns a copy of env with internal GOTOOLCHAIN environment
    76  // variables filtered out.
    77  func FilterEnv(env []string) []string {
    78  	// Note: Don't need to filter out targetEnv because Switch does that.
    79  	var out []string
    80  	for _, e := range env {
    81  		if strings.HasPrefix(e, countEnv+"=") {
    82  			continue
    83  		}
    84  		out = append(out, e)
    85  	}
    86  	return out
    87  }
    88  
    89  var counterErrorsInvalidToolchainInFile = counter.New("go/errors:invalid-toolchain-in-file")
    90  var toolchainTrace = godebug.New("#toolchaintrace").Value() == "1"
    91  
    92  // Select invokes a different Go toolchain if directed by
    93  // the GOTOOLCHAIN environment variable or the user's configuration
    94  // or go.mod file.
    95  // It must be called early in startup.
    96  // See https://go.dev/doc/toolchain#select.
    97  func Select() {
    98  	log.SetPrefix("go: ")
    99  	defer log.SetPrefix("")
   100  
   101  	if !modload.WillBeEnabled() {
   102  		return
   103  	}
   104  
   105  	// As a special case, let "go env GOTOOLCHAIN" and "go env -w GOTOOLCHAIN=..."
   106  	// be handled by the local toolchain, since an older toolchain may not understand it.
   107  	// This provides an easy way out of "go env -w GOTOOLCHAIN=go1.19" and makes
   108  	// sure that "go env GOTOOLCHAIN" always prints the local go command's interpretation of it.
   109  	// We look for these specific command lines in order to avoid mishandling
   110  	//
   111  	//	GOTOOLCHAIN=go1.999 go env -newflag GOTOOLCHAIN
   112  	//
   113  	// where -newflag is a flag known to Go 1.999 but not known to us.
   114  	if (len(os.Args) == 3 && os.Args[1] == "env" && os.Args[2] == "GOTOOLCHAIN") ||
   115  		(len(os.Args) == 4 && os.Args[1] == "env" && os.Args[2] == "-w" && strings.HasPrefix(os.Args[3], "GOTOOLCHAIN=")) {
   116  		return
   117  	}
   118  
   119  	// As a special case, let "go env GOMOD" and "go env GOWORK" be handled by
   120  	// the local toolchain. Users expect to be able to look up GOMOD and GOWORK
   121  	// since the go.mod and go.work file need to be determined to determine
   122  	// the minimum toolchain. See issue #61455.
   123  	if len(os.Args) == 3 && os.Args[1] == "env" && (os.Args[2] == "GOMOD" || os.Args[2] == "GOWORK") {
   124  		return
   125  	}
   126  
   127  	// Interpret GOTOOLCHAIN to select the Go toolchain to run.
   128  	gotoolchain := cfg.Getenv("GOTOOLCHAIN")
   129  	gover.Startup.GOTOOLCHAIN = gotoolchain
   130  	if gotoolchain == "" {
   131  		// cfg.Getenv should fall back to $GOROOT/go.env,
   132  		// so this should not happen, unless a packager
   133  		// has deleted the GOTOOLCHAIN line from go.env.
   134  		// It can also happen if GOROOT is missing or broken,
   135  		// in which case best to let the go command keep running
   136  		// and diagnose the problem.
   137  		return
   138  	}
   139  
   140  	// Note: minToolchain is what https://go.dev/doc/toolchain#select calls the default toolchain.
   141  	minToolchain := gover.LocalToolchain()
   142  	minVers := gover.Local()
   143  	var mode string
   144  	var toolchainTraceBuffer bytes.Buffer
   145  	if gotoolchain == "auto" {
   146  		mode = "auto"
   147  	} else if gotoolchain == "path" {
   148  		mode = "path"
   149  	} else {
   150  		min, suffix, plus := strings.Cut(gotoolchain, "+") // go1.2.3+auto
   151  		if min != "local" {
   152  			v := gover.FromToolchain(min)
   153  			if v == "" {
   154  				if plus {
   155  					base.Fatalf("invalid GOTOOLCHAIN %q: invalid minimum toolchain %q", gotoolchain, min)
   156  				}
   157  				base.Fatalf("invalid GOTOOLCHAIN %q", gotoolchain)
   158  			}
   159  			minToolchain = min
   160  			minVers = v
   161  		}
   162  		if plus && suffix != "auto" && suffix != "path" {
   163  			base.Fatalf("invalid GOTOOLCHAIN %q: only version suffixes are +auto and +path", gotoolchain)
   164  		}
   165  		mode = suffix
   166  		if toolchainTrace {
   167  			fmt.Fprintf(&toolchainTraceBuffer, "go: default toolchain set to %s from GOTOOLCHAIN=%s\n", minToolchain, gotoolchain)
   168  		}
   169  	}
   170  
   171  	gotoolchain = minToolchain
   172  	if mode == "auto" || mode == "path" {
   173  		// Read go.mod to find new minimum and suggested toolchain.
