md/go: fail go clean command when failed to find go cache directory

Currently, if computing of the go cache directory fails it does not expose the error. Commands like go clean, exec, modindex that use go cache directory continue execution producing incorrect or no result.
This patch adds an error to the return values such that it can be validated on call sites. It also introduces such validation in go clean -cache command to fail execution in case when error occurred.

Fixes golang#69997

src/cmd/go/go_test.go

@@ -197,7 +197,7 @@ func TestMain(m *testing.M) {
 		defer removeAll(testTmpDir)
 	}
 
-	testGOCACHE, _ = cache.DefaultDir()
+	testGOCACHE, _, _ = cache.DefaultDir()
 	if testenv.HasGoBuild() {
 		testBin = filepath.Join(testTmpDir, "testbin")
 		if err := os.Mkdir(testBin, 0777); err != nil {

src/cmd/go/internal/cache/default.go

@@ -34,7 +34,7 @@ See golang.org to learn more about Go.
 // initDefaultCache does the work of finding the default cache
 // the first time Default is called.
 func initDefaultCache() Cache {
-	dir, _ := DefaultDir()
+	dir, _, _ := DefaultDir()
 	if dir == "off" {
 		if defaultDirErr != nil {
 			base.Fatalf("build cache is required, but could not be located: %v", defaultDirErr)
@@ -71,7 +71,7 @@ var (
 // DefaultDir returns the effective GOCACHE setting.
 // It returns "off" if the cache is disabled,
 // and reports whether the effective value differs from GOCACHE.
-func DefaultDir() (string, bool) {
+func DefaultDir() (string, bool, error) {
 	// Save the result of the first call to DefaultDir for later use in
 	// initDefaultCache. cmd/go/main.go explicitly sets GOCACHE so that
 	// subprocesses will inherit it, but that means initDefaultCache can't
@@ -100,5 +100,5 @@ func DefaultDir() (string, bool) {
 		defaultDir = filepath.Join(dir, "go-build")
 	})
 
-	return defaultDir, defaultDirChanged
+	return defaultDir, defaultDirChanged, defaultDirErr
 }

src/cmd/go/internal/clean/clean.go

@@ -153,7 +153,10 @@ func runClean(ctx context.Context, cmd *base.Command, args []string) {
 	sh := work.NewShell("", &load.TextPrinter{Writer: os.Stdout})
 
 	if cleanCache {
-		dir, _ := cache.DefaultDir()
+		dir, _, err := cache.DefaultDir()
+		if err != nil {
+			base.Fatal(err)
+		}
 		if dir != "off" {
 			// Remove the cache subdirectories but not the top cache directory.
 			// The top cache directory may have been created with special permissions
@@ -180,7 +183,10 @@ func runClean(ctx context.Context, cmd *base.Command, args []string) {
 		// Instead of walking through the entire cache looking for test results,
 		// we write a file to the cache indicating that all test results from before
 		// right now are to be ignored.
-		dir, _ := cache.DefaultDir()
+		dir, _, err := cache.DefaultDir()
+		if err != nil {
+			base.Fatal(err)
+		}
 		if dir != "off" {
 			f, err := lockedfile.Edit(filepath.Join(dir, "testexpire.txt"))
 			if err == nil {

src/cmd/go/internal/envcmd/env.go

@@ -130,7 +130,7 @@ func MkEnv() []cfg.EnvVar {
 				env[i].Changed = true
 			}
 		case "GOCACHE":
-			env[i].Value, env[i].Changed = cache.DefaultDir()
+			env[i].Value, env[i].Changed, _ = cache.DefaultDir()
 		case "GOTOOLCHAIN":
 			env[i].Value, env[i].Changed = cfg.EnvOrAndChanged("GOTOOLCHAIN", "")
 		case "GODEBUG":

src/cmd/go/internal/help/helpdoc.go

@@ -508,7 +508,7 @@ General-purpose environment variables:
 	GOBIN
 		The directory where 'go install' will install a command.
 	GOCACHE
-		The directory where the go command will store cached
+		The absolute path to the directory where the go command will store cached
 		information for reuse in future builds.
 	GODEBUG
 		Enable various debugging facilities. See https://go.dev/doc/godebug

src/cmd/go/internal/modindex/read.go

@@ -151,7 +151,7 @@ func GetPackage(modroot, pkgdir string) (*IndexPackage, error) {
 // using the index, for instance because the index is disabled, or the package
 // is not in a module.
 func GetModule(modroot string) (*Module, error) {
-	dir, _ := cache.DefaultDir()
+	dir, _, _ := cache.DefaultDir()
 	if !enabled || dir == "off" {
 		return nil, errDisabled
 	}

src/cmd/go/internal/test/test.go

@@ -839,7 +839,7 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 	// Read testcache expiration time, if present.
 	// (We implement go clean -testcache by writing an expiration date
 	// instead of searching out and deleting test result cache entries.)
-	if dir, _ := cache.DefaultDir(); dir != "off" {
+	if dir, _, _ := cache.DefaultDir(); dir != "off" {
 		if data, _ := lockedfile.Read(filepath.Join(dir, "testexpire.txt")); len(data) > 0 && data[len(data)-1] == '\n' {
 			if t, err := strconv.ParseInt(string(data[:len(data)-1]), 10, 64); err == nil {
 				testCacheExpire = time.Unix(0, t)

src/cmd/go/internal/work/shell.go

@@ -127,7 +127,7 @@ func (sh *Shell) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) e
 	// Otherwise fall back to standard copy.
 
 	// If the source is in the build cache, we need to copy it.
-	dir, _ := cache.DefaultDir()
+	dir, _, _ := cache.DefaultDir()
 	if strings.HasPrefix(src, dir) {
 		return sh.CopyFile(dst, src, perm, force)
 	}