cmd/gofmt: don't call Chmod on windows

Fixes #18026

Change-Id: Id510f427ceffb2441c3d6f5bb5c93244e46c6497
Reviewed-on: https://go-review.googlesource.com/33477
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/cmd/gofmt/gofmt.go

@@ -18,6 +18,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"runtime/pprof"
 	"strings"
 )
@@ -252,6 +253,8 @@ func diff(b1, b2 []byte) (data []byte, err error) {
 
 }
 
+const chmodSupported = runtime.GOOS != "windows"
+
 // backupFile writes data to a new file named filename<number> with permissions perm,
 // with <number randomly chosen such that the file name is unique. backupFile returns
 // the chosen file name.
@@ -262,11 +265,13 @@ func backupFile(filename string, data []byte, perm os.FileMode) (string, error)
 		return "", err
 	}
 	bakname := f.Name()
-	err = f.Chmod(perm)
-	if err != nil {
-		f.Close()
-		os.Remove(bakname)
-		return bakname, err
+	if chmodSupported {
+		err = f.Chmod(perm)
+		if err != nil {
+			f.Close()
+			os.Remove(bakname)
+			return bakname, err
+		}
 	}
 
 	// write data to backup file

src/cmd/gofmt/gofmt_test.go

@@ -171,3 +171,16 @@ func TestCRLF(t *testing.T) {
 		t.Errorf("%s contains CR's", golden)
 	}
 }
+
+func TestBackupFile(t *testing.T) {
+	dir, err := ioutil.TempDir("", "gofmt_test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	name, err := backupFile(filepath.Join(dir, "foo.go"), []byte("  package main"), 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Created: %s", name)
+}