Source file src/cmd/go/internal/cfg/cfg.go

     1  // Copyright 2017 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 cfg holds configuration shared by multiple parts
     6  // of the go command.
     7  package cfg
     8  
     9  import (
    10  	"bytes"
    11  	"context"
    12  	"fmt"
    13  	"go/build"
    14  	"internal/buildcfg"
    15  	"internal/cfg"
    16  	"io"
    17  	"os"
    18  	"path/filepath"
    19  	"runtime"
    20  	"strings"
    21  	"sync"
    22  
    23  	"cmd/go/internal/fsys"
    24  )
    25  
    26  // Global build parameters (used during package load)
    27  var (
    28  	Goos   = envOr("GOOS", build.Default.GOOS)
    29  	Goarch = envOr("GOARCH", build.Default.GOARCH)
    30  
    31  	ExeSuffix = exeSuffix()
    32  
    33  	// ModulesEnabled specifies whether the go command is running
    34  	// in module-aware mode (as opposed to GOPATH mode).
    35  	// It is equal to modload.Enabled, but not all packages can import modload.
    36  	ModulesEnabled bool
    37  )
    38  
    39  func exeSuffix() string {
    40  	if Goos == "windows" {
    41  		return ".exe"
    42  	}
    43  	return ""
    44  }
    45  
    46  // Configuration for tools installed to GOROOT/bin.
    47  // Normally these match runtime.GOOS and runtime.GOARCH,
    48  // but when testing a cross-compiled cmd/go they will
    49  // indicate the GOOS and GOARCH of the installed cmd/go
    50  // rather than the test binary.
    51  var (
    52  	installedGOOS   string
    53  	installedGOARCH string
    54  )
    55  
    56  // ToolExeSuffix returns the suffix for executables installed
    57  // in build.ToolDir.
    58  func ToolExeSuffix() string {
    59  	if installedGOOS == "windows" {
    60  		return ".exe"
    61  	}
    62  	return ""
    63  }
    64  
    65  // These are general "build flags" used by build and other commands.
    66  var (
    67  	BuildA                 bool     // -a flag
    68  	BuildBuildmode         string   // -buildmode flag
    69  	BuildBuildvcs          = "auto" // -buildvcs flag: "true", "false", or "auto"
    70  	BuildContext           = defaultContext()
    71  	BuildMod               string                  // -mod flag
    72  	BuildModExplicit       bool                    // whether -mod was set explicitly
    73  	BuildModReason         string                  // reason -mod was set, if set by default
    74  	BuildLinkshared        bool                    // -linkshared flag
    75  	BuildMSan              bool                    // -msan flag
    76  	BuildASan              bool                    // -asan flag
    77  	BuildCover             bool                    // -cover flag
    78  	BuildCoverMode         string                  // -covermode flag
    79  	BuildCoverPkg          []string                // -coverpkg flag
    80  	BuildN                 bool                    // -n flag
    81  	BuildO                 string                  // -o flag
    82  	BuildP                 = runtime.GOMAXPROCS(0) // -p flag
    83  	BuildPGO               string                  // -pgo flag
    84  	BuildPkgdir            string                  // -pkgdir flag
    85  	BuildRace              bool                    // -race flag
    86  	BuildToolexec          []string                // -toolexec flag
    87  	BuildToolchainName     string
    88  	BuildToolchainCompiler func() string
    89  	BuildToolchainLinker   func() string
    90  	BuildTrimpath          bool // -trimpath flag
    91  	BuildV                 bool // -v flag
    92  	BuildWork              bool // -work flag
    93  	BuildX                 bool // -x flag
    94  
    95  	ModCacheRW bool   // -modcacherw flag
    96  	ModFile    string // -modfile flag
    97  
    98  	CmdName string // "build", "install", "list", "mod tidy", etc.
    99  
   100  	DebugActiongraph  string // -debug-actiongraph flag (undocumented, unstable)
   101  	DebugTrace        string // -debug-trace flag
   102  	DebugRuntimeTrace string // -debug-runtime-trace flag (undocumented, unstable)
   103  
   104  	// GoPathError is set when GOPATH is not set. it contains an
   105  	// explanation why GOPATH is unset.
