cmd/go, cmd/link: make builds deterministic

Add the following flags when supported by the compiler:
  -gno-record-gcc-switches
  -fdebug-prefix-map=$WORK=/tmp/go-build

Add an empty NAME symbol to the ELF .symtab. GNU ld will add a NAME
symbol when one is not present; including one of our own prevents it
from adding a reference to the link tempdir.

Fixes #13247 for compilers that support -fdebug-prefix-map. (gcc, clang
in the near future.)

Change-Id: I221c71fc59cd23ee8c99bcc038793ff4623c9ffc
Reviewed-on: https://go-review.googlesource.com/19363Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>

src/cmd/go/build.go

@@ -686,6 +686,7 @@ type builder struct {
 	work        string               // the temporary work directory (ends in filepath.Separator)
 	actionCache map[cacheKey]*action // a cache of already-constructed actions
 	mkdirCache  map[string]bool      // a cache of created directories
+	flagCache   map[string]bool      // a cache of supported compiler flags
 	print       func(args ...interface{}) (int, error)
 
 	output    sync.Mutex
@@ -2927,6 +2928,14 @@ func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 	// disable word wrapping in error messages
 	a = append(a, "-fmessage-length=0")
 
+	// Tell gcc not to include the work directory in object files.
+	if b.gccSupportsFlag("-fdebug-prefix-map=a=b") {
+		// -gno-record-gcc-switches is supported by all gcc/clang
+		// versions that support -fdebug-prefix-map.
+		a = append(a, "-gno-record-gcc-switches")
+		a = append(a, "-fdebug-prefix-map="+b.work+"=/tmp/go-build")
+	}
+
 	// On OS X, some of the compilers behave as if -fno-common
 	// is always set, and the Mach-O linker in 6l/8l assumes this.
 	// See https://golang.org/issue/3253.
@@ -2941,19 +2950,24 @@ func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 // -no-pie must be passed when doing a partial link with -Wl,-r. But -no-pie is
 // not supported by all compilers.
 func (b *builder) gccSupportsNoPie() bool {
-	if goos != "linux" {
-		// On some BSD platforms, error messages from the
-		// compiler make it to the console despite cmd.Std*
-		// all being nil. As -no-pie is only required on linux
-		// systems so far, we only test there.
-		return false
+	return b.gccSupportsFlag("-no-pie")
+}
+
+// gccSupportsFlag checks to see if the compiler supports a flag.
+func (b *builder) gccSupportsFlag(flag string) bool {
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	if b, ok := b.flagCache[flag]; ok {
+		return b
 	}
+	if b.flagCache == nil {
 		src := filepath.Join(b.work, "trivial.c")
 		if err := ioutil.WriteFile(src, []byte{}, 0666); err != nil {
 			return false
 		}
-	cmdArgs := b.gccCmd(b.work)
-	cmdArgs = append(cmdArgs, "-no-pie", "-c", "trivial.c")
+		b.flagCache = make(map[string]bool)
+	}
+	cmdArgs := append(envList("CC", defaultCC), flag, "-c", "trivial.c")
 	if buildN || buildX {
 		b.showcmd(b.work, "%s", joinUnambiguously(cmdArgs))
 		if buildN {
@@ -2964,7 +2978,9 @@ func (b *builder) gccSupportsNoPie() bool {
 	cmd.Dir = b.work
 	cmd.Env = envForDir(cmd.Dir, os.Environ())
 	out, err := cmd.CombinedOutput()
-	return err == nil && !bytes.Contains(out, []byte("unrecognized"))
+	supported := err == nil && !bytes.Contains(out, []byte("unrecognized"))
+	b.flagCache[flag] = supported
+	return supported
 }
 
 // gccArchArgs returns arguments to pass to gcc based on the architecture.

src/cmd/go/go_test.go

@@ -2759,3 +2759,30 @@ func TestParallelTest(t *testing.T) {
 	tg.setenv("GOPATH", tg.path("."))
 	tg.run("test", "-p=4", "p1", "p2", "p3", "p4")
 }
+
+func TestCgoConsistentResults(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	exe1 := tg.path("cgotest1" + exeSuffix)
+	exe2 := tg.path("cgotest2" + exeSuffix)
+	tg.run("build", "-o", exe1, "cgotest")
+	tg.run("build", "-x", "-o", exe2, "cgotest")
+	b1, err := ioutil.ReadFile(exe1)
+	tg.must(err)
+	b2, err := ioutil.ReadFile(exe2)
+	tg.must(err)
+
+	if !tg.doGrepMatch(`-fdebug-prefix-map=\$WORK`, &tg.stderr) {
+		t.Skip("skipping because C compiler does not support -fdebug-prefix-map")
+	}
+	if !bytes.Equal(b1, b2) {
+		t.Error("building cgotest twice did not produce the same output")
+	}
+}

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

@@ -215,6 +215,11 @@ func Asmelfsym() {
 
 	dwarfaddelfsectionsyms()
 
+	// Some linkers will add a FILE sym if one is not present.
+	// Avoid having the working directory inserted into the symbol table.
+	putelfsyment(0, 0, 0, STB_LOCAL<<4|STT_FILE, SHN_ABS, 0)
+	numelfsym++
+
 	elfbind = STB_LOCAL
 	genasmsym(putelfsym)