remotes: add distribution labels to blob data

We can use cross repository push feature to reuse the existing blobs in
the same registry. Before make push fast, we know where the blob comes
from.

Use the `containerd.io/distribution.source. = [,]` as label format. For
example, the blob is downloaded by the docker.io/library/busybox:latest
and the label will be

    containerd.io/distribution.source.docker.io = library/busybox

If the blob is shared by different repos in the same registry, the repo
name will be appended, like:

    containerd.io/distribution.source.docker.io = library/busybox,x/y

NOTE:
1. no need to apply for legacy docker image schema1.
2. the concurrent fetch actions might miss some repo names in label, but
it is ok.
3. it is optional. no need to add label if the engine only uses images
not push.

Signed-off-by: Wei Fu <fuweid89@gmail.com>

pull.go

@@ -112,8 +112,9 @@ func (c *Client) fetch(ctx context.Context, rCtx *RemoteContext, ref string, lim
 		childrenHandler := images.ChildrenHandler(store)
 		// Set any children labels for that content
 		childrenHandler = images.SetChildrenLabels(store, childrenHandler)
-		// Filter children by platforms
-		childrenHandler = images.FilterPlatforms(childrenHandler, rCtx.PlatformMatcher)
+		// Filter manifests by platforms but allow to handle manifest
+		// and configuration for not-target platforms
+		childrenHandler = remotes.FilterManifestByPlatformHandler(childrenHandler, rCtx.PlatformMatcher)
 		// Sort and limit manifests if a finite number is needed
 		if limit > 0 {
 			childrenHandler = images.LimitManifests(childrenHandler, rCtx.PlatformMatcher, limit)
@@ -130,11 +131,23 @@ func (c *Client) fetch(ctx context.Context, rCtx *RemoteContext, ref string, lim
 			},
 		)
 
-		handler = images.Handlers(append(rCtx.BaseHandlers,
+		handlers := append(rCtx.BaseHandlers,
 			remotes.FetchHandler(store, fetcher),
 			convertibleHandler,
 			childrenHandler,
-		)...)
+		)
+
+		// append distribution source label to blob data
+		if rCtx.AppendDistributionSourceLabel {
+			appendDistSrcLabelHandler, err := docker.AppendDistributionSourceLabel(store, ref)
+			if err != nil {
+				return images.Image{}, err
+			}
+
+			handlers = append(handlers, appendDistSrcLabelHandler)
+		}
+
+		handler = images.Handlers(handlers...)
 
 		converterFunc = func(ctx context.Context, desc ocispec.Descriptor) (ocispec.Descriptor, error) {
 			return docker.ConvertManifest(ctx, store, desc)
@@ -148,6 +161,7 @@ func (c *Client) fetch(ctx context.Context, rCtx *RemoteContext, ref string, lim
 	if rCtx.MaxConcurrentDownloads > 0 {
 		limiter = semaphore.NewWeighted(int64(rCtx.MaxConcurrentDownloads))
 	}
+
 	if err := images.Dispatch(ctx, handler, limiter, desc); err != nil {
 		return images.Image{}, err
 	}