Referrers tag index is not updated when all referrers are removed and GC is skipped

[shizhMSFT]

Note

Credits to @qweeah for finding this bug.

Problem statement: Under the mode of referrers tag schema, when all referrers of a manifest are removed the referrers index is not updated if SkipReferrersGC is toggled to true.

Related code:

oras-go/registry/remote/repository.go

Lines 1455 to 1476 in 4a52cfd

	// 3. push the updated referrers list using referrers tag schema
	if len(updatedReferrers) > 0 {
	newIndexDesc, newIndex, err := generateIndex(updatedReferrers)
	if err != nil {
	return fmt.Errorf("failed to generate referrers index for referrers tag %s: %w", referrersTag, err)
	}
	if err := s.push(ctx, newIndexDesc, bytes.NewReader(newIndex), referrersTag); err != nil {
	return fmt.Errorf("failed to push referrers index tagged by %s: %w", referrersTag, err)
	}
	}
	// 4. delete the dangling original referrers index
	if !skipDelete {
	if err := s.repo.delete(ctx, oldIndexDesc, true); err != nil {
	return &ReferrersError{
	Op: opDeleteReferrersIndex,
	Err: fmt.Errorf("failed to delete dangling referrers index %s for referrers tag %s: %w", oldIndexDesc.Digest.String(), referrersTag, err),
	Subject: subject,
	}
	}
	}
	return nil

If there are no referrers (i.e. len(updatedReferrers) == 0), we should remove the referrers index if GC is enabled as the above code does.

However, when GC is disabled / skipped, we need to keep the previous index. In this case, a new index with no manifest should be generated and pushed. Note that the image-spec allows the index to have zero manifests.