service/snapshotter: move default to client

In order to enforce strict handling of snapshotter values on the
container object, the defaults have been moved to the client side. This
ensures that we correctly qualify the snapshotter under use when from
the container at the time it was created, rather than possibly losing
the metadata on a change of default.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

services/snapshot/default_linux.go

@@ -1,5 +0,0 @@
-package snapshot
-
-const (
-	defaultSnapshotter = "overlayfs"
-)

services/snapshot/default_unix.go

@@ -1,7 +0,0 @@
-// +build darwin freebsd solaris
-
-package snapshot
-
-const (
-	defaultSnapshotter = "naive"
-)

services/snapshot/default_windows.go

@@ -1,5 +0,0 @@
-package snapshot
-
-const (
-	defaultSnapshotter = "windows"
-)

services/snapshot/service.go

@@ -20,11 +20,6 @@ import (
 	"google.golang.org/grpc"
 )
 
-type config struct {
-	// Default is the default snapshotter to use for the service
-	Default string `toml:"default,omitempty"`
-}
-
 func init() {
 	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
@@ -33,9 +28,6 @@ func init() {
 			plugin.SnapshotPlugin,
 			plugin.MetadataPlugin,
 		},
-		Config: &config{
-			Default: defaultSnapshotter,
-		},
 		Init: newService,
 	})
 }
@@ -43,9 +35,8 @@ func init() {
 var empty = &protoempty.Empty{}
 
 type service struct {
-	snapshotters           map[string]snapshot.Snapshotter
-	defaultSnapshotterName string
-	publisher              events.Publisher
+	snapshotters map[string]snapshot.Snapshotter
+	publisher    events.Publisher
 }
 
 func newService(ic *plugin.InitContext) (interface{}, error) {
@@ -62,26 +53,24 @@ func newService(ic *plugin.InitContext) (interface{}, error) {
 		snapshotters[name] = metadata.NewSnapshotter(md.(*bolt.DB), name, sn.(snapshot.Snapshotter))
 	}
 
-	cfg := ic.Config.(*config)
-	_, ok := snapshotters[cfg.Default]
-	if !ok {
-		return nil, errors.Errorf("default snapshotter not loaded: %s", cfg.Default)
+	if len(snapshotters) == 0 {
+		return nil, errors.Errorf("failed to create snapshotter service: no snapshotters loaded")
 	}
 
 	return &service{
-		snapshotters:           snapshotters,
-		defaultSnapshotterName: cfg.Default,
-		publisher:              ic.Events,
+		snapshotters: snapshotters,
+		publisher:    ic.Events,
 	}, nil
 }
 
 func (s *service) getSnapshotter(name string) (snapshot.Snapshotter, error) {
 	if name == "" {
-		name = s.defaultSnapshotterName
+		return nil, errdefs.ToGRPCf(errdefs.ErrInvalidArgument, "snapshotter argument missing")
 	}
+
 	sn, ok := s.snapshotters[name]
 	if !ok {
-		return nil, errors.Errorf("snapshotter not loaded: %s", name)
+		return nil, errdefs.ToGRPCf(errdefs.ErrInvalidArgument, "snapshotter not loaded: %s", name)
 	}
 	return sn, nil
 }