   106  	GoPathError string
   107  )
   108  
   109  func defaultContext() build.Context {
   110  	ctxt := build.Default
   111  
   112  	ctxt.JoinPath = filepath.Join // back door to say "do not use go command"
   113  
   114  	// Override defaults computed in go/build with defaults
   115  	// from go environment configuration file, if known.
   116  	ctxt.GOPATH = envOr("GOPATH", gopath(ctxt))
   117  	ctxt.GOOS = Goos
   118  	ctxt.GOARCH = Goarch
   119  
   120  	// Clear the GOEXPERIMENT-based tool tags, which we will recompute later.
   121  	var save []string
   122  	for _, tag := range ctxt.ToolTags {
   123  		if !strings.HasPrefix(tag, "goexperiment.") {
   124  			save = append(save, tag)
   125  		}
   126  	}
   127  	ctxt.ToolTags = save
   128  
   129  	// The go/build rule for whether cgo is enabled is:
   130  	//	1. If $CGO_ENABLED is set, respect it.
   131  	//	2. Otherwise, if this is a cross-compile, disable cgo.
   132  	//	3. Otherwise, use built-in default for GOOS/GOARCH.
   133  	// Recreate that logic here with the new GOOS/GOARCH setting.
   134  	if v := Getenv("CGO_ENABLED"); v == "0" || v == "1" {
   135  		ctxt.CgoEnabled = v[0] == '1'
   136  	} else if ctxt.GOOS != runtime.GOOS || ctxt.GOARCH != runtime.GOARCH {
   137  		ctxt.CgoEnabled = false
   138  	} else {
   139  		// Use built-in default cgo setting for GOOS/GOARCH.
   140  		// Note that ctxt.GOOS/GOARCH are derived from the preference list
   141  		// (1) environment, (2) go/env file, (3) runtime constants,
   142  		// while go/build.Default.GOOS/GOARCH are derived from the preference list
   143  		// (1) environment, (2) runtime constants.
   144  		//
   145  		// We know ctxt.GOOS/GOARCH == runtime.GOOS/GOARCH;
   146  		// no matter how that happened, go/build.Default will make the
   147  		// same decision (either the environment variables are set explicitly
   148  		// to match the runtime constants, or else they are unset, in which
   149  		// case go/build falls back to the runtime constants), so
   150  		// go/build.Default.GOOS/GOARCH == runtime.GOOS/GOARCH.
   151  		// So ctxt.CgoEnabled (== go/build.Default.CgoEnabled) is correct
   152  		// as is and can be left unmodified.
   153  		//
   154  		// All that said, starting in Go 1.20 we layer one more rule
   155  		// on top of the go/build decision: if CC is unset and
   156  		// the default C compiler we'd look for is not in the PATH,
   157  		// we automatically default cgo to off.
   158  		// This makes go builds work automatically on systems
   159  		// without a C compiler installed.
   160  		if ctxt.CgoEnabled {
   161  			if os.Getenv("CC") == "" {
   162  				cc := DefaultCC(ctxt.GOOS, ctxt.GOARCH)
   163  				if _, err := LookPath(cc); err != nil {
   164  					ctxt.CgoEnabled = false
   165  				}
   166  			}
   167  		}
   168  	}
   169  
   170  	ctxt.OpenFile = func(path string) (io.ReadCloser, error) {
   171  		return fsys.Open(path)
   172  	}
   173  	ctxt.ReadDir = fsys.ReadDir
   174  	ctxt.IsDir = func(path string) bool {
   175  		isDir, err := fsys.IsDir(path)
   176  		return err == nil && isDir
   177  	}
   178  
   179  	return ctxt
   180  }
   181  
   182  func init() {
   183  	SetGOROOT(Getenv("GOROOT"), false)
   184  	BuildToolchainCompiler = func() string { return "missing-compiler" }
   185  	BuildToolchainLinker = func() string { return "missing-linker" }
   186  }
   187  
   188  // SetGOROOT sets GOROOT and associated variables to the given values.
   189  //
   190  // If isTestGo is true, build.ToolDir is set based on the TESTGO_GOHOSTOS and
   191  // TESTGO_GOHOSTARCH environment variables instead of runtime.GOOS and
   192  // runtime.GOARCH.
