image/gif: make Decode only keep the first frame in memory

Decode decodes entire GIF image and returns the first frame as an
image.Image. There's no need for it to keep every decoded frame in
memory except for the one it returns.

Fixes #22199

Change-Id: I76b4bd31608ebc76a1a3df02e85c20eb80df7877
Reviewed-on: https://go-review.googlesource.com/69890
Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/gif/reader.go

diff --git a/src/image/gif/reader.go b/src/image/gif/reader.go
index b1335e6..19f3c61 100644
--- a/src/image/gif/reader.go
+++ b/src/image/gif/reader.go
@@ -150,7 +150,7 @@
 }
 
 // decode reads a GIF image from r and stores the result in d.
-func (d *decoder) decode(r io.Reader, configOnly bool) error {
+func (d *decoder) decode(r io.Reader, configOnly, keepAllFrames bool) error {
 	// Add buffering if r does not provide ReadByte.
 	if rr, ok := r.(reader); ok {
 		d.r = rr
@@ -279,9 +279,11 @@
 				uninterlace(m)
 			}
 
-			d.image = append(d.image, m)
-			d.delay = append(d.delay, d.delayTime)
-			d.disposal = append(d.disposal, d.disposalMethod)
+			if keepAllFrames || len(d.image) == 0 {
+				d.image = append(d.image, m)
+				d.delay = append(d.delay, d.delayTime)
+				d.disposal = append(d.disposal, d.disposalMethod)
+			}
 			// The GIF89a spec, Section 23 (Graphic Control Extension) says:
 			// "The scope of this extension is the first graphic rendering block
 			// to follow." We therefore reset the GCE fields to zero.
@@ -491,7 +493,7 @@
 // image as an image.Image.
 func Decode(r io.Reader) (image.Image, error) {
 	var d decoder
-	if err := d.decode(r, false); err != nil {
+	if err := d.decode(r, false, false); err != nil {
 		return nil, err
 	}
 	return d.image[0], nil
@@ -526,7 +528,7 @@
 // and timing information.
 func DecodeAll(r io.Reader) (*GIF, error) {
 	var d decoder
-	if err := d.decode(r, false); err != nil {
+	if err := d.decode(r, false, true); err != nil {
 		return nil, err
 	}
 	gif := &GIF{
@@ -548,7 +550,7 @@
 // without decoding the entire image.
 func DecodeConfig(r io.Reader) (image.Config, error) {
 	var d decoder
-	if err := d.decode(r, true); err != nil {
+	if err := d.decode(r, true, false); err != nil {
 		return image.Config{}, err
 	}
 	return image.Config{