   174  		file, goVers, toolchain := modGoToolchain()
   175  		gover.Startup.AutoFile = file
   176  		if toolchain == "default" {
   177  			// "default" means always use the default toolchain,
   178  			// which is already set, so nothing to do here.
   179  			// Note that if we have Go 1.21 installed originally,
   180  			// GOTOOLCHAIN=go1.30.0+auto or GOTOOLCHAIN=go1.30.0,
   181  			// and the go.mod  says "toolchain default", we use Go 1.30, not Go 1.21.
   182  			// That is, default overrides the "auto" part of the calculation
   183  			// but not the minimum that the user has set.
   184  			// Of course, if the go.mod also says "go 1.35", using Go 1.30
   185  			// will provoke an error about the toolchain being too old.
   186  			// That's what people who use toolchain default want:
   187  			// only ever use the toolchain configured by the user
   188  			// (including its environment and go env -w file).
   189  			gover.Startup.AutoToolchain = toolchain
   190  		} else {
   191  			if toolchain != "" {
   192  				// Accept toolchain only if it is > our min.
   193  				// (If it is equal, then min satisfies it anyway: that can matter if min
   194  				// has a suffix like "go1.21.1-foo" and toolchain is "go1.21.1".)
   195  				toolVers := gover.FromToolchain(toolchain)
   196  				if toolVers == "" || (!strings.HasPrefix(toolchain, "go") && !strings.Contains(toolchain, "-go")) {
   197  					counterErrorsInvalidToolchainInFile.Inc()
   198  					base.Fatalf("invalid toolchain %q in %s", toolchain, base.ShortPath(file))
   199  				}
   200  				if gover.Compare(toolVers, minVers) > 0 {
   201  					if toolchainTrace {
   202  						modeFormat := mode
   203  						if strings.Contains(cfg.Getenv("GOTOOLCHAIN"), "+") { // go1.2.3+auto
   204  							modeFormat = fmt.Sprintf("<name>+%s", mode)
   205  						}
   206  						fmt.Fprintf(&toolchainTraceBuffer, "go: upgrading toolchain to %s (required by toolchain line in %s; upgrade allowed by GOTOOLCHAIN=%s)\n", toolchain, base.ShortPath(file), modeFormat)
   207  					}
   208  					gotoolchain = toolchain
   209  					minVers = toolVers
   210  					gover.Startup.AutoToolchain = toolchain
   211  				}
   212  			}
   213  			if gover.Compare(goVers, minVers) > 0 {
   214  				gotoolchain = "go" + goVers
   215  				minVers = goVers
   216  				// Starting with Go 1.21, the first released version has a .0 patch version suffix.
   217  				// Don't try to download a language version (sans patch component), such as go1.22.
   218  				// Instead, use the first toolchain of that language version, such as 1.22.0.
   219  				// See golang.org/issue/62278.
   220  				if gover.IsLang(goVers) && gover.Compare(goVers, "1.21") >= 0 {
   221  					gotoolchain += ".0"
   222  				}
   223  				gover.Startup.AutoGoVersion = goVers
   224  				gover.Startup.AutoToolchain = "" // in case we are overriding it for being too old
   225  				if toolchainTrace {
   226  					modeFormat := mode
   227  					if strings.Contains(cfg.Getenv("GOTOOLCHAIN"), "+") { // go1.2.3+auto
   228  						modeFormat = fmt.Sprintf("<name>+%s", mode)
   229  					}
   230  					fmt.Fprintf(&toolchainTraceBuffer, "go: upgrading toolchain to %s (required by go line in %s; upgrade allowed by GOTOOLCHAIN=%s)\n", gotoolchain, base.ShortPath(file), modeFormat)
   231  				}
   232  			}
   233  		}
   234  		maybeSwitchForGoInstallVersion(minVers)
   235  	}
   236  
   237  	// If we are invoked as a target toolchain, confirm that
   238  	// we provide the expected version and then run.