   193  func SetGOROOT(goroot string, isTestGo bool) {
   194  	BuildContext.GOROOT = goroot
   195  
   196  	GOROOT = goroot
   197  	if goroot == "" {
   198  		GOROOTbin = ""
   199  		GOROOTpkg = ""
   200  		GOROOTsrc = ""
   201  	} else {
   202  		GOROOTbin = filepath.Join(goroot, "bin")
   203  		GOROOTpkg = filepath.Join(goroot, "pkg")
   204  		GOROOTsrc = filepath.Join(goroot, "src")
   205  	}
   206  	GOROOT_FINAL = findGOROOT_FINAL(goroot)
   207  
   208  	installedGOOS = runtime.GOOS
   209  	installedGOARCH = runtime.GOARCH
   210  	if isTestGo {
   211  		if testOS := os.Getenv("TESTGO_GOHOSTOS"); testOS != "" {
   212  			installedGOOS = testOS
   213  		}
   214  		if testArch := os.Getenv("TESTGO_GOHOSTARCH"); testArch != "" {
   215  			installedGOARCH = testArch
   216  		}
   217  	}
   218  
   219  	if runtime.Compiler != "gccgo" {
   220  		if goroot == "" {
   221  			build.ToolDir = ""
   222  		} else {
   223  			// Note that we must use the installed OS and arch here: the tool
   224  			// directory does not move based on environment variables, and even if we
   225  			// are testing a cross-compiled cmd/go all of the installed packages and
   226  			// tools would have been built using the native compiler and linker (and
   227  			// would spuriously appear stale if we used a cross-compiled compiler and
   228  			// linker).
   229  			//
   230  			// This matches the initialization of ToolDir in go/build, except for
   231  			// using ctxt.GOROOT and the installed GOOS and GOARCH rather than the
   232  			// GOROOT, GOOS, and GOARCH reported by the runtime package.
   233  			build.ToolDir = filepath.Join(GOROOTpkg, "tool", installedGOOS+"_"+installedGOARCH)
   234  		}
   235  	}
   236  }
   237  
   238  // Experiment configuration.
   239  var (
   240  	// RawGOEXPERIMENT is the GOEXPERIMENT value set by the user.
   241  	RawGOEXPERIMENT = envOr("GOEXPERIMENT", buildcfg.DefaultGOEXPERIMENT)
   242  	// CleanGOEXPERIMENT is the minimal GOEXPERIMENT value needed to reproduce the
   243  	// experiments enabled by RawGOEXPERIMENT.
   244  	CleanGOEXPERIMENT = RawGOEXPERIMENT
   245  
   246  	Experiment    *buildcfg.ExperimentFlags
   247  	ExperimentErr error
   248  )
   249  
   250  func init() {
   251  	Experiment, ExperimentErr = buildcfg.ParseGOEXPERIMENT(Goos, Goarch, RawGOEXPERIMENT)
   252  	if ExperimentErr != nil {
   253  		return
   254  	}
   255  
   256  	// GOEXPERIMENT is valid, so convert it to canonical form.
   257  	CleanGOEXPERIMENT = Experiment.String()
   258  
   259  	// Add build tags based on the experiments in effect.
   260  	exps := Experiment.Enabled()
   261  	expTags := make([]string, 0, len(exps)+len(BuildContext.ToolTags))
   262  	for _, exp := range exps {
   263  		expTags = append(expTags, "goexperiment."+exp)
   264  	}
   265  	BuildContext.ToolTags = append(expTags, BuildContext.ToolTags...)
   266  }
   267  
   268  // An EnvVar is an environment variable Name=Value.
   269  type EnvVar struct {
   270  	Name  string
   271  	Value string
   272  }
   273  
   274  // OrigEnv is the original environment of the program at startup.
   275  var OrigEnv []string
   276  
   277  // CmdEnv is the new environment for running go tool commands.
   278  // User binaries (during go test or go run) are run with OrigEnv,
   279  // not CmdEnv.
   280  var CmdEnv []EnvVar
   281  
   282  var envCache struct {
   283  	once sync.Once
   284  	m    map[string]string
   285  }
   286  
   287  // EnvFile returns the name of the Go environment configuration file.
