ensure wrappers support seeking to continue partial downloads

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/contentutil/copy.go

@@ -46,6 +46,18 @@ func (r *rc) Read(b []byte) (int, error) {
 	return n, err
 }
 
+func (r *rc) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		r.offset = int(offset)
+	case io.SeekCurrent:
+		r.offset += int(offset)
+	case io.SeekEnd:
+		r.offset = int(r.Size()) - int(offset)
+	}
+	return int64(r.offset), nil
+}
+
 func CopyChain(ctx context.Context, ingester content.Ingester, provider content.Provider, desc ocispec.Descriptor) error {
 	var m sync.Mutex
 	manifestStack := []ocispec.Descriptor{}

util/resolver/limited/group.go

@@ -149,6 +149,7 @@ func (f *fetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCl
 		release()
 		return nil, err
 	}
+
 	rcw := &readCloser{ReadCloser: rc}
 	closer := func() {
 		if !rcw.closed {
@@ -161,9 +162,18 @@ func (f *fetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCl
 		rc.close()
 	})
 
+	if s, ok := rc.(io.Seeker); ok {
+		return &readCloserSeeker{rcw, s}, nil
+	}
+
 	return rcw, nil
 }
 
+type readCloserSeeker struct {
+	*readCloser
+	io.Seeker
+}
+
 type readCloser struct {
 	io.ReadCloser
 	once    sync.Once