image/png: always set up palette during DecodeConfig

The old code would decode the palette only for 8-bit
images during a DecodeConfig.
This CL keeps the behavior for 8-bit images and sets
up the decoded palette also for 1, 2 and 4-bit images.

Fixes #4279.

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/7421048

src/pkg/image/png/reader.go

@@ -652,10 +652,11 @@ func DecodeConfig(r io.Reader) (image.Config, error) {
 			}
 			return image.Config{}, err
 		}
-		if d.stage == dsSeenIHDR && d.cb != cbP8 {
+		paletted := d.cb == cbP8 || d.cb == cbP4 || d.cb == cbP2 || d.cb == cbP1
+		if d.stage == dsSeenIHDR && !paletted {
 			break
 		}
-		if d.stage == dsSeenPLTE && d.cb == cbP8 {
+		if d.stage == dsSeenPLTE && paletted {
 			break
 		}
 	}

src/pkg/image/png/reader_test.go

@@ -38,6 +38,14 @@ var filenames = []string{
 	"basn6a16",
 }
 
+var filenamesPaletted = []string{
+	"basn3p01",
+	"basn3p02",
+	"basn3p04",
+	"basn3p08",
+	"basn3p08-trns",
+}
+
 var filenamesShort = []string{
 	"basn0g01",
 	"basn0g04-31",
@@ -278,6 +286,31 @@ func TestReaderError(t *testing.T) {
 	}
 }
 
+func TestPalettedDecodeConfig(t *testing.T) {
+	for _, fn := range filenamesPaletted {
+		f, err := os.Open("testdata/pngsuite/" + fn + ".png")
+		if err != nil {
+			t.Errorf("%s: open failed: %v", fn, err)
+			continue
+		}
+		defer f.Close()
+		cfg, err := DecodeConfig(f)
+		if err != nil {
+			t.Errorf("%s: %v", fn, err)
+			continue
+		}
+		pal, ok := cfg.ColorModel.(color.Palette)
+		if !ok {
+			t.Errorf("%s: expected paletted color model", fn)
+			continue
+		}
+		if pal == nil {
+			t.Errorf("%s: palette not initialized", fn)
+			continue
+		}
+	}
+}
+
 func benchmarkDecode(b *testing.B, filename string, bytesPerPixel int) {
 	b.StopTimer()
 	data, err := ioutil.ReadFile(filename)