Separate overlay implementation from plugin

Put the overlay plugin in a separate package to allow the overlay package to be
used without needing to import and initialize the plugin.

Signed-off-by: Derek McGowan <derek@mcg.dev>

snapshots/overlay/plugin/plugin.go

@@ -0,0 +1,57 @@
+// +build linux
+
+/*
+   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 overlay
+
+import (
+	"errors"
+
+	"github.com/containerd/containerd/platforms"
+	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/snapshots/overlay"
+)
+
+// Config represents configuration for the overlay plugin.
+type Config struct {
+	// Root directory for the plugin
+	RootPath string `toml:"root_path"`
+}
+
+func init() {
+	plugin.Register(&plugin.Registration{
+		Type:   plugin.SnapshotPlugin,
+		ID:     "overlayfs",
+		Config: &Config{},
+		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
+			ic.Meta.Platforms = append(ic.Meta.Platforms, platforms.DefaultSpec())
+
+			config, ok := ic.Config.(*Config)
+			if !ok {
+				return nil, errors.New("invalid overlay configuration")
+			}
+
+			root := ic.Root
+			if config.RootPath != "" {
+				root = config.RootPath
+			}
+
+			ic.Meta.Exports["root"] = root
+			return overlay.NewSnapshotter(root, overlay.AsynchronousRemove)
+		},
+	})
+}