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