   239  	// This check is delayed until after the handling of auto and path
   240  	// so that we have initialized gover.Startup for use in error messages.
   241  	if target := os.Getenv(targetEnv); target != "" && TestVersionSwitch != "loop" {
   242  		if gover.LocalToolchain() != target {
   243  			base.Fatalf("toolchain %v invoked to provide %v", gover.LocalToolchain(), target)
   244  		}
   245  		os.Unsetenv(targetEnv)
   246  
   247  		// Note: It is tempting to check that if gotoolchain != "local"
   248  		// then target == gotoolchain here, as a sanity check that
   249  		// the child has made the same version determination as the parent.
   250  		// This turns out not always to be the case. Specifically, if we are
   251  		// running Go 1.21 with GOTOOLCHAIN=go1.22+auto, which invokes
   252  		// Go 1.22, then 'go get go@1.23.0' or 'go get needs_go_1_23'
   253  		// will invoke Go 1.23, but as the Go 1.23 child the reason for that
   254  		// will not be apparent here: it will look like we should be using Go 1.22.
   255  		// We rely on the targetEnv being set to know not to downgrade.
   256  		// A longer term problem with the sanity check is that the exact details
   257  		// may change over time: there may be other reasons that a future Go
   258  		// version might invoke an older one, and the older one won't know why.
   259  		// Best to just accept that we were invoked to provide a specific toolchain
   260  		// (which we just checked) and leave it at that.
   261  		return
   262  	}
   263  
   264  	if toolchainTrace {
   265  		// Flush toolchain tracing buffer only in the parent process (targetEnv is unset).
   266  		io.Copy(os.Stderr, &toolchainTraceBuffer)
   267  	}
   268  
   269  	if gotoolchain == "local" || gotoolchain == gover.LocalToolchain() {
   270  		// Let the current binary handle the command.
   271  		if toolchainTrace {
   272  			fmt.Fprintf(os.Stderr, "go: using local toolchain %s\n", gover.LocalToolchain())
   273  		}
   274  		return
   275  	}
   276  
   277  	// Minimal sanity check of GOTOOLCHAIN setting before search.
   278  	// We want to allow things like go1.20.3 but also gccgo-go1.20.3.
   279  	// We want to disallow mistakes / bad ideas like GOTOOLCHAIN=bash,
   280  	// since we will find that in the path lookup.
   281  	if !strings.HasPrefix(gotoolchain, "go1") && !strings.Contains(gotoolchain, "-go1") {
   282  		base.Fatalf("invalid GOTOOLCHAIN %q", gotoolchain)
   283  	}
   284  
   285  	counterSelectExec.Inc()
   286  	Exec(gotoolchain)
   287  }
   288  
   289  var counterSelectExec = counter.New("go/toolchain/select-exec")
   290  
   291  // TestVersionSwitch is set in the test go binary to the value in $TESTGO_VERSION_SWITCH.
   292  // Valid settings are:
   293  //
   294  //	"switch" - simulate version switches by reinvoking the test go binary with a different TESTGO_VERSION.
   295  //	"mismatch" - like "switch" but forget to set TESTGO_VERSION, so it looks like we invoked a mismatched toolchain
   296  //	"loop" - like "mismatch" but forget the target check, causing a toolchain switching loop
   297  var TestVersionSwitch string
   298  
   299  // Exec invokes the specified Go toolchain or else prints an error and exits the process.
   300  // If $GOTOOLCHAIN is set to path or min+path, Exec only considers the PATH
   301  // as a source of Go toolchains. Otherwise Exec tries the PATH but then downloads
   302  // a toolchain if necessary.
