Update unpacker to always fetch all

When a set of layers are provided to the unpacker, then the unpacker
should still fetch them regardless of whether they will be used for
unpack. The image handler filters are responsible for removing content
which is not intended to be fetched. Currently there is no way to use an
unpacker and also fetch all platforms.

Signed-off-by: Derek McGowan <derek@mcg.dev>

core/unpack/unpacker.go

@@ -262,7 +262,8 @@ func (u *Unpacker) unpack(
 	}
 
 	if unpack == nil {
-		return fmt.Errorf("unpacker does not support platform %s for image %s", imgPlatform, config.Digest)
+		log.G(ctx).WithField("image", config.Digest).WithField("platform", platforms.Format(imgPlatform)).Debugf("unpacker does not support platform, only fetching layers")
+		return u.fetch(ctx, h, layers, nil)
 	}
 
 	atomic.AddInt32(&u.unpacks, 1)
@@ -460,12 +461,18 @@ func (u *Unpacker) fetch(ctx context.Context, h images.Handler, layers []ocispec
 			tracing.Attribute("layer.media.digest", desc.Digest.String()),
 		)
 		desc := desc
-		i := i
+		var ch chan struct{}
+		if done != nil {
+			ch = done[i]
+		}
+
 		if err := u.acquire(ctx); err != nil {
 			return err
 		}
 
 		eg.Go(func() error {
+			defer layerSpan.End()
+
 			unlock, err := u.lockBlobDescriptor(ctx2, desc)
 			if err != nil {
 				u.release()
@@ -480,11 +487,12 @@ func (u *Unpacker) fetch(ctx context.Context, h images.Handler, layers []ocispec
 			if err != nil && !errors.Is(err, images.ErrSkipDesc) {
 				return err
 			}
-			close(done[i])
+			if ch != nil {
+				close(ch)
+			}
 
 			return nil
 		})
-		layerSpan.End()
 	}
 
 	return eg.Wait()