   288  func EnvFile() (string, error) {
   289  	if file := os.Getenv("GOENV"); file != "" {
   290  		if file == "off" {
   291  			return "", fmt.Errorf("GOENV=off")
   292  		}
   293  		return file, nil
   294  	}
   295  	dir, err := os.UserConfigDir()
   296  	if err != nil {
   297  		return "", err
   298  	}
   299  	if dir == "" {
   300  		return "", fmt.Errorf("missing user-config dir")
   301  	}
   302  	return filepath.Join(dir, "go/env"), nil
   303  }
   304  
   305  func initEnvCache() {
   306  	envCache.m = make(map[string]string)
   307  	if file, _ := EnvFile(); file != "" {
   308  		readEnvFile(file, "user")
   309  	}
   310  	goroot := findGOROOT(envCache.m["GOROOT"])
   311  	if goroot != "" {
   312  		readEnvFile(filepath.Join(goroot, "go.env"), "GOROOT")
   313  	}
   314  
   315  	// Save the goroot for func init calling SetGOROOT,
   316  	// and also overwrite anything that might have been in go.env.
   317  	// It makes no sense for GOROOT/go.env to specify
   318  	// a different GOROOT.
   319  	envCache.m["GOROOT"] = goroot
   320  }
   321  
   322  func readEnvFile(file string, source string) {
   323  	if file == "" {
   324  		return
   325  	}
   326  	data, err := os.ReadFile(file)
   327  	if err != nil {
   328  		return
   329  	}
   330  
   331  	for len(data) > 0 {
   332  		// Get next line.
   333  		line := data
   334  		i := bytes.IndexByte(data, '\n')
   335  		if i >= 0 {
   336  			line, data = line[:i], data[i+1:]
   337  		} else {
   338  			data = nil
   339  		}
   340  
   341  		i = bytes.IndexByte(line, '=')
   342  		if i < 0 || line[0] < 'A' || 'Z' < line[0] {
   343  			// Line is missing = (or empty) or a comment or not a valid env name. Ignore.
   344  			// This should not happen in the user file, since the file should be maintained almost
   345  			// exclusively by "go env -w", but better to silently ignore than to make
   346  			// the go command unusable just because somehow the env file has
   347  			// gotten corrupted.
   348  			// In the GOROOT/go.env file, we expect comments.
   349  			continue
   350  		}
   351  		key, val := line[:i], line[i+1:]
   352  
   353  		if source == "GOROOT" {
   354  			// In the GOROOT/go.env file, do not overwrite fields loaded from the user's go/env file.
   355  			if _, ok := envCache.m[string(key)]; ok {
   356  				continue
   357  			}
   358  		}
   359  		envCache.m[string(key)] = string(val)
   360  	}
   361  }
   362  
   363  // Getenv gets the value for the configuration key.
   364  // It consults the operating system environment
   365  // and then the go/env file.
   366  // If Getenv is called for a key that cannot be set
   367  // in the go/env file (for example GODEBUG), it panics.
   368  // This ensures that CanGetenv is accurate, so that
   369  // 'go env -w' stays in sync with what Getenv can retrieve.
   370  func Getenv(key string) string {
   371  	if !CanGetenv(key) {
   372  		switch key {
   373  		case "CGO_TEST_ALLOW", "CGO_TEST_DISALLOW", "CGO_test_ALLOW", "CGO_test_DISALLOW":
   374  			// used by internal/work/security_test.go; allow
   375  		default:
   376  			panic("internal error: invalid Getenv " + key)
   377  		}
   378  	}
   379  	val := os.Getenv(key)
   380  	if val != "" {
   381  		return val
   382  	}
   383  	envCache.once.Do(initEnvCache)
   384  	return envCache.m[key]
   385  }
   386  
   387  // CanGetenv reports whether key is a valid go/env configuration key.
   388  func CanGetenv(key string) bool {
   389  	envCache.once.Do(initEnvCache)
   390  	if _, ok := envCache.m[key]; ok {
   391  		// Assume anything in the user file or go.env file is valid.
   392  		return true
   393  	}
   394  	return strings.Contains(cfg.KnownEnv, "\t"+key+"\n")
   395  }
   396  
   397  var (
   398  	GOROOT string
   399  
   400  	// Either empty or produced by filepath.Join(GOROOT, …).
   401  	GOROOTbin string
   402  	GOROOTpkg string
   403  	GOROOTsrc string
   404  
   405  	GOROOT_FINAL string
   406  
   407  	GOBIN      = Getenv("GOBIN")
   408  	GOMODCACHE = envOr("GOMODCACHE", gopathDir("pkg/mod"))
   409  
   410  	// Used in envcmd.MkEnv and build ID computations.
