cmd/link: directly exec archive command if external tmpdir

When linking a Go archive, if the archiver invocation is the very last
thing that needs to happen in the link (no "atexit" cleanups required
remove the locally created tmpdir) then call syscall.Exec to invoke
the archiver command instead of the usual exec.Command. This has the
effect of reducing peak memory use for the linker overall, since we
don't be holding onto all of the linker's live memory while the
archiver is running.

Change-Id: Ibbe22d8d67a70cc2a4f91c68aab56d19fb77c393
Reviewed-on: https://go-review.googlesource.com/c/go/+/203821
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -0,0 +1,37 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !wasm,!windows
+
+package ld
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"syscall"
+)
+
+const syscallExecSupported = true
+
+// execArchive invokes the archiver tool with syscall.Exec(), with
+// the expectation that this is the last thing that takes place
+// in the linking operation.
+func (ctxt *Link) execArchive(argv []string) {
+	var err error
+	argv0 := argv[0]
+	if filepath.Base(argv0) == argv0 {
+		argv0, err = exec.LookPath(argv0)
+		if err != nil {
+			Exitf("cannot find %s: %v", argv[0], err)
+		}
+	}
+	if ctxt.Debugvlog != 0 {
+		ctxt.Logf("invoking archiver with syscall.Exec()\n")
+	}
+	err = syscall.Exec(argv0, argv, os.Environ())
+	if err != nil {
+		Exitf("running %s failed: %v", argv[0], err)
+	}
+}

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

@@ -0,0 +1,13 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build wasm windows
+
+package ld
+
+const syscallExecSupported = false
+
+func (ctxt *Link) execArchive(argv []string) {
+	panic("should never arrive here")
+}

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

@@ -5,10 +5,13 @@
 package ld
 
 import (
+	"fmt"
 	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -69,3 +72,70 @@ func TestUndefinedRelocErrors(t *testing.T) {
 		t.Errorf("unexpected error: %s (x%d)", unexpected, n)
 	}
 }
+
+const carchiveSrcText = `
+package main
+
+// int fortytwo;
+import "C"
+
+var v int
+
+//export GoFunc
+func GoFunc() {
+	v = int(C.fortytwo)
+}
+
+func main() {
+}
+`
+
+func TestArchiveBuildInvokeWithExec(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveCGO(t)
+
+	// run this test on just a small set of platforms (no need to test it
+	// across the board given the nature of the test).
+	pair := runtime.GOOS + "-" + runtime.GOARCH
+	switch pair {
+	case "darwin-amd64", "darwin-arm64", "linux-amd64", "freebsd-amd64":
+	default:
+		t.Skip("no need for test on " + pair)
+	}
+	switch runtime.GOOS {
+	case "openbsd", "windows":
+		t.Skip("c-archive unsupported")
+	}
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	srcfile := filepath.Join(dir, "test.go")
+	arfile := filepath.Join(dir, "test.a")
+	if err := ioutil.WriteFile(srcfile, []byte(carchiveSrcText), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	ldf := fmt.Sprintf("-ldflags=-v -tmpdir=%s", dir)
+	argv := []string{"build", "-buildmode=c-archive", "-o", arfile, ldf, srcfile}
+	out, err := exec.Command(testenv.GoToolPath(t), argv...).CombinedOutput()
+	if err != nil {
+		t.Fatalf("build failure: %s\n%s\n", err, string(out))
+	}
+
+	found := false
+	const want = "invoking archiver with syscall.Exec"
+	for _, l := range strings.Split(string(out), "\n") {
+		if strings.HasPrefix(l, want) {
+			found = true
+			break
+		}
+	}
+
+	if !found {
+		t.Errorf("expected '%s' in -v output, got:\n%s\n", want, string(out))
+	}
+}

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

@@ -294,13 +294,16 @@ func libinit(ctxt *Link) {
 	}
 }
 
+func exitIfErrors() {
+	if nerrors != 0 || checkStrictDups > 1 && strictDupMsgCount > 0 {
+		mayberemoveoutfile()
+		Exit(2)
+	}
+
+}
+
 func errorexit() {
-	if nerrors != 0 {
-		Exit(2)
-	}
-	if checkStrictDups > 1 && strictDupMsgCount > 0 {
-		Exit(2)
-	}
+	exitIfErrors()
 	Exit(0)
 }
 
@@ -1021,6 +1024,7 @@ func hostlinksetup(ctxt *Link) {
 			log.Fatal(err)
 		}
 		*flagTmpdir = dir
+		ownTmpDir = true
 		AtExit(func() {
 			ctxt.Out.f.Close()
 			os.RemoveAll(*flagTmpdir)
@@ -1114,6 +1118,8 @@ func (ctxt *Link) archive() {
 		return
 	}
 
+	exitIfErrors()
+
 	if *flagExtar == "" {
 		*flagExtar = "ar"
 	}
@@ -1140,6 +1146,18 @@ func (ctxt *Link) archive() {
 		ctxt.Logf("archive: %s\n", strings.Join(argv, " "))
 	}
 
+	// If supported, use syscall.Exec() to invoke the archive command,
+	// which should be the final remaining step needed for the link.
+	// This will reduce peak RSS for the link (and speed up linking of
+	// large applications), since when the archive command runs we
+	// won't be holding onto all of the linker's live memory.
+	if syscallExecSupported && !ownTmpDir {
+		runAtExitFuncs()
+		ctxt.execArchive(argv)
+		panic("should not get here")
+	}
+
+	// Otherwise invoke 'ar' in the usual way (fork + exec).
 	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
 		Exitf("running %s failed: %v\n%s", argv[0], err, out)
 	}

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

@@ -46,6 +46,7 @@ import (
 var (
 	pkglistfornote []byte
 	windowsgui     bool // writes a "GUI binary" instead of a "console binary"
+	ownTmpDir      bool // set to true if tmp dir created by linker (e.g. no -tmpdir)
 )
 
 func init() {
@@ -272,13 +273,12 @@ func Main(arch *sys.Arch, theArch Arch) {
 
 	ctxt.undef()
 	ctxt.hostlink()
-	ctxt.archive()
 	if ctxt.Debugvlog != 0 {
 		ctxt.Logf("%d symbols\n", len(ctxt.Syms.Allsym))
 		ctxt.Logf("%d liveness data\n", liveness)
 	}
-
 	ctxt.Bso.Flush()
+	ctxt.archive()
 
 	errorexit()
 }

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

@@ -17,11 +17,17 @@ func AtExit(f func()) {
 	atExitFuncs = append(atExitFuncs, f)
 }
 
-// Exit exits with code after executing all atExitFuncs.
-func Exit(code int) {
+// runAtExitFuncs runs the queued set of AtExit functions.
+func runAtExitFuncs() {
 	for i := len(atExitFuncs) - 1; i >= 0; i-- {
 		atExitFuncs[i]()
 	}
+	atExitFuncs = nil
+}
+
+// Exit exits with code after executing all atExitFuncs.
+func Exit(code int) {
+	runAtExitFuncs()
 	os.Exit(code)
 }