image/gif: accept an out-of-bounds transparent color index.

This is an error according to the spec, but Firefox and Google Chrome
seem OK with this.

Fixes #15059.

Change-Id: I841cf44e96655e91a2481555f38fbd7055a32202
Reviewed-on: https://go-review.googlesource.com/22546Reviewed-by: Rob Pike <r@golang.org>

src/image/gif/reader.go

@@ -178,12 +178,25 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 				}
 				m.Palette = d.globalColorTable
 			}
-			if d.hasTransparentIndex && int(d.transparentIndex) < len(m.Palette) {
+			if d.hasTransparentIndex {
 				if !useLocalColorTable {
 					// Clone the global color table.
 					m.Palette = append(color.Palette(nil), d.globalColorTable...)
 				}
-				m.Palette[d.transparentIndex] = color.RGBA{}
+				if ti := int(d.transparentIndex); ti < len(m.Palette) {
+					m.Palette[ti] = color.RGBA{}
+				} else {
+					// The transparentIndex is out of range, which is an error
+					// according to the spec, but Firefox and Google Chrome
+					// seem OK with this, so we enlarge the palette with
+					// transparent colors. See golang.org/issue/15059.
+					p := make(color.Palette, ti+1)
+					copy(p, m.Palette)
+					for i := len(m.Palette); i < len(p); i++ {
+						p[i] = color.RGBA{}
+					}
+					m.Palette = p
+				}
 			}
 			litWidth, err := d.r.ReadByte()
 			if err != nil {

src/image/gif/reader_test.go

@@ -22,12 +22,16 @@ const (
 	trailerStr = "\x3b"
 )
 
-// lzwEncode returns an LZW encoding (with 2-bit literals) of n zeroes.
-func lzwEncode(n int) []byte {
+// lzwEncode returns an LZW encoding (with 2-bit literals) of in.
+func lzwEncode(in []byte) []byte {
 	b := &bytes.Buffer{}
 	w := lzw.NewWriter(b, lzw.LSB, 2)
-	w.Write(make([]byte, n))
-	w.Close()
+	if _, err := w.Write(in); err != nil {
+		panic(err)
+	}
+	if err := w.Close(); err != nil {
+		panic(err)
+	}
 	return b.Bytes()
 }
 
@@ -53,7 +57,7 @@ func TestDecode(t *testing.T) {
 		// byte, and 2-bit LZW literals.
 		b.WriteString("\x2c\x00\x00\x00\x00\x02\x00\x01\x00\x00\x02")
 		if tc.nPix > 0 {
-			enc := lzwEncode(tc.nPix)
+			enc := lzwEncode(make([]byte, tc.nPix))
 			if len(enc) > 0xff {
 				t.Errorf("nPix=%d, extra=%t: compressed length %d is too large", tc.nPix, tc.extra, len(enc))
 				continue
@@ -103,7 +107,7 @@ func TestTransparentIndex(t *testing.T) {
 		}
 		// Write an image with bounds 2x1, as per TestDecode.
 		b.WriteString("\x2c\x00\x00\x00\x00\x02\x00\x01\x00\x00\x02")
-		enc := lzwEncode(2)
+		enc := lzwEncode([]byte{0x00, 0x00})
 		if len(enc) > 0xff {
 			t.Fatalf("compressed length %d is too large", len(enc))
 		}
@@ -196,21 +200,13 @@ func TestNoPalette(t *testing.T) {
 	b.WriteString(headerStr[:len(headerStr)-3])
 	b.WriteString("\x00\x00\x00") // No global palette.
 
-	// Image descriptor: 2x1, no local palette.
+	// Image descriptor: 2x1, no local palette, and 2-bit LZW literals.
 	b.WriteString("\x2c\x00\x00\x00\x00\x02\x00\x01\x00\x00\x02")
 
 	// Encode the pixels: neither is in range, because there is no palette.
-	pix := []byte{0, 3}
-	enc := &bytes.Buffer{}
-	w := lzw.NewWriter(enc, lzw.LSB, 2)
-	if _, err := w.Write(pix); err != nil {
-		t.Fatalf("Write: %v", err)
-	}
-	if err := w.Close(); err != nil {
-		t.Fatalf("Close: %v", err)
-	}
-	b.WriteByte(byte(len(enc.Bytes())))
-	b.Write(enc.Bytes())
+	enc := lzwEncode([]byte{0x00, 0x03})
+	b.WriteByte(byte(len(enc)))
+	b.Write(enc)
 	b.WriteByte(0x00) // An empty block signifies the end of the image data.
 
 	b.WriteString(trailerStr)
@@ -226,21 +222,13 @@ func TestPixelOutsidePaletteRange(t *testing.T) {
 		b.WriteString(headerStr)
 		b.WriteString(paletteStr)
 
-		// Image descriptor: 2x1, no local palette.
+		// Image descriptor: 2x1, no local palette, and 2-bit LZW literals.
 		b.WriteString("\x2c\x00\x00\x00\x00\x02\x00\x01\x00\x00\x02")
 
 		// Encode the pixels; some pvals trigger the expected error.
-		pix := []byte{pval, pval}
-		enc := &bytes.Buffer{}
-		w := lzw.NewWriter(enc, lzw.LSB, 2)
-		if _, err := w.Write(pix); err != nil {
-			t.Fatalf("Write: %v", err)
-		}
-		if err := w.Close(); err != nil {
-			t.Fatalf("Close: %v", err)
-		}
-		b.WriteByte(byte(len(enc.Bytes())))
-		b.Write(enc.Bytes())
+		enc := lzwEncode([]byte{pval, pval})
+		b.WriteByte(byte(len(enc)))
+		b.Write(enc)
 		b.WriteByte(0x00) // An empty block signifies the end of the image data.
 
 		b.WriteString(trailerStr)
@@ -254,6 +242,36 @@ func TestPixelOutsidePaletteRange(t *testing.T) {
 	}
 }
 
+func TestTransparentPixelOutsidePaletteRange(t *testing.T) {
+	b := &bytes.Buffer{}
+
+	// Manufacture a GIF with a 2 color palette.
+	b.WriteString(headerStr)
+	b.WriteString(paletteStr)
+
+	// Graphic Control Extension: transparency, transparent color index = 3.
+	//
+	// This index, 3, is out of range of the global palette and there is no
+	// local palette in the subsequent image descriptor. This is an error
+	// according to the spec, but Firefox and Google Chrome seem OK with this.
+	//
+	// See golang.org/issue/15059.
+	b.WriteString("\x21\xf9\x04\x01\x00\x00\x03\x00")
+
+	// Image descriptor: 2x1, no local palette, and 2-bit LZW literals.
+	b.WriteString("\x2c\x00\x00\x00\x00\x02\x00\x01\x00\x00\x02")
+
+	// Encode the pixels.
+	enc := lzwEncode([]byte{0x03, 0x03})
+	b.WriteByte(byte(len(enc)))
+	b.Write(enc)
+	b.WriteByte(0x00) // An empty block signifies the end of the image data.
+
+	b.WriteString(trailerStr)
+
+	try(t, b.Bytes(), "")
+}
+
 func TestLoopCount(t *testing.T) {
 	data := []byte("GIF89a000\x00000,0\x00\x00\x00\n\x00" +
 		"\n\x00\x80000000\x02\b\xf01u\xb9\xfdal\x05\x00;")