   411  	GOARM    = envOr("GOARM", fmt.Sprint(buildcfg.GOARM))
   412  	GO386    = envOr("GO386", buildcfg.GO386)
   413  	GOAMD64  = envOr("GOAMD64", fmt.Sprintf("%s%d", "v", buildcfg.GOAMD64))
   414  	GOMIPS   = envOr("GOMIPS", buildcfg.GOMIPS)
   415  	GOMIPS64 = envOr("GOMIPS64", buildcfg.GOMIPS64)
   416  	GOPPC64  = envOr("GOPPC64", fmt.Sprintf("%s%d", "power", buildcfg.GOPPC64))
   417  	GOWASM   = envOr("GOWASM", fmt.Sprint(buildcfg.GOWASM))
   418  
   419  	GOPROXY    = envOr("GOPROXY", "")
   420  	GOSUMDB    = envOr("GOSUMDB", "")
   421  	GOPRIVATE  = Getenv("GOPRIVATE")
   422  	GONOPROXY  = envOr("GONOPROXY", GOPRIVATE)
   423  	GONOSUMDB  = envOr("GONOSUMDB", GOPRIVATE)
   424  	GOINSECURE = Getenv("GOINSECURE")
   425  	GOVCS      = Getenv("GOVCS")
   426  )
   427  
   428  var SumdbDir = gopathDir("pkg/sumdb")
   429  
   430  // GetArchEnv returns the name and setting of the
   431  // GOARCH-specific architecture environment variable.
   432  // If the current architecture has no GOARCH-specific variable,
   433  // GetArchEnv returns empty key and value.
   434  func GetArchEnv() (key, val string) {
   435  	switch Goarch {
   436  	case "arm":
   437  		return "GOARM", GOARM
   438  	case "386":
   439  		return "GO386", GO386
   440  	case "amd64":
   441  		return "GOAMD64", GOAMD64
   442  	case "mips", "mipsle":
   443  		return "GOMIPS", GOMIPS
   444  	case "mips64", "mips64le":
   445  		return "GOMIPS64", GOMIPS64
   446  	case "ppc64", "ppc64le":
   447  		return "GOPPC64", GOPPC64
   448  	case "wasm":
   449  		return "GOWASM", GOWASM
   450  	}
   451  	return "", ""
   452  }
   453  
   454  // envOr returns Getenv(key) if set, or else def.
   455  func envOr(key, def string) string {
   456  	val := Getenv(key)
   457  	if val == "" {
   458  		val = def
   459  	}
   460  	return val
   461  }
   462  
   463  // There is a copy of findGOROOT, isSameDir, and isGOROOT in
   464  // x/tools/cmd/godoc/goroot.go.
   465  // Try to keep them in sync for now.
   466  
   467  // findGOROOT returns the GOROOT value, using either an explicitly
   468  // provided environment variable, a GOROOT that contains the current
   469  // os.Executable value, or else the GOROOT that the binary was built
   470  // with from runtime.GOROOT().
   471  //
   472  // There is a copy of this code in x/tools/cmd/godoc/goroot.go.
   473  func findGOROOT(env string) string {
   474  	if env == "" {
   475  		// Not using Getenv because findGOROOT is called
   476  		// to find the GOROOT/go.env file. initEnvCache
   477  		// has passed in the setting from the user go/env file.
   478  		env = os.Getenv("GOROOT")
   479  	}
   480  	if env != "" {
   481  		return filepath.Clean(env)
   482  	}
   483  	def := ""
   484  	if r := runtime.GOROOT(); r != "" {
   485  		def = filepath.Clean(r)
   486  	}
   487  	if runtime.Compiler == "gccgo" {
   488  		// gccgo has no real GOROOT, and it certainly doesn't
   489  		// depend on the executable's location.
   490  		return def
   491  	}
   492  
   493  	// canonical returns a directory path that represents
   494  	// the same directory as dir,
   495  	// preferring the spelling in def if the two are the same.
   496  	canonical := func(dir string) string {
   497  		if isSameDir(def, dir) {
   498  			return def
   499  		}
   500  		return dir
   501  	}
   502  
   503  	exe, err := os.Executable()
   504  	if err == nil {
   505  		exe, err = filepath.Abs(exe)
   506  		if err == nil {
   507  			// cmd/go may be installed in GOROOT/bin or GOROOT/bin/GOOS_GOARCH,
   508  			// depending on whether it was cross-compiled with a different
   509  			// GOHOSTOS (see https://go.dev/issue/62119). Try both.
