cmd/gofmt: use actual filename in gofmt -d output

By using actual filename, diff output of "gofmt -d" can be used with
other commands like "diffstat" and "patch".

Example:
  $ gofmt -d path/to/file.go | diffstat
  $ gofmt -d path/to/file.go > gofmt.patch
  $ patch -u -p0 < gofmt.patch

Fixes #18932

Change-Id: I21ce15eb77870d72f2c14bfd5e7c21e2c77dc9ab
Reviewed-on: https://go-review.googlesource.com/36374
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/gofmt/gofmt.go

@@ -139,11 +139,11 @@ func processFile(filename string, in io.Reader, out io.Writer, stdin bool) error
 			}
 		}
 		if *doDiff {
-			data, err := diff(src, res)
+			data, err := diff(src, res, filename)
 			if err != nil {
 				return fmt.Errorf("computing diff: %s", err)
 			}
-			fmt.Printf("diff %s gofmt/%s\n", filename, filename)
+			fmt.Printf("diff -u %s %s\n", filepath.ToSlash(filename+".orig"), filepath.ToSlash(filename))
 			out.Write(data)
 		}
 	}
@@ -225,7 +225,7 @@ func gofmtMain() {
 	}
 }
 
-func diff(b1, b2 []byte) (data []byte, err error) {
+func diff(b1, b2 []byte, filename string) (data []byte, err error) {
 	f1, err := ioutil.TempFile("", "gofmt")
 	if err != nil {
 		return
@@ -248,9 +248,39 @@ func diff(b1, b2 []byte) (data []byte, err error) {
 		// diff exits with a non-zero status when the files don't match.
 		// Ignore that failure as long as we get output.
 		err = nil
+
+		data, err = replaceTempFilename(data, filename)
 	}
 	return
+}
 
+// replaceTempFilename replaces temporary filenames in diff with actual one.
+//
+// --- /tmp/gofmt316145376	2017-02-03 19:13:00.280468375 -0500
+// +++ /tmp/gofmt617882815	2017-02-03 19:13:00.280468375 -0500
+// ...
+// ->
+// --- path/to/file.go.orig	2017-02-03 19:13:00.280468375 -0500
+// +++ path/to/file.go	2017-02-03 19:13:00.280468375 -0500
+// ...
+func replaceTempFilename(diff []byte, filename string) ([]byte, error) {
+	bs := bytes.SplitN(diff, []byte{'\n'}, 3)
+	if len(bs) < 3 {
+		return nil, fmt.Errorf("got unexpected diff for %s", filename)
+	}
+	// Preserve timestamps.
+	var t0, t1 []byte
+	if i := bytes.LastIndexByte(bs[0], '\t'); i != -1 {
+		t0 = bs[0][i:]
+	}
+	if i := bytes.LastIndexByte(bs[1], '\t'); i != -1 {
+		t1 = bs[1][i:]
+	}
+	// Always print filepath with slash separator.
+	f := filepath.ToSlash(filename)
+	bs[0] = []byte(fmt.Sprintf("--- %s%s", f+".orig", t0))
+	bs[1] = []byte(fmt.Sprintf("+++ %s%s", f, t1))
+	return bytes.Join(bs, []byte{'\n'}), nil
 }
 
 const chmodSupported = runtime.GOOS != "windows"

src/cmd/gofmt/gofmt_test.go

@@ -9,7 +9,9 @@ import (
 	"flag"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 	"text/scanner"
@@ -110,7 +112,7 @@ func runTest(t *testing.T, in, out string) {
 		}
 
 		t.Errorf("(gofmt %s) != %s (see %s.gofmt)", in, out, in)
-		d, err := diff(expected, got)
+		d, err := diff(expected, got, in)
 		if err == nil {
 			t.Errorf("%s", d)
 		}
@@ -184,3 +186,64 @@ func TestBackupFile(t *testing.T) {
 	}
 	t.Logf("Created: %s", name)
 }
+
+func TestDiff(t *testing.T) {
+	if _, err := exec.LookPath("diff"); err != nil {
+		t.Skipf("skip test on %s: diff command is required", runtime.GOOS)
+	}
+	in := []byte("first\nsecond\n")
+	out := []byte("first\nthird\n")
+	filename := "difftest.txt"
+	b, err := diff(in, out, filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	bs := bytes.SplitN(b, []byte{'\n'}, 3)
+	line0, line1 := bs[0], bs[1]
+
+	if prefix := "--- difftest.txt.orig"; !bytes.HasPrefix(line0, []byte(prefix)) {
+		t.Errorf("diff: first line should start with `%s`\ngot: %s", prefix, line0)
+	}
+
+	if prefix := "+++ difftest.txt"; !bytes.HasPrefix(line1, []byte(prefix)) {
+		t.Errorf("diff: second line should start with `%s`\ngot: %s", prefix, line1)
+	}
+
+	want := `@@ -1,2 +1,2 @@
+ first
+-second
++third
+`
+
+	if got := string(bs[2]); got != want {
+		t.Errorf("diff: got:\n%s\nwant:\n%s", got, want)
+	}
+}
+
+func TestReplaceTempFilename(t *testing.T) {
+	diff := []byte(`--- /tmp/tmpfile1	2017-02-08 00:53:26.175105619 +0900
++++ /tmp/tmpfile2	2017-02-08 00:53:38.415151275 +0900
+@@ -1,2 +1,2 @@
+ first
+-second
++third
+`)
+	want := []byte(`--- path/to/file.go.orig	2017-02-08 00:53:26.175105619 +0900
++++ path/to/file.go	2017-02-08 00:53:38.415151275 +0900
+@@ -1,2 +1,2 @@
+ first
+-second
++third
+`)
+	// Check path in diff output is always slash regardless of the
+	// os.PathSeparator (`/` or `\`).
+	sep := string(os.PathSeparator)
+	filename := strings.Join([]string{"path", "to", "file.go"}, sep)
+	got, err := replaceTempFilename(diff, filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(got, want) {
+		t.Errorf("os.PathSeparator='%s': replacedDiff:\ngot:\n%s\nwant:\n%s", sep, got, want)
+	}
+}