feat: added blob mounting support

Implements a WithMount method on CopyGraphOptions

Also allows for getContent to return ErrUnsupported to fall back to default behavior.

Signed-off-by: Kyle M. Tarplee <[email protected]>

copy.go

@@ -38,6 +38,7 @@ import (
 const defaultConcurrency int = 3 // This value is consistent with dockerd and containerd.
 
 // ErrSkipDesc signal to stop copying a descriptor.  When returned from PreCopy the blob must exist in the target.
+// This can be used to signal that a blob has been "Mounted" into the target repository.
 var ErrSkipDesc = errors.New("skip descriptor")
 
 // DefaultCopyOptions provides the default CopyOptions.
@@ -111,6 +112,50 @@ type CopyGraphOptions struct {
 	FindSuccessors func(ctx context.Context, fetcher content.Fetcher, desc ocispec.Descriptor) ([]ocispec.Descriptor, error)
 }
 
+// WithMount enabled cross repository blob mounting.
+// sourceReference is the repository to use for mounting (the mount point).
+// mounter is the destination for the mount (a well-known implementation of this is *registry.Repository representing the target).
+// onMounted is called (if provided) when the blob is mounted.
+// The original PreCopy hook is called only on copy, and there fore not when the blob is mounted.
+func (opts *CopyGraphOptions) WithMount(sourceRepository string, mounter registry.Mounter, onMounted func(context.Context, ocispec.Descriptor)) {
+	preCopy := opts.PreCopy
+	opts.PreCopy = func(ctx context.Context, desc ocispec.Descriptor) error {
+		// Only care to mount blobs
+		if descriptor.IsManifest(desc) {
+			return preCopy(ctx, desc)
+		}
+
+		var mountFailed bool
+		getContent := func() (io.ReadCloser, error) {
+			// call the original PreCopy function if it exists
+			if preCopy != nil {
+				if err := preCopy(ctx, desc); err != nil {
+					return nil, err
+				}
+			}
+			// the invocation of getContent indicates that mounting has failed
+			mountFailed = true
+
+			// To avoid needing a content.Fetcher as an input argument we simply fall back to the default behavior
+			// as if getContent was nil
+			return nil, errdef.ErrUnsupported
+		}
+
+		// Mount or copy
+		if err := mounter.Mount(ctx, desc, sourceRepository, getContent); err != nil {
+			return err
+		}
+
+		if !mountFailed && onMounted != nil {
+			onMounted(ctx, desc)
+		}
+
+		// Mount succeeded or we copied it
+		// either way we return ErrSkipDesc to signal that the descriptor now exists
+		return ErrSkipDesc
+	}
+}
+
 // Copy copies a rooted directed acyclic graph (DAG) with the tagged root node
 // in the source Target to the destination Target.
 // The destination reference will be the same as the source reference if the

registry/remote/repository.go

@@ -797,6 +797,10 @@ func (s *blobStore) Mount(ctx context.Context, desc ocispec.Descriptor, fromRepo
 	var r io.ReadCloser
 	if getContent != nil {
 		r, err = getContent()
+		if errors.Is(err, errdef.ErrUnsupported) {
+			// getContent can return a ErrUnsupported to fallback to the default copy operation
+			r, err = s.sibling(fromRepo).Fetch(ctx, desc)
+		}
 	} else {
 		r, err = s.sibling(fromRepo).Fetch(ctx, desc)
 	}