   510  			if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
   511  				return canonical(dir)
   512  			}
   513  			if dir := filepath.Join(exe, "../../.."); isGOROOT(dir) {
   514  				return canonical(dir)
   515  			}
   516  
   517  			// Depending on what was passed on the command line, it is possible
   518  			// that os.Executable is a symlink (like /usr/local/bin/go) referring
   519  			// to a binary installed in a real GOROOT elsewhere
   520  			// (like /usr/lib/go/bin/go).
   521  			// Try to find that GOROOT by resolving the symlinks.
   522  			exe, err = filepath.EvalSymlinks(exe)
   523  			if err == nil {
   524  				if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
   525  					return canonical(dir)
   526  				}
   527  				if dir := filepath.Join(exe, "../../.."); isGOROOT(dir) {
   528  					return canonical(dir)
   529  				}
   530  			}
   531  		}
   532  	}
   533  	return def
   534  }
   535  
   536  func findGOROOT_FINAL(goroot string) string {
   537  	// $GOROOT_FINAL is only for use during make.bash
   538  	// so it is not settable using go/env, so we use os.Getenv here.
   539  	def := goroot
   540  	if env := os.Getenv("GOROOT_FINAL"); env != "" {
   541  		def = filepath.Clean(env)
   542  	}
   543  	return def
   544  }
   545  
   546  // isSameDir reports whether dir1 and dir2 are the same directory.
   547  func isSameDir(dir1, dir2 string) bool {
   548  	if dir1 == dir2 {
   549  		return true
   550  	}
   551  	info1, err1 := os.Stat(dir1)
   552  	info2, err2 := os.Stat(dir2)
   553  	return err1 == nil && err2 == nil && os.SameFile(info1, info2)
   554  }
   555  
   556  // isGOROOT reports whether path looks like a GOROOT.
   557  //
   558  // It does this by looking for the path/pkg/tool directory,
   559  // which is necessary for useful operation of the cmd/go tool,
   560  // and is not typically present in a GOPATH.
   561  //
   562  // There is a copy of this code in x/tools/cmd/godoc/goroot.go.
   563  func isGOROOT(path string) bool {
   564  	stat, err := os.Stat(filepath.Join(path, "pkg", "tool"))
   565  	if err != nil {
   566  		return false
   567  	}
   568  	return stat.IsDir()
   569  }
   570  
   571  func gopathDir(rel string) string {
   572  	list := filepath.SplitList(BuildContext.GOPATH)
   573  	if len(list) == 0 || list[0] == "" {
   574  		return ""
   575  	}
   576  	return filepath.Join(list[0], rel)
   577  }
   578  
   579  func gopath(ctxt build.Context) string {
   580  	if len(ctxt.GOPATH) > 0 {
   581  		return ctxt.GOPATH
   582  	}
   583  	env := "HOME"
   584  	if runtime.GOOS == "windows" {
   585  		env = "USERPROFILE"
   586  	} else if runtime.GOOS == "plan9" {
   587  		env = "home"
   588  	}
   589  	if home := os.Getenv(env); home != "" {
   590  		def := filepath.Join(home, "go")
   591  		if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
   592  			GoPathError = "cannot set GOROOT as GOPATH"
   593  		}
   594  		return ""
   595  	}
   596  	GoPathError = fmt.Sprintf("%s is not set", env)
   597  	return ""
   598  }
   599  
   600  // WithBuildXWriter returns a Context in which BuildX output is written
   601  // to given io.Writer.
   602  func WithBuildXWriter(ctx context.Context, xLog io.Writer) context.Context {
   603  	return context.WithValue(ctx, buildXContextKey{}, xLog)
   604  }
   605  
   606  type buildXContextKey struct{}
   607  
   608  // BuildXWriter returns nil if BuildX is false, or
   609  // the writer to which BuildX output should be written otherwise.
   610  func BuildXWriter(ctx context.Context) (io.Writer, bool) {
   611  	if !BuildX {
   612  		return nil, false
   613  	}
   614  	if v := ctx.Value(buildXContextKey{}); v != nil {
   615  		return v.(io.Writer), true
   616  	}
   617  	return os.Stderr, true
   618  }
   619  

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