fix: correctly handle OnCopySkipped (#609)

Fix: #552
Signed-off-by: Lixia (Sylvia) Lei <lixlei@microsoft.com>

copy.go

@@ -393,18 +393,26 @@ func prepareCopy(ctx context.Context, dst Target, dstRef string, proxy *cas.Prox
 
 	onCopySkipped := opts.OnCopySkipped
 	opts.OnCopySkipped = func(ctx context.Context, desc ocispec.Descriptor) error {
-		if onCopySkipped != nil {
-			if err := onCopySkipped(ctx, desc); err != nil {
-				return err
-			}
-		}
 		if !content.Equal(desc, root) {
+			if onCopySkipped != nil {
+				return onCopySkipped(ctx, desc)
+			}
 			return nil
 		}
-		// enforce tagging when root is skipped
+
+		// enforce tagging when the skipped node is root
 		if refPusher, ok := dst.(registry.ReferencePusher); ok {
+			// NOTE: refPusher tags the node by copying it with the reference,
+			// so onCopySkipped shouldn't be invoked in this case
 			return copyCachedNodeWithReference(ctx, proxy, refPusher, desc, dstRef)
 		}
+
+		// invoke onCopySkipped before tagging
+		if onCopySkipped != nil {
+			if err := onCopySkipped(ctx, desc); err != nil {
+				return err
+			}
+		}
 		return dst.Tag(ctx, root, dstRef)
 	}