   303  func Exec(gotoolchain string) {
   304  	log.SetPrefix("go: ")
   305  
   306  	writeBits = sysWriteBits()
   307  
   308  	count, _ := strconv.Atoi(os.Getenv(countEnv))
   309  	if count >= maxSwitch-10 {
   310  		fmt.Fprintf(os.Stderr, "go: switching from go%v to %v [depth %d]\n", gover.Local(), gotoolchain, count)
   311  	}
   312  	if count >= maxSwitch {
   313  		base.Fatalf("too many toolchain switches")
   314  	}
   315  	os.Setenv(countEnv, fmt.Sprint(count+1))
   316  
   317  	env := cfg.Getenv("GOTOOLCHAIN")
   318  	pathOnly := env == "path" || strings.HasSuffix(env, "+path")
   319  
   320  	// For testing, if TESTGO_VERSION is already in use
   321  	// (only happens in the cmd/go test binary)
   322  	// and TESTGO_VERSION_SWITCH=switch is set,
   323  	// "switch" toolchains by changing TESTGO_VERSION
   324  	// and reinvoking the current binary.
   325  	// The special cases =loop and =mismatch skip the
   326  	// setting of TESTGO_VERSION so that it looks like we
   327  	// accidentally invoked the wrong toolchain,
   328  	// to test detection of that failure mode.
   329  	switch TestVersionSwitch {
   330  	case "switch":
   331  		os.Setenv("TESTGO_VERSION", gotoolchain)
   332  		fallthrough
   333  	case "loop", "mismatch":
   334  		exe, err := os.Executable()
   335  		if err != nil {
   336  			base.Fatalf("%v", err)
   337  		}
   338  		execGoToolchain(gotoolchain, os.Getenv("GOROOT"), exe)
   339  	}
   340  
   341  	// Look in PATH for the toolchain before we download one.
   342  	// This allows custom toolchains as well as reuse of toolchains
   343  	// already installed using go install golang.org/dl/go1.2.3@latest.
   344  	if exe, err := pathcache.LookPath(gotoolchain); err == nil {
   345  		execGoToolchain(gotoolchain, "", exe)
   346  	}
   347  
   348  	// GOTOOLCHAIN=auto looks in PATH and then falls back to download.
   349  	// GOTOOLCHAIN=path only looks in PATH.
   350  	if pathOnly {
   351  		base.Fatalf("cannot find %q in PATH", gotoolchain)
   352  	}
   353  
   354  	// Set up modules without an explicit go.mod, to download distribution.
   355  	modload.Reset()
   356  	modload.ForceUseModules = true
   357  	modload.RootMode = modload.NoRoot
   358  	modload.Init()
   359  
   360  	// Download and unpack toolchain module into module cache.
   361  	// Note that multiple go commands might be doing this at the same time,
   362  	// and that's OK: the module cache handles that case correctly.
   363  	m := module.Version{
   364  		Path:    gotoolchainModule,
   365  		Version: gotoolchainVersion + "-" + gotoolchain + "." + runtime.GOOS + "-" + runtime.GOARCH,
   366  	}
   367  	dir, err := modfetch.Download(context.Background(), m)
   368  	if err != nil {
   369  		if errors.Is(err, fs.ErrNotExist) {
   370  			toolVers := gover.FromToolchain(gotoolchain)
   371  			if gover.IsLang(toolVers) && gover.Compare(toolVers, "1.21") >= 0 {
   372  				base.Fatalf("invalid toolchain: %s is a language version but not a toolchain version (%s.x)", gotoolchain, gotoolchain)
   373  			}
   374  			base.Fatalf("download %s for %s/%s: toolchain not available", gotoolchain, runtime.GOOS, runtime.GOARCH)
   375  		}
   376  		base.Fatalf("download %s: %v", gotoolchain, err)
   377  	}
   378  
   379  	// On first use after download, set the execute bits on the commands
   380  	// so that we can run them. Note that multiple go commands might be
   381  	// doing this at the same time, but if so no harm done.
   382  	if runtime.GOOS != "windows" {
   383  		info, err := os.Stat(filepath.Join(dir, "bin/go"))
   384  		if err != nil {
   385  			base.Fatalf("download %s: %v", gotoolchain, err)
   386  		}
   387  		if info.Mode()&0111 == 0 {
   388  			// allowExec sets the exec permission bits on all files found in dir if pattern is the empty string,
   389  			// or only those files that match the pattern if it's non-empty.
   390  			allowExec := func(dir, pattern string) {
   391  				err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
   392  					if err != nil {
   393  						return err
   394  					}
   395  					if !d.IsDir() {
   396  						if pattern != "" {
   397  							if matched, _ := filepath.Match(pattern, d.Name()); !matched {
   398  								// Skip file.
