cmd/go: update BuildContext.GOROOT and build.Tooldir with computed GOROOT

This is necessary to make a relocated GOROOT work correctly.

Fixes #20997

Change-Id: I18624bd2e109721066cd9e4a887a12583ab79f5d
Reviewed-on: https://go-review.googlesource.com/48550
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -434,6 +434,38 @@ func (t *tester) registerTests() {
 		})
 	}
 
+	// On the builders only, test that a moved GOROOT still works.
+	if os.Getenv("GO_BUILDER_NAME") != "" {
+		t.tests = append(t.tests, distTest{
+			name:    "moved_goroot",
+			heading: "moved GOROOT",
+			fn: func(dt *distTest) error {
+				t.runPending(dt)
+				moved := t.goroot + "-moved"
+				if err := os.Rename(t.goroot, moved); err != nil {
+					return err
+				}
+
+				// Run `go test fmt` in the moved GOROOT.
+				cmd := exec.Command(filepath.Join(moved, "bin", "go"), "test", "fmt")
+				cmd.Stdout = os.Stdout
+				cmd.Stderr = os.Stderr
+				// Don't set GOROOT in the environment.
+				for _, e := range os.Environ() {
+					if !strings.HasPrefix(e, "GOROOT=") {
+						cmd.Env = append(cmd.Env, e)
+					}
+				}
+				err := cmd.Run()
+
+				if rerr := os.Rename(moved, t.goroot); rerr != nil {
+					log.Fatalf("failed to restore GOROOT: %v", rerr)
+				}
+				return err
+			},
+		})
+	}
+
 	// Test that internal linking of standard packages does not
 	// require libgcc. This ensures that we can install a Go
 	// release on a system that does not have a C compiler

src/cmd/go/internal/cfg/cfg.go

@@ -60,12 +60,18 @@ var CmdEnv []EnvVar
 
 // Global build parameters (used during package load)
 var (
-	Goarch    string
-	Goos      string
+	Goarch    = BuildContext.GOARCH
+	Goos      = BuildContext.GOOS
 	ExeSuffix string
-	Gopath    []string
+	Gopath    = filepath.SplitList(BuildContext.GOPATH)
 )
 
+func init() {
+	if Goos == "windows" {
+		ExeSuffix = ".exe"
+	}
+}
+
 var (
 	GOROOT    = findGOROOT()
 	GOBIN     = os.Getenv("GOBIN")
@@ -78,6 +84,16 @@ var (
 	GO386 = objabi.GO386
 )
 
+// Update build context to use our computed GOROOT.
+func init() {
+	BuildContext.GOROOT = GOROOT
+	// Note that we must use runtime.GOOS and runtime.GOARCH here,
+	// as the tool directory does not move based on environment variables.
+	// This matches the initialization of ToolDir in go/build,
+	// except for using GOROOT rather than runtime.GOROOT().
+	build.ToolDir = filepath.Join(GOROOT, "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+}
+
 func findGOROOT() string {
 	if env := os.Getenv("GOROOT"); env != "" {
 		return filepath.Clean(env)

src/cmd/go/internal/work/build.go

@@ -643,16 +643,6 @@ func InstallPackages(args []string, forGet bool) {
 	}
 }
 
-func init() {
-	cfg.Goarch = cfg.BuildContext.GOARCH
-	cfg.Goos = cfg.BuildContext.GOOS
-
-	if cfg.Goos == "windows" {
-		cfg.ExeSuffix = ".exe"
-	}
-	cfg.Gopath = filepath.SplitList(cfg.BuildContext.GOPATH)
-}
-
 // A Builder holds global state about a build.
 // It does not hold per-package state, because we
 // build packages in parallel, and the builder is shared.