2 files changed, 71 insertions, 29 deletions

diff --git a/libgo/go/image/gif/reader.go b/libgo/go/image/gif/reader.go
index 6181a946fad..e61112817b7 100644
--- a/libgo/go/image/gif/reader.go
+++ b/libgo/go/image/gif/reader.go
@@ -63,6 +63,22 @@ const (
 	eApplication    = 0xFF // Application
 )
 
+func readFull(r io.Reader, b []byte) error {
+	_, err := io.ReadFull(r, b)
+	if err == io.EOF {
+		err = io.ErrUnexpectedEOF
+	}
+	return err
+}
+
+func readByte(r io.ByteReader) (byte, error) {
+	b, err := r.ReadByte()
+	if err == io.EOF {
+		err = io.ErrUnexpectedEOF
+	}
+	return b, err
+}
+
 // decoder is the type used to decode a GIF file.
 type decoder struct {
 	r reader
@@ -124,7 +140,7 @@ func (b *blockReader) Read(p []byte) (int, error) {
 			return 0, b.err
 		}
 		b.slice = b.tmp[:blockLen]
-		if _, b.err = io.ReadFull(b.r, b.slice); b.err != nil {
+		if b.err = readFull(b.r, b.slice); b.err != nil {
 			return 0, b.err
 		}
 	}
@@ -151,9 +167,9 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 	}
 
 	for {
-		c, err := d.r.ReadByte()
+		c, err := readByte(d.r)
 		if err != nil {
-			return err
+			return fmt.Errorf("gif: reading frames: %v", err)
 		}
 		switch c {
 		case sExtension:
@@ -198,9 +214,9 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 					m.Palette = p
 				}
 			}
-			litWidth, err := d.r.ReadByte()
+			litWidth, err := readByte(d.r)
 			if err != nil {
-				return err
+				return fmt.Errorf("gif: reading image data: %v", err)
 			}
 			if litWidth < 2 || litWidth > 8 {
 				return fmt.Errorf("gif: pixel size in decode out of range: %d", litWidth)
@@ -209,9 +225,9 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 			br := &blockReader{r: d.r}
 			lzwr := lzw.NewReader(br, lzw.LSB, int(litWidth))
 			defer lzwr.Close()
-			if _, err = io.ReadFull(lzwr, m.Pix); err != nil {
+			if err = readFull(lzwr, m.Pix); err != nil {
 				if err != io.ErrUnexpectedEOF {
-					return err
+					return fmt.Errorf("gif: reading image data: %v", err)
 				}
 				return errNotEnough
 			}
@@ -228,13 +244,13 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 			// See https://golang.org/issue/9856 for an example GIF.
 			if n, err := lzwr.Read(d.tmp[:1]); n != 0 || (err != io.EOF && err != io.ErrUnexpectedEOF) {
 				if err != nil {
-					return err
+					return fmt.Errorf("gif: reading image data: %v", err)
 				}
 				return errTooMuch
 			}
 			if n, err := br.Read(d.tmp[:1]); n != 0 || err != io.EOF {
 				if err != nil {
-					return err
+					return fmt.Errorf("gif: reading image data: %v", err)
 				}
 				return errTooMuch
 			}
@@ -264,7 +280,7 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 
 		case sTrailer:
 			if len(d.image) == 0 {
-				return io.ErrUnexpectedEOF
+				return fmt.Errorf("gif: missing image data")
 			}
 			return nil
 
@@ -275,13 +291,13 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 }
 
 func (d *decoder) readHeaderAndScreenDescriptor() error {
-	_, err := io.ReadFull(d.r, d.tmp[:13])
+	err := readFull(d.r, d.tmp[:13])
 	if err != nil {
-		return err
+		return fmt.Errorf("gif: reading header: %v", err)
 	}
 	d.vers = string(d.tmp[:6])
 	if d.vers != "GIF87a" && d.vers != "GIF89a" {
-		return fmt.Errorf("gif: can't recognize format %s", d.vers)
+		return fmt.Errorf("gif: can't recognize format %q", d.vers)
 	}
 	d.width = int(d.tmp[6]) + int(d.tmp[7])<<8
 	d.height = int(d.tmp[8]) + int(d.tmp[9])<<8
@@ -298,9 +314,9 @@ func (d *decoder) readHeaderAndScreenDescriptor() error {
 
 func (d *decoder) readColorTable(fields byte) (color.Palette, error) {
 	n := 1 << (1 + uint(fields&fColorTableBitsMask))
-	_, err := io.ReadFull(d.r, d.tmp[:3*n])
+	err := readFull(d.r, d.tmp[:3*n])
 	if err != nil {
-		return nil, fmt.Errorf("gif: short read on color table: %s", err)
+		return nil, fmt.Errorf("gif: reading color table: %s", err)
 	}
 	j, p := 0, make(color.Palette, n)
 	for i := range p {
@@ -311,9 +327,9 @@ func (d *decoder) readColorTable(fields byte) (color.Palette, error) {
 }
 
