Add a default differ that matches the snapshotter

Signed-off-by: James Sturtevant <jsturtevant@gmail.com>

plugins/transfer/plugin.go

@@ -22,7 +22,6 @@ import (
 	"github.com/containerd/containerd/v2/core/diff"
 	"github.com/containerd/containerd/v2/core/leases"
 	"github.com/containerd/containerd/v2/core/metadata"
-	"github.com/containerd/containerd/v2/defaults"
 	"github.com/containerd/containerd/v2/pkg/imageverifier"
 	"github.com/containerd/containerd/v2/pkg/transfer/local"
 	"github.com/containerd/containerd/v2/pkg/unpack"
@@ -81,12 +80,7 @@ func init() {
 			// If UnpackConfiguration is not defined, set the default.
 			// If UnpackConfiguration is defined and empty, ignore.
 			if config.UnpackConfiguration == nil {
-				config.UnpackConfiguration = []unpackConfiguration{
-					{
-						Platform:    platforms.Format(platforms.DefaultSpec()),
-						Snapshotter: defaults.DefaultSnapshotter,
-					},
-				}
+				config.UnpackConfiguration = defaultUnpackConfig()
 			}
 			for _, uc := range config.UnpackConfiguration {
 				p, err := platforms.Parse(uc.Platform)
@@ -122,7 +116,7 @@ func init() {
 							continue
 						}
 						if applier != nil {
-							log.G(ic.Context).Warnf("multiple differs match for platform, set `differ` option to choose, skipping %q", name)
+							log.G(ic.Context).Warnf("multiple differs match for platform, set `differ` option to choose, skipping %q", plugin.Registration.ID)
 							continue
 						}
 						inst, err := plugin.Instance()

plugins/transfer/plugin_defaults_other.go

@@ -0,0 +1,33 @@
+//go:build !windows
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package transfer
+
+import (
+	"github.com/containerd/containerd/v2/defaults"
+	"github.com/containerd/platforms"
+)
+
+func defaultUnpackConfig() []unpackConfiguration {
+	return []unpackConfiguration{
+		{
+			Platform:    platforms.Format(platforms.DefaultSpec()),
+			Snapshotter: defaults.DefaultSnapshotter,
+		},
+	}
+}

plugins/transfer/plugin_defaults_windows.go

@@ -0,0 +1,32 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package transfer
+
+import (
+	"github.com/containerd/containerd/v2/defaults"
+	"github.com/containerd/platforms"
+)
+
+func defaultUnpackConfig() []unpackConfiguration {
+	return []unpackConfiguration{
+		{
+			Platform:    platforms.Format(platforms.DefaultSpec()),
+			Snapshotter: defaults.DefaultSnapshotter,
+			Differ:      defaults.DefaultDiffer,
+		},
+	}
+}