   399  								return nil
   400  							}
   401  						}
   402  						info, err := os.Stat(path)
   403  						if err != nil {
   404  							return err
   405  						}
   406  						if err := os.Chmod(path, info.Mode()&0777|0111); err != nil {
   407  							return err
   408  						}
   409  					}
   410  					return nil
   411  				})
   412  				if err != nil {
   413  					base.Fatalf("download %s: %v", gotoolchain, err)
   414  				}
   415  			}
   416  
   417  			// Set the bits in pkg/tool before bin/go.
   418  			// If we are racing with another go command and do bin/go first,
   419  			// then the check of bin/go above might succeed, the other go command
   420  			// would skip its own mode-setting, and then the go command might
   421  			// try to run a tool before we get to setting the bits on pkg/tool.
   422  			// Setting pkg/tool and lib before bin/go avoids that ordering problem.
   423  			// The only other tool the go command invokes is gofmt,
   424  			// so we set that one explicitly before handling bin (which will include bin/go).
   425  			allowExec(filepath.Join(dir, "pkg/tool"), "")
   426  			allowExec(filepath.Join(dir, "lib"), "go_?*_?*_exec")
   427  			allowExec(filepath.Join(dir, "bin/gofmt"), "")
   428  			allowExec(filepath.Join(dir, "bin"), "")
   429  		}
   430  	}
   431  
   432  	srcUGoMod := filepath.Join(dir, "src/_go.mod")
   433  	srcGoMod := filepath.Join(dir, "src/go.mod")
   434  	if size(srcGoMod) != size(srcUGoMod) {
   435  		err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
   436  			if err != nil {
   437  				return err
   438  			}
   439  			if path == srcUGoMod {
   440  				// Leave for last, in case we are racing with another go command.
   441  				return nil
   442  			}
   443  			if pdir, name := filepath.Split(path); name == "_go.mod" {
   444  				if err := raceSafeCopy(path, pdir+"go.mod"); err != nil {
   445  					return err
   446  				}
   447  			}
   448  			return nil
   449  		})
   450  		// Handle src/go.mod; this is the signal to other racing go commands
   451  		// that everything is okay and they can skip this step.
   452  		if err == nil {
   453  			err = raceSafeCopy(srcUGoMod, srcGoMod)
   454  		}
   455  		if err != nil {
   456  			base.Fatalf("download %s: %v", gotoolchain, err)
   457  		}
   458  	}
   459  
   460  	// Reinvoke the go command.
   461  	execGoToolchain(gotoolchain, dir, filepath.Join(dir, "bin/go"))
   462  }
   463  
   464  func size(path string) int64 {
   465  	info, err := os.Stat(path)
   466  	if err != nil {
   467  		return -1
   468  	}
   469  	return info.Size()
   470  }
   471  
   472  var writeBits fs.FileMode
   473  
   474  // raceSafeCopy copies the file old to the file new, being careful to ensure
   475  // that if multiple go commands call raceSafeCopy(old, new) at the same time,
   476  // they don't interfere with each other: both will succeed and return and
   477  // later observe the correct content in new. Like in the build cache, we arrange
   478  // this by opening new without truncation and then writing the content.
   479  // Both go commands can do this simultaneously and will write the same thing
   480  // (old never changes content).
   481  func raceSafeCopy(old, new string) error {
   482  	oldInfo, err := os.Stat(old)
   483  	if err != nil {
   484  		return err
   485  	}
   486  	newInfo, err := os.Stat(new)
   487  	if err == nil && newInfo.Size() == oldInfo.Size() {
   488  		return nil
   489  	}
   490  	data, err := os.ReadFile(old)
   491  	if err != nil {
   492  		return err
   493  	}
   494  	// The module cache has unwritable directories by default.
   495  	// Restore the user write bit in the directory so we can create
   496  	// the new go.mod file. We clear it again at the end on a
   497  	// best-effort basis (ignoring failures).
   498  	dir := filepath.Dir(old)
   499  	info, err := os.Stat(dir)
   500  	if err != nil {
   501  		return err
   502  	}
   503  	if err := os.Chmod(dir, info.Mode()|writeBits); err != nil {
   504  		return err
   505  	}
   506  	defer os.Chmod(dir, info.Mode())
   507  	// Note: create the file writable, so that a racing go command
   508  	// doesn't get an error before we store the actual data.
