Fix object not found error in OFS deltas

Unfortunately the code in go-git#303 was incorrect for large objects in OFS deltas.

The underlying fault is that the seeker in the packfile was seeking away whilst the
delta was being read.

This PR simply reads the delta into a buffer.

Fix go-git#323

Signed-off-by: Andrew Thornton <art27@cantab.net>

plumbing/format/packfile/packfile.go

@@ -293,24 +293,28 @@ func (p *Packfile) getReaderDirect(h *ObjectHeader) (io.ReadCloser, error) {
 	case plumbing.CommitObject, plumbing.TreeObject, plumbing.BlobObject, plumbing.TagObject:
 		return p.s.ReadObject()
 	case plumbing.REFDeltaObject:
-		deltaRC, err := p.s.ReadObject()
+		deltaBuf := bufPool.Get().(*bytes.Buffer)
+		defer bufPool.Put(deltaBuf)
+		deltaBuf.Reset()
+		_, _, err := p.s.NextObject(deltaBuf)
 		if err != nil {
 			return nil, err
 		}
-		r, err := p.readREFDeltaObjectContent(h, deltaRC)
+		r, err := p.readREFDeltaObjectContent(h, deltaBuf)
 		if err != nil {
-			_ = deltaRC.Close()
 			return nil, err
 		}
 		return r, nil
 	case plumbing.OFSDeltaObject:
-		deltaRC, err := p.s.ReadObject()
+		deltaBuf := bufPool.Get().(*bytes.Buffer)
+		defer bufPool.Put(deltaBuf)
+		deltaBuf.Reset()
+		_, _, err := p.s.NextObject(deltaBuf)
 		if err != nil {
 			return nil, err
 		}
-		r, err := p.readOFSDeltaObjectContent(h, deltaRC)
+		r, err := p.readOFSDeltaObjectContent(h, deltaBuf)
 		if err != nil {
-			_ = deltaRC.Close()
 			return nil, err
 		}
 		return r, nil
@@ -371,7 +375,7 @@ func (p *Packfile) fillREFDeltaObjectContent(obj plumbing.EncodedObject, ref plu
 	return p.fillREFDeltaObjectContentWithBuffer(obj, ref, buf)
 }
 
-func (p *Packfile) readREFDeltaObjectContent(h *ObjectHeader, deltaRC io.ReadCloser) (io.ReadCloser, error) {
+func (p *Packfile) readREFDeltaObjectContent(h *ObjectHeader, deltaRC io.Reader) (io.ReadCloser, error) {
 	var err error
 
 	base, ok := p.cacheGet(h.Reference)
@@ -415,7 +419,7 @@ func (p *Packfile) fillOFSDeltaObjectContent(obj plumbing.EncodedObject, offset
 	return p.fillOFSDeltaObjectContentWithBuffer(obj, offset, buf)
 }
 
-func (p *Packfile) readOFSDeltaObjectContent(h *ObjectHeader, deltaRC io.ReadCloser) (io.ReadCloser, error) {
+func (p *Packfile) readOFSDeltaObjectContent(h *ObjectHeader, deltaRC io.Reader) (io.ReadCloser, error) {
 	hash, err := p.FindHash(h.OffsetReference)
 	if err != nil {
 		return nil, err
@@ -426,14 +430,6 @@ func (p *Packfile) readOFSDeltaObjectContent(h *ObjectHeader, deltaRC io.ReadClo
 		return nil, err
 	}
 
-	base, ok := p.cacheGet(h.Reference)
-	if !ok {
-		base, err = p.Get(h.Reference)
-		if err != nil {
-			return nil, err
-		}
-	}
-
 	return ReaderFromDelta(h, base, deltaRC)
 }
 

plumbing/format/packfile/patch_delta.go

@@ -75,7 +75,7 @@ func PatchDelta(src, delta []byte) ([]byte, error) {
 	return b.Bytes(), nil
 }
 
-func ReaderFromDelta(h *ObjectHeader, base plumbing.EncodedObject, deltaRC io.ReadCloser) (io.ReadCloser, error) {
+func ReaderFromDelta(h *ObjectHeader, base plumbing.EncodedObject, deltaRC io.Reader) (io.ReadCloser, error) {
 	deltaBuf := bufio.NewReaderSize(deltaRC, 1024)
 	srcSz, err := decodeLEB128ByteReader(deltaBuf)
 	if err != nil {