cmd/link: set runtime.GOROOT default during link

Suppose you build the Go toolchain in directory A,
move the whole thing to directory B, and then use
it from B to build a new program hello.exe, and then
run hello.exe, and hello.exe crashes with a stack
trace into the standard library.

Long ago, you'd have seen hello.exe print file names
in the A directory tree, even though the files had moved
to the B directory tree. About two years ago we changed
the compiler to write down these files with the name
"$GOROOT" (that literal string) instead of A, so that the
final link from B could replace "$GOROOT" with B,
so that hello.exe's crash would show the correct source
file paths in the stack trace. (golang.org/cl/18200)

Now suppose that you do the same thing but hello.exe
doesn't crash: it prints fmt.Println(runtime.GOROOT()).
And you run hello.exe after clearing $GOROOT from the
environment.

Long ago, you'd have seen hello.exe print A instead of B.
Before this CL, you'd still see hello.exe print A instead of B.
This case is the one instance where a moved toolchain
still divulges its origin. Not anymore. After this CL, hello.exe
will print B, because the linker sets runtime/internal/sys.DefaultGoroot
with the effective GOROOT from link time.
This makes the default result of runtime.GOROOT once again
match the file names recorded in the binary, after two years
of divergence.

With that cleared up, we can reintroduce GOROOT into the
link action ID and also reenable TestExecutableGOROOT/RelocatedExe.

When $GOROOT_FINAL is set during link, it is used
in preference to $GOROOT, as always, but it was easier
to explain the behavior above without introducing that
complication.

Fixes #22155.
Fixes #20284.
Fixes #22475.

Change-Id: Ifdaeb77fd4678fdb337cf59ee25b2cd873ec1016
Reviewed-on: https://go-review.googlesource.com/86835
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: rsc

src/cmd/dist/buildruntime.go

@@ -18,7 +18,6 @@ import (
 // mkzversion writes zversion.go:
 //
 //	package sys
-//	var DefaultGoroot = <goroot>
 //
 //	const TheVersion = <version>
 //	const Goexperiment = <goexperiment>
@@ -30,8 +29,6 @@ func mkzversion(dir, file string) {
 	fmt.Fprintln(&buf)
 	fmt.Fprintf(&buf, "package sys\n")
 	fmt.Fprintln(&buf)
-	fmt.Fprintf(&buf, "var DefaultGoroot = `%s`\n", goroot_final)
-	fmt.Fprintln(&buf)
 	fmt.Fprintf(&buf, "const TheVersion = `%s`\n", findgoversion())
 	fmt.Fprintf(&buf, "const Goexperiment = `%s`\n", os.Getenv("GOEXPERIMENT"))
 	fmt.Fprintf(&buf, "const StackGuardMultiplier = %d\n", stackGuardMultiplier())
@@ -71,7 +68,6 @@ func mkzbootstrap(file string) {
 	fmt.Fprintln(&buf)
 	fmt.Fprintf(&buf, "import \"runtime\"\n")
 	fmt.Fprintln(&buf)
-	fmt.Fprintf(&buf, "const defaultGOROOT = `%s`\n", goroot_final)
 	fmt.Fprintf(&buf, "const defaultGO386 = `%s`\n", go386)
 	fmt.Fprintf(&buf, "const defaultGOARM = `%s`\n", goarm)
 	fmt.Fprintf(&buf, "const defaultGOMIPS = `%s`\n", gomips)

src/cmd/dist/test.go

@@ -172,9 +172,13 @@ func (t *tester) run() {
 		return
 	}
 
-	// we must unset GOROOT_FINAL before tests, because runtime/debug requires
+	// We must unset GOROOT_FINAL before tests, because runtime/debug requires
 	// correct access to source code, so if we have GOROOT_FINAL in effect,
 	// at least runtime/debug test will fail.
+	// If GOROOT_FINAL was set before, then now all the commands will appear stale.
+	// Nothing we can do about that other than not checking them below.
+	// (We call checkNotStale but only with "std" not "cmd".)
+	os.Setenv("GOROOT_FINAL_OLD", os.Getenv("GOROOT_FINAL")) // for cmd/link test
 	os.Unsetenv("GOROOT_FINAL")
 
