copy: compute blob compression on reused blobs based on source MediaType

When copying blobs, if we end up reusing a blob from the destination
with the same digest as that given in the srcInfo, but don't know how
that reused blob was compressed, compute the compression information
from the MediaType included in the srcInfo, if one was supplied.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

copy/copy.go

@@ -1069,6 +1069,19 @@ type diffIDResult struct {
 // copyLayer copies a layer with srcInfo (with known Digest and Annotations and possibly known Size) in src to dest, perhaps (de/re/)compressing it,
 // and returns a complete blobInfo of the copied layer, and a value for LayerDiffIDs if diffIDIsNeeded
 func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, toEncrypt bool, pool *mpb.Progress) (types.BlobInfo, digest.Digest, error) {
+	// If the srcInfo doesn't contain compression information, try to compute it from the
+	// MediaType, which was usually read from a manifest by way of LayerInfos(), so that
+	// we can use the information to ensure that the eventual call to UpdatedImage() that
+	// uses the blobInfo we return can correctly compute updated MediaType values.
+	if srcInfo.CompressionAlgorithm == nil {
+		switch srcInfo.MediaType {
+		case manifest.DockerV2Schema2LayerMediaType, imgspecv1.MediaTypeImageLayerGzip:
+			srcInfo.CompressionAlgorithm = &compression.Gzip
+		case imgspecv1.MediaTypeImageLayerZstd:
+			srcInfo.CompressionAlgorithm = &compression.Zstd
+		}
+	}
+
 	cachedDiffID := ic.c.blobInfoCache.UncompressedDigest(srcInfo.Digest) // May be ""
 	// Diffs are needed if we are encrypting an image or trying to decrypt an image
 	diffIDIsNeeded := ic.diffIDsAreNeeded && cachedDiffID == "" || toEncrypt || (isOciEncrypted(srcInfo.MediaType) && ic.c.ociDecryptConfig != nil)
@@ -1097,6 +1110,14 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 					Artifact: srcInfo,
 				}
 			}
+
+			// If the reused blob has the same digest as the one we asked for, but
+			// the transport didn't/couldn't supply compression info, fill it in based
+			// on what we know from the srcInfo.
+			if blobInfo.Digest == srcInfo.Digest && blobInfo.CompressionAlgorithm == nil {
+				blobInfo.CompressionOperation = srcInfo.CompressionOperation
+				blobInfo.CompressionAlgorithm = srcInfo.CompressionAlgorithm
+			}
 			return blobInfo, cachedDiffID, nil
 		}
 	}