Skip ID3v2 data prepended to flac files on parsing (#21)

* Change signature to flacSignature

In order to disambiguate the introduction of the ID3v2 signature.

* Add check flac files containing ID3v2 data

* Implement skipId3v2

Note: a new import for decoding synchronized integers from ID3v2 headers
was introduced.

* Add error checking on the r.Discard calls

* Fix comments

* Use limited scope for error handling

* Capitalize ID in skipID3v2

* Fix comments

* Add testcase for skipping id3 data

enc.go

@@ -24,7 +24,7 @@ func Encode(w io.Writer, stream *Stream) error {
 	enc := &encoder{bw: bitio.NewWriter(buf)}
 
 	// Store FLAC signature.
-	if _, err := enc.bw.Write(signature); err != nil {
+	if _, err := enc.bw.Write(flacSignature); err != nil {
 		return errutil.Err(err)
 	}
 

flac.go

@@ -30,6 +30,8 @@ import (
 
 	"github.com/mewkiz/flac/frame"
 	"github.com/mewkiz/flac/meta"
+
+	"github.com/mikkyang/id3-go/encodedbytes"
 )
 
 // A Stream contains the metadata blocks and provides access to the audio frames
@@ -79,8 +81,11 @@ func New(r io.Reader) (stream *Stream, err error) {
 	return stream, nil
 }
 
-// signature marks the beginning of a FLAC stream.
-var signature = []byte("fLaC")
+// flacSignature marks the beginning of a FLAC stream.
+var flacSignature = []byte("fLaC")
+
+// id3Signature marks the beginning of an ID3 stream, used to skip over ID3 data.
+var id3Signature = []byte("ID3")
 
 // parseStreamInfo verifies the signature which marks the beginning of a FLAC
 // stream, and parses the StreamInfo metadata block. It returns a boolean value
@@ -90,12 +95,24 @@ func (stream *Stream) parseStreamInfo() (isLast bool, err error) {
 	// Verify FLAC signature.
 	r := stream.r
 	var buf [4]byte
-	_, err = io.ReadFull(r, buf[:])
-	if err != nil {
+	if _, err = io.ReadFull(r, buf[:]); err != nil {
 		return false, err
 	}
-	if !bytes.Equal(buf[:], signature) {
-		return false, fmt.Errorf("flac.parseStreamInfo: invalid FLAC signature; expected %q, got %q", signature, buf)
+
+	// Skip prepended ID3v2 data.
+	if bytes.Equal(buf[:3], id3Signature) {
+		if err := stream.skipID3v2(); err != nil {
+			return false, err
+		}
+
+		// Second attempt at verifying signature.
+		if _, err = io.ReadFull(r, buf[:]); err != nil {
+			return false, err
+		}
+	}
+
+	if !bytes.Equal(buf[:], flacSignature) {
+		return false, fmt.Errorf("flac.parseStreamInfo: invalid FLAC signature; expected %q, got %q", flacSignature, buf)
 	}
 
 	// Parse StreamInfo metadata block.
@@ -111,6 +128,31 @@ func (stream *Stream) parseStreamInfo() (isLast bool, err error) {
 	return block.IsLast, nil
 }
 
+// skipId3v2 skips ID3v2 data prepended to flac files.
+func (stream *Stream) skipID3v2() error {
+	r := bufio.NewReader(stream.r)
+
+	// Discard unnecessary data from the ID3v2 header.
+	if _, err := r.Discard(2); err != nil {
+		return err
+	}
+
+	// Read the size from the ID3v2 header.
+	var sizeBuf [4]byte
+	if _, err := r.Read(sizeBuf[:]); err != nil {
+		return err
+	}
+
+	// The size is encoded as a synchsafe integer.
+	size, err := encodedbytes.SynchInt(sizeBuf[:])
+	if err != nil {
+		return err
+	}
+
+	_, err = r.Discard(int(size))
+	return err
+}
+
 // Parse creates a new Stream for accessing the metadata blocks and audio
 // samples of r. It reads and parses the FLAC signature and all metadata blocks.
 //

flac_test.go

@@ -0,0 +1,13 @@
+package flac_test
+
+import (
+	"testing"
+
+	"github.com/mewkiz/flac"
+)
+
+func TestSkipID3v2(t *testing.T) {
+	if _, err := flac.ParseFile("testdata/id3.flac"); err != nil {
+		t.Fatal(err)
+	}
+}