Merge pull request #11936 from k8s-infra-cherrypick-robot/cherry-pick-11915-to-release/2.1

[release/2.1] Fix transfer differ selection

core/unpack/unpacker.go

@@ -428,7 +428,7 @@ func (u *Unpacker) unpack(
 		diff, err := a.Apply(ctx, desc, mounts, unpack.ApplyOpts...)
 		if err != nil {
 			cleanup.Do(ctx, abort)
-			return fmt.Errorf("failed to extract layer %s: %w", diffIDs[i], err)
+			return fmt.Errorf("failed to extract layer (%s %s) to %s as %q: %w", desc.MediaType, desc.Digest, unpack.SnapshotterKey, key, err)
 		}
 		if diff.Digest != diffIDs[i] {
 			cleanup.Do(ctx, abort)

plugins/services/transfer/service.go

@@ -138,6 +138,7 @@ func (s *service) Transfer(ctx context.Context, req *transferapi.TransferRequest
 		} else if !errdefs.IsNotImplemented(err) {
 			return nil, errgrpc.ToGRPC(err)
 		}
+		log.G(ctx).WithError(err).Debugf("transfer not implemented for %T to %T", src, dst)
 	}
 	return nil, status.Errorf(codes.Unimplemented, "method Transfer not implemented for %s to %s", req.Source.GetTypeUrl(), req.Destination.GetTypeUrl())
 }

plugins/transfer/plugin.go

@@ -30,6 +30,7 @@ import (
 	"github.com/containerd/containerd/v2/core/metadata"
 	"github.com/containerd/containerd/v2/core/transfer/local"
 	"github.com/containerd/containerd/v2/core/unpack"
+	"github.com/containerd/containerd/v2/defaults"
 	"github.com/containerd/containerd/v2/internal/kmutex"
 	"github.com/containerd/containerd/v2/pkg/imageverifier"
 	"github.com/containerd/containerd/v2/plugins"
@@ -115,6 +116,7 @@ func init() {
 					}
 					applier = inst.(diff.Applier)
 				} else {
+					var applierID string
 					for name, plugin := range ic.GetAll() {
 						if plugin.Registration.Type != plugins.DiffPlugin {
 							continue
@@ -129,14 +131,25 @@ func init() {
 							continue
 						}
 						if applier != nil {
-							log.G(ic.Context).Warnf("multiple differs match for platform, set `differ` option to choose, skipping %q", plugin.Registration.ID)
-							continue
+							skippedApplier := plugin.Registration.ID
+
+							// Prefer the default when multiple plugins match
+							if skippedApplier == defaults.DefaultDiffer {
+								skippedApplier = applierID
+							}
+
+							log.G(ic.Context).Warnf("multiple differs match for platform, set `differ` option to choose, skipping %q", skippedApplier)
+
+							if plugin.Registration.ID == skippedApplier {
+								continue
+							}
 						}
 						inst, err := plugin.Instance()
 						if err != nil {
 							return nil, fmt.Errorf("failed to get instance for diff plugin %q: %w", name, err)
 						}
 						applier = inst.(diff.Applier)
+						applierID = plugin.Registration.ID
 					}
 				}
 				if applier == nil {