Add snapshot plugin type

Update existing snapshot drivers to register as plugins.
Load snapshot driver at containerd startup.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

cmd/containerd/builtins.go

@@ -5,4 +5,6 @@ import (
 	_ "github.com/docker/containerd/linux"
 	_ "github.com/docker/containerd/services/content"
 	_ "github.com/docker/containerd/services/execution"
+	_ "github.com/docker/containerd/snapshot/btrfs"
+	_ "github.com/docker/containerd/snapshot/overlay"
 )

cmd/containerd/config.go

@@ -13,6 +13,7 @@ func defaultConfig() *config {
 			Level:  "info",
 			Socket: "/run/containerd/debug.sock",
 		},
+		Snapshotter: "overlay",
 	}
 }
 
@@ -39,6 +40,8 @@ type config struct {
 	Debug debug `toml:"debug"`
 	// Metrics and monitoring settings
 	Metrics metricsConfig `toml:"metrics"`
+	// Snapshotter specifies which snapshot driver to use
+	Snapshotter string `toml:"snapshotter"`
 	// Plugins provides plugin specific configuration for the initialization of a plugin
 	Plugins map[string]toml.Primitive `toml:"plugins"`
 

cmd/containerd/main.go

@@ -21,6 +21,7 @@ import (
 	"github.com/docker/containerd/content"
 	"github.com/docker/containerd/log"
 	"github.com/docker/containerd/plugin"
+	"github.com/docker/containerd/snapshot"
 	"github.com/docker/containerd/utils"
 	metrics "github.com/docker/go-metrics"
 	"github.com/pkg/errors"
@@ -101,7 +102,11 @@ func main() {
 		if err != nil {
 			return err
 		}
-		services, err := loadServices(runtimes, store)
+		snapshotter, err := loadSnapshotter(store)
+		if err != nil {
+			return err
+		}
+		services, err := loadServices(runtimes, store, snapshotter)
 		if err != nil {
 			return err
 		}
@@ -248,12 +253,45 @@ func loadRuntimes() (map[string]containerd.Runtime, error) {
 	return o, nil
 }
 
+func loadSnapshotter(store *content.Store) (snapshot.Snapshotter, error) {
+	for name, sr := range plugin.Registrations() {
+		if sr.Type != plugin.SnapshotPlugin {
+			continue
+		}
+		moduleName := fmt.Sprintf("snapshot-%s", conf.Snapshotter)
+		if name != moduleName {
+			continue
+		}
+
+		log.G(global).Infof("loading snapshot plugin %q...", name)
+		ic := &plugin.InitContext{
+			Root:    conf.Root,
+			State:   conf.State,
+			Store:   store,
+			Context: log.WithModule(global, moduleName),
+		}
+		if sr.Config != nil {
+			if err := conf.decodePlugin(name, sr.Config); err != nil {
+				return nil, err
+			}
+			ic.Config = sr.Config
+		}
+		sn, err := sr.Init(ic)
+		if err != nil {
+			return nil, err
+		}
+
+		return sn.(snapshot.Snapshotter), nil
+	}
+	return nil, fmt.Errorf("snapshotter not loaded: %v", conf.Snapshotter)
+}
+
 func newGRPCServer() *grpc.Server {
 	s := grpc.NewServer(grpc.UnaryInterceptor(interceptor))
 	return s
 }
 
-func loadServices(runtimes map[string]containerd.Runtime, store *content.Store) ([]plugin.Service, error) {
+func loadServices(runtimes map[string]containerd.Runtime, store *content.Store, sn snapshot.Snapshotter) ([]plugin.Service, error) {
 	var o []plugin.Service
 	for name, sr := range plugin.Registrations() {
 		if sr.Type != plugin.GRPCPlugin {
@@ -261,11 +299,12 @@ func loadServices(runtimes map[string]containerd.Runtime, store *content.Store)
 		}
 		log.G(global).Infof("loading grpc service plugin %q...", name)
 		ic := &plugin.InitContext{
-			Root:     conf.Root,
-			State:    conf.State,
-			Context:  log.WithModule(global, fmt.Sprintf("service-%s", name)),
-			Runtimes: runtimes,
-			Store:    store,
+			Root:        conf.Root,
+			State:       conf.State,
+			Context:     log.WithModule(global, fmt.Sprintf("service-%s", name)),
+			Runtimes:    runtimes,
+			Store:       store,
+			Snapshotter: sn,
 		}
 		if sr.Config != nil {
 			if err := conf.decodePlugin(name, sr.Config); err != nil {