os: fix passing long paths to Chmod on Windows

os.Chmod returns an error when passed a long path (>=260) characters on
Windows. CL 32451 fixed most file functions in os. This change applies the
same fix to os.Chmod.

Fixes #20829

Change-Id: I3270db8317ce6e06e6d77070a32a5df6ab2491e0
Reviewed-on: https://go-review.googlesource.com/47010Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file_posix.go

@@ -48,7 +48,7 @@ func syscallMode(i FileMode) (o uint32) {
 // If the file is a symbolic link, it changes the mode of the link's target.
 // If there is an error, it will be of type *PathError.
 func Chmod(name string, mode FileMode) error {
-	if e := syscall.Chmod(name, syscallMode(mode)); e != nil {
+	if e := syscall.Chmod(fixLongPath(name), syscallMode(mode)); e != nil {
 		return &PathError{"chmod", name, e}
 	}
 	return nil

src/os/os_test.go

@@ -1990,6 +1990,10 @@ func TestLongPath(t *testing.T) {
 					if dir.Size() != filesize || filesize != wantSize {
 						t.Errorf("Size(%q) is %d, len(ReadFile()) is %d, want %d", path, dir.Size(), filesize, wantSize)
 					}
+					err = Chmod(path, dir.Mode())
+					if err != nil {
+						t.Fatalf("Chmod(%q) failed: %v", path, err)
+					}
 				}
 				if err := Truncate(sizedTempDir+"/bar.txt", 0); err != nil {
 					t.Fatalf("Truncate failed: %v", err)