   509  	f, err := os.OpenFile(new, os.O_CREATE|os.O_WRONLY, writeBits&^0o111)
   510  	if err != nil {
   511  		// If OpenFile failed because a racing go command completed our work
   512  		// (and then OpenFile failed because the directory or file is now read-only),
   513  		// count that as a success.
   514  		if size(old) == size(new) {
   515  			return nil
   516  		}
   517  		return err
   518  	}
   519  	defer os.Chmod(new, oldInfo.Mode())
   520  	if _, err := f.Write(data); err != nil {
   521  		f.Close()
   522  		return err
   523  	}
   524  	return f.Close()
   525  }
   526  
   527  // modGoToolchain finds the enclosing go.work or go.mod file
   528  // and returns the go version and toolchain lines from the file.
   529  // The toolchain line overrides the version line
   530  func modGoToolchain() (file, goVers, toolchain string) {
   531  	wd := base.UncachedCwd()
   532  	file = modload.FindGoWork(wd)
   533  	// $GOWORK can be set to a file that does not yet exist, if we are running 'go work init'.
   534  	// Do not try to load the file in that case
   535  	if _, err := os.Stat(file); err != nil {
   536  		file = ""
   537  	}
   538  	if file == "" {
   539  		file = modload.FindGoMod(wd)
   540  	}
   541  	if file == "" {
   542  		return "", "", ""
   543  	}
   544  
   545  	data, err := os.ReadFile(file)
   546  	if err != nil {
   547  		base.Fatalf("%v", err)
   548  	}
   549  	return file, gover.GoModLookup(data, "go"), gover.GoModLookup(data, "toolchain")
   550  }
   551  
   552  // maybeSwitchForGoInstallVersion reports whether the command line is go install m@v or go run m@v.
   553  // If so, switch to the go version required to build m@v if it's higher than minVers.
   554  func maybeSwitchForGoInstallVersion(minVers string) {
   555  	// Note: We assume there are no flags between 'go' and 'install' or 'run'.
   556  	// During testing there are some debugging flags that are accepted
   557  	// in that position, but in production go binaries there are not.
   558  	if len(os.Args) < 3 {
   559  		return
   560  	}
   561  
   562  	var cmdFlags *flag.FlagSet
   563  	switch os.Args[1] {
   564  	default:
   565  		// Command doesn't support a pkg@version as the main module.
   566  		return
   567  	case "install":
   568  		cmdFlags = &work.CmdInstall.Flag
   569  	case "run":
   570  		cmdFlags = &run.CmdRun.Flag
   571  	}
   572  
   573  	// The modcachrw flag is unique, in that it affects how we fetch the
   574  	// requested module to even figure out what toolchain it needs.
   575  	// We need to actually set it before we check the toolchain version.
   576  	// (See https://go.dev/issue/64282.)
   577  	modcacherwFlag := cmdFlags.Lookup("modcacherw")
   578  	if modcacherwFlag == nil {
   579  		base.Fatalf("internal error: modcacherw flag not registered for command")
   580  	}
   581  	modcacherwVal, ok := modcacherwFlag.Value.(interface {
   582  		IsBoolFlag() bool
   583  		flag.Value
   584  	})
   585  	if !ok || !modcacherwVal.IsBoolFlag() {
   586  		base.Fatalf("internal error: modcacherw is not a boolean flag")
   587  	}
   588  
   589  	// Make a best effort to parse the command's args to find the pkg@version
   590  	// argument and the -modcacherw flag.
   591  	var (
   592  		pkgArg         string
   593  		modcacherwSeen bool
   594  	)
   595  	for args := os.Args[2:]; len(args) > 0; {
   596  		a := args[0]
   597  		args = args[1:]
   598  		if a == "--" {
   599  			if len(args) == 0 {
   600  				return
   601  			}
   602  			pkgArg = args[0]
   603  			break
   604  		}
   605  
   606  		a, ok := strings.CutPrefix(a, "-")
   607  		if !ok {
   608  			// Not a flag argument. Must be a package.
   609  			pkgArg = a
   610  			break
   611  		}
   612  		a = strings.TrimPrefix(a, "-") // Treat --flag as -flag.
