Allow dockerImageDestination to reuse zstd:chunked blobs

- Return the required annotations, if we have them
- If we have a zstd blob and the BIC contains the annotations,
  we don't check for the blob's presence initially. In that case,
  don't skip it if we find the BIC annotations.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

docker/docker_image_dest.go

@@ -332,6 +332,7 @@ func (d *dockerImageDestination) TryReusingBlobWithOptions(ctx context.Context,
 		return false, private.ReusedBlob{}, errors.New("Can not check for a blob with unknown digest")
 	}
 
+	originalCandidateKnownToBeMissing := false
 	if impl.OriginalCandidateMatchesTryReusingBlobOptions(options) {
 		// First, check whether the blob happens to already exist at the destination.
 		haveBlob, reusedInfo, err := d.tryReusingExactBlob(ctx, info, options.Cache)
@@ -341,9 +342,17 @@ func (d *dockerImageDestination) TryReusingBlobWithOptions(ctx context.Context,
 		if haveBlob {
 			return true, reusedInfo, nil
 		}
+		originalCandidateKnownToBeMissing = true
 	} else {
 		logrus.Debugf("Ignoring exact blob match, compression %s does not match required %s or MIME types %#v",
 			optionalCompressionName(options.OriginalCompression), optionalCompressionName(options.RequiredCompression), options.PossibleManifestFormats)
+		// We can get here with a blob detected to be zstd when the user wants a zstd:chunked.
+		// In that case we keep originalCandiateKnownToBeMissing = false, so that if we find
+		// a BIC entry for this blob, we do use that entry and return a zstd:chunked entry
+		// with the BIC’s annotations.
+		// This is not quite correct, it only works if the BIC also contains an acceptable _location_.
+		// Ideally, we could look up just the compression algorithm/annotations for info.digest,
+		// and use it even if no location candidate exists and the original dandidate is present.
 	}
 
 	// Then try reusing blobs from other locations.
@@ -387,7 +396,8 @@ func (d *dockerImageDestination) TryReusingBlobWithOptions(ctx context.Context,
 			// for it in the current repo.
 			candidateRepo = reference.TrimNamed(d.ref.ref)
 		}
-		if candidateRepo.Name() == d.ref.ref.Name() && candidate.Digest == info.Digest {
+		if originalCandidateKnownToBeMissing &&
+			candidateRepo.Name() == d.ref.ref.Name() && candidate.Digest == info.Digest {
 			logrus.Debug("... Already tried the primary destination")
 			continue
 		}
@@ -427,10 +437,12 @@ func (d *dockerImageDestination) TryReusingBlobWithOptions(ctx context.Context,
 		options.Cache.RecordKnownLocation(d.ref.Transport(), bicTransportScope(d.ref), candidate.Digest, newBICLocationReference(d.ref))
 
 		return true, private.ReusedBlob{
-			Digest:               candidate.Digest,
-			Size:                 size,
-			CompressionOperation: candidate.CompressionOperation,
-			CompressionAlgorithm: candidate.CompressionAlgorithm}, nil
+			Digest:                 candidate.Digest,
+			Size:                   size,
+			CompressionOperation:   candidate.CompressionOperation,
+			CompressionAlgorithm:   candidate.CompressionAlgorithm,
+			CompressionAnnotations: candidate.CompressionAnnotations,
+		}, nil
 	}
 
 	return false, private.ReusedBlob{}, nil