 func (d *decoder) readExtension() error {
-	extension, err := d.r.ReadByte()
+	extension, err := readByte(d.r)
 	if err != nil {
-		return err
+		return fmt.Errorf("gif: reading extension: %v", err)
 	}
 	size := 0
 	switch extension {
@@ -324,9 +340,9 @@ func (d *decoder) readExtension() error {
 	case eComment:
 		// nothing to do but read the data.
 	case eApplication:
-		b, err := d.r.ReadByte()
+		b, err := readByte(d.r)
 		if err != nil {
-			return err
+			return fmt.Errorf("gif: reading extension: %v", err)
 		}
 		// The spec requires size be 11, but Adobe sometimes uses 10.
 		size = int(b)
@@ -334,8 +350,8 @@ func (d *decoder) readExtension() error {
 		return fmt.Errorf("gif: unknown extension 0x%.2x", extension)
 	}
 	if size > 0 {
-		if _, err := io.ReadFull(d.r, d.tmp[:size]); err != nil {
-			return err
+		if err := readFull(d.r, d.tmp[:size]); err != nil {
+			return fmt.Errorf("gif: reading extension: %v", err)
 		}
 	}
 
@@ -343,8 +359,11 @@ func (d *decoder) readExtension() error {
 	// this extension defines a loop count.
 	if extension == eApplication && string(d.tmp[:size]) == "NETSCAPE2.0" {
 		n, err := d.readBlock()
-		if n == 0 || err != nil {
-			return err
+		if err != nil {
+			return fmt.Errorf("gif: reading extension: %v", err)
+		}
+		if n == 0 {
+			return nil
 		}
 		if n == 3 && d.tmp[0] == 1 {
 			d.loopCount = int(d.tmp[1]) | int(d.tmp[2])<<8
@@ -352,14 +371,17 @@ func (d *decoder) readExtension() error {
 	}
 	for {
 		n, err := d.readBlock()
-		if n == 0 || err != nil {
-			return err
+		if err != nil {
+			return fmt.Errorf("gif: reading extension: %v", err)
+		}
+		if n == 0 {
+			return nil
 		}
 	}
 }
 
 func (d *decoder) readGraphicControl() error {
-	if _, err := io.ReadFull(d.r, d.tmp[:6]); err != nil {
+	if err := readFull(d.r, d.tmp[:6]); err != nil {
 		return fmt.Errorf("gif: can't read graphic control: %s", err)
 	}
 	if d.tmp[0] != 4 {
@@ -379,7 +401,7 @@ func (d *decoder) readGraphicControl() error {
 }
 
 func (d *decoder) newImageFromDescriptor() (*image.Paletted, error) {
-	if _, err := io.ReadFull(d.r, d.tmp[:9]); err != nil {
+	if err := readFull(d.r, d.tmp[:9]); err != nil {
 		return nil, fmt.Errorf("gif: can't read image descriptor: %s", err)
 	}
 	left := int(d.tmp[0]) + int(d.tmp[1])<<8
@@ -399,11 +421,14 @@ func (d *decoder) newImageFromDescriptor() (*image.Paletted, error) {
 }
 
 func (d *decoder) readBlock() (int, error) {
-	n, err := d.r.ReadByte()
+	n, err := readByte(d.r)
 	if n == 0 || err != nil {
 		return 0, err
 	}
-	return io.ReadFull(d.r, d.tmp[:n])
+	if err := readFull(d.r, d.tmp[:n]); err != nil {
+		return 0, err
+	}
+	return int(n), nil
 }
 
 // interlaceScan defines the ordering for a pass of the interlace algorithm.
diff --git a/libgo/go/image/gif/reader_test.go b/libgo/go/image/gif/reader_test.go
index 90c81493cba..1267ba06a9d 100644
--- a/libgo/go/image/gif/reader_test.go
+++ b/libgo/go/image/gif/reader_test.go
@@ -10,6 +10,7 @@ import (
 	"image"
 	"image/color"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -292,3 +293,19 @@ func TestLoopCount(t *testing.T) {
 		t.Errorf("loop count mismatch: %d vs %d", img.LoopCount, img1.LoopCount)
 	}
 }
+
+func TestUnexpectedEOF(t *testing.T) {
+	for i := len(testGIF) - 1; i >= 0; i-- {
+		_, err := Decode(bytes.NewReader(testGIF[:i]))
+		if err == errNotEnough {
+			continue
+		}
+		text := ""
+		if err != nil {
+			text = err.Error()
+		}
+		if !strings.HasPrefix(text, "gif:") || !strings.HasSuffix(text, ": unexpected EOF") {
+			t.Errorf("Decode(testGIF[:%d]) = %v, want gif: ...: unexpected EOF", i, err)
+		}
+	}
+}