 	for _, name := range t.runNames {
@@ -1044,7 +1048,7 @@ func (t *tester) cgoTest(dt *distTest) error {
 // running in parallel with earlier tests, or if it has some other reason
 // for needing the earlier tests to be done.
 func (t *tester) runPending(nextTest *distTest) {
-	checkNotStale("go", "std", "cmd")
+	checkNotStale("go", "std")
 	worklist := t.worklist
 	t.worklist = nil
 	for _, w := range worklist {
@@ -1097,7 +1101,7 @@ func (t *tester) runPending(nextTest *distTest) {
 			log.Printf("Failed: %v", w.err)
 			t.failed = true
 		}
-		checkNotStale("go", "std", "cmd")
+		checkNotStale("go", "std")
 	}
 	if t.failed && !t.keepGoing {
 		log.Fatal("FAILED")

src/cmd/go/go_test.go

@@ -102,6 +102,7 @@ func TestMain(m *testing.M) {
 		fmt.Printf("SKIP\n")
 		return
 	}
+	os.Unsetenv("GOROOT_FINAL")
 
 	if canRun {
 		args := []string{"build", "-tags", "testgo", "-o", "testgo" + exeSuffix}
@@ -4511,19 +4512,9 @@ func TestExecutableGOROOT(t *testing.T) {
 	newRoot := tg.path("new")
 
 	t.Run("RelocatedExe", func(t *testing.T) {
-		t.Skip("TODO: skipping known broken test; see golang.org/issue/20284")
-
-		// Should fall back to default location in binary.
-		// No way to dig out other than look at source code.
-		data, err := ioutil.ReadFile("../../runtime/internal/sys/zversion.go")
-		if err != nil {
-			t.Fatal(err)
-		}
-		m := regexp.MustCompile("var DefaultGoroot = `([^`]+)`").FindStringSubmatch(string(data))
-		if m == nil {
-			t.Fatal("cannot find DefaultGoroot in ../../runtime/internal/sys/zversion.go")
-		}
-		check(t, newGoTool, m[1])
+		// Should fall back to default location in binary,
+		// which is the GOROOT we used when building testgo.exe.
+		check(t, newGoTool, testGOROOT)
 	})
 
 	// If the binary is sitting in a bin dir next to ../pkg/tool, that counts as a GOROOT,
@@ -4548,9 +4539,7 @@ func TestExecutableGOROOT(t *testing.T) {
 	tg.must(os.RemoveAll(tg.path("new/pkg")))
 
 	// Binaries built in the new tree should report the
-	// new tree when they call runtime.GOROOT().
-	// This is implemented by having the go tool pass a -X option
-	// to the linker setting runtime/internal/sys.DefaultGoroot.
+	// new tree when they call runtime.GOROOT.
 	t.Run("RuntimeGoroot", func(t *testing.T) {
 		// Build a working GOROOT the easy way, with symlinks.
 		testenv.MustHaveSymlink(t)

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

@@ -76,11 +76,12 @@ func init() {
 }
 
 var (
-	GOROOT    = findGOROOT()
-	GOBIN     = os.Getenv("GOBIN")
-	GOROOTbin = filepath.Join(GOROOT, "bin")
-	GOROOTpkg = filepath.Join(GOROOT, "pkg")
-	GOROOTsrc = filepath.Join(GOROOT, "src")
+	GOROOT       = findGOROOT()
+	GOBIN        = os.Getenv("GOBIN")
+	GOROOTbin    = filepath.Join(GOROOT, "bin")
+	GOROOTpkg    = filepath.Join(GOROOT, "pkg")
+	GOROOTsrc    = filepath.Join(GOROOT, "src")
+	GOROOT_FINAL = findGOROOT_FINAL()
 
 	// Used in envcmd.MkEnv and build ID computations.
 	GOARM  = fmt.Sprint(objabi.GOARM)
@@ -129,6 +130,14 @@ func findGOROOT() string {
 	return def
 }
 
+func findGOROOT_FINAL() string {
+	def := GOROOT
+	if env := os.Getenv("GOROOT_FINAL"); env != "" {
+		def = filepath.Clean(env)
+	}
+	return def
+}
+
 // isSameDir reports whether dir1 and dir2 are the same directory.
 func isSameDir(dir1, dir2 string) bool {
 	if dir1 == dir2 {

src/cmd/go/internal/work/exec.go

@@ -790,15 +790,8 @@ func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
 		}
 		fmt.Fprintf(h, "GO$GOARCH=%s\n", os.Getenv("GO"+strings.ToUpper(cfg.BuildContext.GOARCH))) // GO386, GOARM, etc
 
