storage, dest: clarify when TOCDigest is used

This update introduces an enhancement in the blob handling mechanism,
specifically by separating the TOC digest from the
uncompressed/compressed digest.

Follow-up for: #1080.

Signed-off-by: Giuseppe Scrivano <[email protected]>

copy/single.go

@@ -696,11 +696,16 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 			originalCompression = srcInfo.CompressionAlgorithm
 		}
 
+		var tocDigest digest.Digest
+
 		// Check if we have a chunked layer in storage that's based on that blob.  These layers are stored by their TOC digest.
-		tocDigest, err := chunkedToc.GetTOCDigest(srcInfo.Annotations)
+		d, err := chunkedToc.GetTOCDigest(srcInfo.Annotations)
 		if err != nil {
 			return types.BlobInfo{}, "", err
 		}
+		if d != nil {
+			tocDigest = *d
+		}
 
 		reused, reusedBlob, err := ic.c.dest.TryReusingBlobWithOptions(ctx, srcInfo, private.TryReusingBlobOptions{
 			Cache:               ic.c.blobInfoCache,
@@ -718,7 +723,12 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 		if reused {
 			logrus.Debugf("Skipping blob %s (already present):", srcInfo.Digest)
 			func() { // A scope for defer
-				bar := ic.c.createProgressBar(pool, false, types.BlobInfo{Digest: reusedBlob.Digest, Size: 0}, "blob", "skipped: already exists")
+				d := reusedBlob.Digest
+				label := "skipped: already exists"
+				if reusedBlob.TOCDigest != "" {
+					label = "skipped: already exists (found by TOC)"
+				}
+				bar := ic.c.createProgressBar(pool, false, types.BlobInfo{Digest: d, Size: 0}, "blob", label)
 				defer bar.Abort(false)
 				bar.mark100PercentComplete()
 			}()

internal/private/private.go

@@ -81,8 +81,9 @@ type ImageDestination interface {
 // UploadedBlob is information about a blob written to a destination.
 // It is the subset of types.BlobInfo fields the transport is responsible for setting; all fields must be provided.
 type UploadedBlob struct {
-	Digest digest.Digest
-	Size   int64
+	Digest    digest.Digest
+	Size      int64
+	TOCDigest digest.Digest
 }
 
 // PutBlobOptions are used in PutBlobWithOptions.
@@ -117,14 +118,15 @@ type TryReusingBlobOptions struct {
 	EmptyLayer          bool                   // True if the blob is an "empty"/"throwaway" layer, and may not necessarily be physically represented.
 	LayerIndex          *int                   // If the blob is a layer, a zero-based index of the layer within the image; nil otherwise.
 	SrcRef              reference.Named        // A reference to the source image that contains the input blob.
-	TOCDigest           *digest.Digest         // If specified, the blob can be looked up in the destination also by its TOC digest.
+	TOCDigest           digest.Digest          // If specified, the blob can be looked up in the destination also by its TOC digest.
 }
 
 // ReusedBlob is information about a blob reused in a destination.
 // It is the subset of types.BlobInfo fields the transport is responsible for setting.
 type ReusedBlob struct {
-	Digest digest.Digest // Must be provided
-	Size   int64         // Must be provided
+	Digest    digest.Digest // Must be provided, can be empty if TOCDigest is present
+	TOCDigest digest.Digest // Must be provided, can be empty if Digest is present
+	Size      int64         // Must be provided
 	// The following compression fields should be set when the reuse substitutes
 	// a differently-compressed blob.
 	CompressionOperation types.LayerCompression // Compress/Decompress, matching the reused blob; PreserveOriginal if N/A