image/gif: support non-looping animated gifs (LoopCount=-1)

The Netscape looping application extension encodes how many
times the animation should restart, and if it's present
there is no way to signal that a GIF should play only once.

Use LoopCount=-1 to signal when a decoded GIF had no looping
extension, and update the encoder to omit that extension
block when LoopCount=-1.

Fixes #15768

GitHub-Last-Rev: 249744f0e28ef8907aa876070a102cb5493f5084
GitHub-Pull-Request: golang/go#23761
Change-Id: Ic915268505bf12bdad690b59148983a7d78d693b
Reviewed-on: https://go-review.googlesource.com/93076
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

3 files changed, 66 insertions, 23 deletions

diff --git a/src/image/gif/reader.go b/src/image/gif/reader.go
index c1c9562067..763146ecc4 100644
--- a/src/image/gif/reader.go
+++ b/src/image/gif/reader.go
@@ -224,6 +224,8 @@ func (d *decoder) decode(r io.Reader, configOnly, keepAllFrames bool) error {
 		d.r = bufio.NewReader(r)
 	}
 
+	d.loopCount = -1
+
 	err := d.readHeaderAndScreenDescriptor()
 	if err != nil {
 		return err
@@ -566,9 +568,14 @@ func Decode(r io.Reader) (image.Image, error) {
 
 // GIF represents the possibly multiple images stored in a GIF file.
 type GIF struct {
-	Image     []*image.Paletted // The successive images.
-	Delay     []int             // The successive delay times, one per frame, in 100ths of a second.
-	LoopCount int               // The loop count.
+	Image []*image.Paletted // The successive images.
+	Delay []int             // The successive delay times, one per frame, in 100ths of a second.
+	// LoopCount controls the number of times an animation will be
+	// restarted during display.
+	// A LoopCount of 0 means to loop forever.
+	// A LoopCount of -1 means to show each frame only once.
+	// Otherwise, the animation is looped LoopCount+1 times.
+	LoopCount int
 	// Disposal is the successive disposal methods, one per frame. For
 	// backwards compatibility, a nil Disposal is valid to pass to EncodeAll,
 	// and implies that each frame's disposal method is 0 (no disposal
diff --git a/src/image/gif/reader_test.go b/src/image/gif/reader_test.go
index 220e8f52d4..29f47b6c08 100644
--- a/src/image/gif/reader_test.go
+++ b/src/image/gif/reader_test.go
@@ -318,23 +318,62 @@ func TestTransparentPixelOutsidePaletteRange(t *testing.T) {
 }
 
 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;")
-	img, err := DecodeAll(bytes.NewReader(data))
-	if err != nil {
-		t.Fatal("DecodeAll:", err)
-	}
-	w := new(bytes.Buffer)
-	err = EncodeAll(w, img)
-	if err != nil {
-		t.Fatal("EncodeAll:", err)
-	}
-	img1, err := DecodeAll(w)
-	if err != nil {
-		t.Fatal("DecodeAll:", err)
+	testCases := []struct {
+		name      string
+		data      []byte
+		loopCount int
+	}{
+		{
+			"loopcount-missing",
+			[]byte("GIF89a000\x00000" +
+				",0\x00\x00\x00\n\x00\n\x00\x80000000" + // image 0 descriptor & color table
+				"\x02\b\xf01u\xb9\xfdal\x05\x00;"), // image 0 image data & trailer
+			-1,
+		},
+		{
+			"loopcount-0",
+			[]byte("GIF89a000\x00000" +
+				"!\xff\vNETSCAPE2.0\x03\x01\x00\x00\x00" + // loop count = 0
+				",0\x00\x00\x00\n\x00\n\x00\x80000000" + // image 0 descriptor & color table
+				"\x02\b\xf01u\xb9\xfdal\x05\x00" + // image 0 image data
+				",0\x00\x00\x00\n\x00\n\x00\x80000000" + // image 1 descriptor & color table
+				"\x02\b\xf01u\xb9\xfdal\x05\x00;"), // image 1 image data & trailer
+			0,
+		},
+		{
+			"loopcount-1",
+			[]byte("GIF89a000\x00000" +
+				"!\xff\vNETSCAPE2.0\x03\x01\x01\x00\x00" + // loop count = 1
+				",0\x00\x00\x00\n\x00\n\x00\x80000000" + // image 0 descriptor & color table
+				"\x02\b\xf01u\xb9\xfdal\x05\x00" + // image 0 image data
+				",0\x00\x00\x00\n\x00\n\x00\x80000000" + // image 1 descriptor & color table
+				"\x02\b\xf01u\xb9\xfdal\x05\x00;"), // image 1 image data & trailer
+			1,
+		},
 	}
-	if img.LoopCount != img1.LoopCount {
-		t.Errorf("loop count mismatch: %d vs %d", img.LoopCount, img1.LoopCount)
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			img, err := DecodeAll(bytes.NewReader(tc.data))
+			if err != nil {
+				t.Fatal("DecodeAll:", err)
+			}
+			w := new(bytes.Buffer)
+			err = EncodeAll(w, img)
+			if err != nil {
+				t.Fatal("EncodeAll:", err)
+			}
+			img1, err := DecodeAll(w)
+			if err != nil {
+				t.Fatal("DecodeAll:", err)
+			}
+			if img.LoopCount != tc.loopCount {
+				t.Errorf("loop count mismatch: %d vs %d", img.LoopCount, tc.loopCount)
+			}
+			if img.LoopCount != img1.LoopCount {
+				t.Errorf("loop count failed round-trip: %d vs %d", img.LoopCount, img1.LoopCount)
+			}
+		})
 	}
 }
 
diff --git a/src/image/gif/writer.go b/src/image/gif/writer.go
index f26af8be47..5e80feb33f 100644
--- a/src/image/gif/writer.go
+++ b/src/image/gif/writer.go
@@ -178,7 +178,7 @@ func (e *encoder) writeHeader() {
 	}
 
 	// Add animation info if necessary.
-	if len(e.g.Image) > 1 {
+	if len(e.g.Image) > 1 && e.g.LoopCount >= 0 {
 		e.buf[0] = 0x21 // Extension Introducer.
 		e.buf[1] = 0xff // Application Label.
 		e.buf[2] = 0x0b // Block Size.
@@ -377,9 +377,6 @@ func EncodeAll(w io.Writer, g *GIF) error {
 	if len(g.Image) != len(g.Delay) {
 		return errors.New("gif: mismatched image and delay lengths")
 	}
-	if g.LoopCount < 0 {
-		g.LoopCount = 0
-	}
 
 	e := encoder{g: *g}
 	// The GIF.Disposal, GIF.Config and GIF.BackgroundIndex fields were added