cmd/go: respect group sticky bit on install.

When installing a package to a different directory using `go build`,
`mv` cannot be used if the destination directory has the group sticky
bit set.  Instead, `cp` should be used to make sure the destination
file has the correct permissions.

Fixes golang/go#18878.

Change-Id: I5423f559e7f84df080ed47816e19a22c6d00ab6d
Reviewed-on: https://go-review.googlesource.com/36797
Run-TryBot: Chris Manghane <cmang@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1655,6 +1655,15 @@ func (b *Builder) moveOrCopyFile(a *Action, dst, src string, perm os.FileMode, f
 	// If we can update the mode and rename to the dst, do it.
 	// Otherwise fall back to standard copy.
 
+	// If the destination directory has the group sticky bit set,
+	// we have to copy the file to retain the correct permissions.
+	// https://golang.org/issue/18878
+	if fi, err := os.Stat(filepath.Dir(dst)); err == nil {
+		if fi.IsDir() && (fi.Mode()&os.ModeSetgid) != 0 {
+			return b.copyFile(a, dst, src, perm, force)
+		}
+	}
+
 	// The perm argument is meant to be adjusted according to umask,
 	// but we don't know what the umask is.
 	// Create a dummy file to find out.

src/cmd/go/internal/work/build_test.go

@@ -5,10 +5,15 @@
 package work
 
 import (
+	"bytes"
+	"fmt"
 	"io/ioutil"
 	"os"
+	"path"
 	"path/filepath"
 	"reflect"
+	"runtime"
+	"strings"
 	"testing"
 
 	"cmd/go/internal/base"
@@ -165,3 +170,54 @@ func pkgImportPath(pkgpath string) *load.Package {
 		},
 	}
 }
+
+// When installing packages, the installed package directory should
+// respect the group sticky bit and group name of the destination
+// directory.
+// See https://golang.org/issue/18878.
+func TestRespectGroupSticky(t *testing.T) {
+	if runtime.GOOS == "nacl" {
+		t.Skip("can't set group sticky bit with chmod on nacl")
+	}
+
+	var b Builder
+	b.Init()
+
+	// Check that `cp` is called instead of `mv` by looking at the output
+	// of `(*Builder).ShowCmd` afterwards as a sanity check.
+	cfg.BuildX = true
+	var cmdBuf bytes.Buffer
+	b.Print = func(a ...interface{}) (int, error) {
+		return cmdBuf.WriteString(fmt.Sprint(a...))
+	}
+
+	stickydir := path.Join(os.TempDir(), "GroupSticky")
+	if err := os.Mkdir(stickydir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(stickydir)
+
+	// Mkdir doesn't always correctly set the group sticky bit.
+	// Change stickydir's permissions to include group sticky bit.
+	if err := os.Chmod(stickydir, 0755|os.ModeSetgid); err != nil {
+		t.Fatal(err)
+	}
+
+	pkgfile, err := ioutil.TempFile(b.WorkDir, "")
+	if err != nil {
+		t.Fatalf("ioutil.TempFile(%q): %v", b.WorkDir, err)
+	}
+	defer os.Remove(pkgfile.Name())
+	defer pkgfile.Close()
+
+	stickyFile := filepath.Join(stickydir, "sticky")
+	if err := b.moveOrCopyFile(nil, stickyFile, pkgfile.Name(), 0666, true); err != nil {
+		t.Fatalf("moveOrCopyFile: %v", err)
+	}
+
+	got := strings.TrimSpace(cmdBuf.String())
+	want := b.fmtcmd("", "cp %s %s", pkgfile.Name(), stickyFile)
+	if got != want {
+		t.Fatalf("moveOrCopyFile(%q, %q): want %q, got %q", stickyFile, pkgfile.Name(), want, got)
+	}
+}