-		/*
-			// TODO(rsc): Enable this code.
-			// golang.org/issue/22475.
-			goroot := cfg.BuildContext.GOROOT
-			if final := os.Getenv("GOROOT_FINAL"); final != "" {
-				goroot = final
-			}
-			fmt.Fprintf(h, "GOROOT=%s\n", goroot)
-		*/
+		// The linker writes source file paths that say GOROOT_FINAL.
+		fmt.Fprintf(h, "GOROOT=%s\n", cfg.GOROOT_FINAL)
 
 		// TODO(rsc): Convince linker team not to add more magic environment variables,
 		// or perhaps restrict the environment variables passed to subprocesses.

src/cmd/go/internal/work/gc.go

@@ -418,11 +418,6 @@ func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string)
 		ldflags = append(ldflags, "-pluginpath", pluginPath(root))
 	}
 
-	// TODO(rsc): This is probably wrong - see golang.org/issue/22155.
-	if cfg.GOROOT != runtime.GOROOT() {
-		ldflags = append(ldflags, "-X=runtime/internal/sys.DefaultGoroot="+cfg.GOROOT)
-	}
-
 	// Store BuildID inside toolchain binaries as a unique identifier of the
 	// tool being run, for use by content-based staleness determination.
 	if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {

src/cmd/internal/objabi/util.go

@@ -19,6 +19,8 @@ func envOr(key, value string) string {
 }
 
 var (
+	defaultGOROOT string // set by linker
+
 	GOROOT  = envOr("GOROOT", defaultGOROOT)
 	GOARCH  = envOr("GOARCH", defaultGOARCH)
 	GOOS    = envOr("GOOS", defaultGOOS)

src/cmd/link/dwarf_test.go

@@ -32,6 +32,9 @@ func TestDWARF(t *testing.T) {
 		t.Fatalf("go list: %v\n%s", err, out)
 	}
 	if string(out) != "false\n" {
+		if os.Getenv("GOROOT_FINAL_OLD") != "" {
+			t.Skip("cmd/link is stale, but $GOROOT_FINAL_OLD is set")
+		}
 		t.Fatalf("cmd/link is stale - run go install cmd/link")
 	}
 

src/cmd/link/internal/ld/main.go

@@ -110,6 +110,10 @@ func Main(arch *sys.Arch, theArch Arch) {
 		}
 	}
 
+	final := gorootFinal()
+	addstrdata1(ctxt, "runtime/internal/sys.DefaultGoroot="+final)
+	addstrdata1(ctxt, "cmd/internal/objabi.defaultGOROOT="+final)
+
 	// TODO(matloob): define these above and then check flag values here
 	if ctxt.Arch.Family == sys.AMD64 && objabi.GOOS == "plan9" {
 		flag.BoolVar(&Flag8, "8", false, "use 64-bit addresses in symbol table")

src/cmd/link/internal/ld/pcln.go

@@ -421,14 +421,18 @@ func (ctxt *Link) pclntab() {
 	}
 }
 
+func gorootFinal() string {
+	root := objabi.GOROOT
+	if final := os.Getenv("GOROOT_FINAL"); final != "" {
+		root = final
+	}
+	return root
+}
+
 func expandGoroot(s string) string {
 	const n = len("$GOROOT")
 	if len(s) >= n+1 && s[:n] == "$GOROOT" && (s[n] == '/' || s[n] == '\\') {
-		root := objabi.GOROOT
-		if final := os.Getenv("GOROOT_FINAL"); final != "" {
-			root = final
-		}
-		return filepath.ToSlash(filepath.Join(root, s[n:]))
+		return filepath.ToSlash(filepath.Join(gorootFinal(), s[n:]))
 	}
 	return s
 }

src/runtime/internal/sys/stubs.go

@@ -9,3 +9,5 @@ package sys
 const PtrSize = 4 << (^uintptr(0) >> 63)           // unsafe.Sizeof(uintptr(0)) but an ideal const
 const RegSize = 4 << (^Uintreg(0) >> 63)           // unsafe.Sizeof(uintreg(0)) but an ideal const
 const SpAlign = 1*(1-GoarchArm64) + 16*GoarchArm64 // SP alignment: 1 normally, 16 for ARM64
+
+var DefaultGoroot string // set at link time