remotecache: fix missing CheckDescriptor method

This method is needed by GHA cache backend to detect
if Github deletes blobs from the cache because of
storage caps. It got missing when Info() support was
added.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

cache/remotecache/v1/chains.go

@@ -122,6 +122,8 @@ type DescriptorProviderPair struct {
 	Provider   content.Provider
 }
 
+var _ withCheckDescriptor = DescriptorProviderPair{}
+
 func (p DescriptorProviderPair) ReaderAt(ctx context.Context, desc ocispecs.Descriptor) (content.ReaderAt, error) {
 	return p.Provider.ReaderAt(ctx, desc)
 }
@@ -156,6 +158,13 @@ func (p DescriptorProviderPair) SnapshotLabels(descs []ocispecs.Descriptor, inde
 	return nil
 }
 
+func (p DescriptorProviderPair) CheckDescriptor(ctx context.Context, desc ocispecs.Descriptor) error {
+	if cd, ok := p.Provider.(withCheckDescriptor); ok {
+		return cd.CheckDescriptor(ctx, desc)
+	}
+	return nil
+}
+
 // item is an implementation of a record in the cache chain. After validation,
 // normalization and marshalling into the cache config, the item results form
 // into the "layers", while the digests and the links form into the "records".

cache/remotecache/v1/utils.go

@@ -12,6 +12,11 @@ import (
 	"github.com/pkg/errors"
 )
 
+type withCheckDescriptor interface {
+	// CheckDescriptor is additional method on Provider to check if the descriptor is available without opening the reader
+	CheckDescriptor(context.Context, ocispecs.Descriptor) error
+}
+
 // sortConfig sorts the config structure to make sure it is deterministic
 func sortConfig(cc *CacheConfig) {
 	type indexedLayer struct {
@@ -279,9 +284,7 @@ func marshalRemote(ctx context.Context, r *solver.Remote, state *marshalState) s
 		return ""
 	}
 
-	if cd, ok := r.Provider.(interface {
-		CheckDescriptor(context.Context, ocispecs.Descriptor) error
-	}); ok && len(r.Descriptors) > 0 {
+	if cd, ok := r.Provider.(withCheckDescriptor); ok && len(r.Descriptors) > 0 {
 		for _, d := range r.Descriptors {
 			if cd.CheckDescriptor(ctx, d) != nil {
 				return ""