   613  
   614  		name, val, hasEq := strings.Cut(a, "=")
   615  
   616  		if name == "modcacherw" {
   617  			if !hasEq {
   618  				val = "true"
   619  			}
   620  			if err := modcacherwVal.Set(val); err != nil {
   621  				return
   622  			}
   623  			modcacherwSeen = true
   624  			continue
   625  		}
   626  
   627  		if hasEq {
   628  			// Already has a value; don't bother parsing it.
   629  			continue
   630  		}
   631  
   632  		f := run.CmdRun.Flag.Lookup(a)
   633  		if f == nil {
   634  			// We don't know whether this flag is a boolean.
   635  			if os.Args[1] == "run" {
   636  				// We don't know where to find the pkg@version argument.
   637  				// For run, the pkg@version can be anywhere on the command line,
   638  				// because it is preceded by run flags and followed by arguments to the
   639  				// program being run. Since we don't know whether this flag takes
   640  				// an argument, we can't reliably identify the end of the run flags.
   641  				// Just give up and let the user clarify using the "=" form.
   642  				return
   643  			}
   644  
   645  			// We would like to let 'go install -newflag pkg@version' work even
   646  			// across a toolchain switch. To make that work, assume by default that
   647  			// the pkg@version is the last argument and skip the remaining args unless
   648  			// we spot a plausible "-modcacherw" flag.
   649  			for len(args) > 0 {
   650  				a := args[0]
   651  				name, _, _ := strings.Cut(a, "=")
   652  				if name == "-modcacherw" || name == "--modcacherw" {
   653  					break
   654  				}
   655  				if len(args) == 1 && !strings.HasPrefix(a, "-") {
   656  					pkgArg = a
   657  				}
   658  				args = args[1:]
   659  			}
   660  			continue
   661  		}
   662  
   663  		if bf, ok := f.Value.(interface{ IsBoolFlag() bool }); !ok || !bf.IsBoolFlag() {
   664  			// The next arg is the value for this flag. Skip it.
   665  			args = args[1:]
   666  			continue
   667  		}
   668  	}
   669  
   670  	if !strings.Contains(pkgArg, "@") || build.IsLocalImport(pkgArg) || filepath.IsAbs(pkgArg) {
   671  		return
   672  	}
   673  	path, version, _ := strings.Cut(pkgArg, "@")
   674  	if path == "" || version == "" || gover.IsToolchain(path) {
   675  		return
   676  	}
   677  
   678  	if !modcacherwSeen && base.InGOFLAGS("-modcacherw") {
   679  		fs := flag.NewFlagSet("goInstallVersion", flag.ExitOnError)
   680  		fs.Var(modcacherwVal, "modcacherw", modcacherwFlag.Usage)
   681  		base.SetFromGOFLAGS(fs)
   682  	}
   683  
   684  	// It would be correct to do nothing here, and let "go run" or "go install"
   685  	// do the toolchain switch.
   686  	// Our goal instead is, since we have gone to the trouble of handling
   687  	// unknown flags to some degree, to run the switch now, so that
   688  	// these commands can switch to a newer toolchain directed by the
   689  	// go.mod which may actually understand the flag.
   690  	// This was brought up during the go.dev/issue/57001 proposal discussion
   691  	// and may end up being common in self-contained "go install" or "go run"
   692  	// command lines if we add new flags in the future.
   693  
   694  	// Set up modules without an explicit go.mod, to download go.mod.
   695  	modload.ForceUseModules = true
   696  	modload.RootMode = modload.NoRoot
   697  	modload.Init()
   698  	defer modload.Reset()
   699  
   700  	// See internal/load.PackagesAndErrorsOutsideModule
   701  	ctx := context.Background()
   702  	allowed := modload.CheckAllowed
   703  	if modload.IsRevisionQuery(path, version) {
   704  		// Don't check for retractions if a specific revision is requested.
   705  		allowed = nil
   706  	}
   707  	noneSelected := func(path string) (version string) { return "none" }
   708  	_, err := modload.QueryPackages(ctx, path, version, noneSelected, allowed)
   709  	if errors.Is(err, gover.ErrTooNew) {
   710  		// Run early switch, same one go install or go run would eventually do,
   711  		// if it understood all the command-line flags.
   712  		var s Switcher
   713  		s.Error(err)
   714  		if s.TooNew != nil && gover.Compare(s.TooNew.GoVersion, minVers) > 0 {
   715  			SwitchOrFatal(ctx, err)
   716  		}
   717  	}
   718  }
   719
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