Update metadata plugin initialization

Updates metadata plugin to require content and
snapshotter plugins be loaded and initializes with
those plugins, keeping the metadata database structure
static after initialization. Service plugins now only
require metadata plugin access snapshotter or content
stores through metadata, which was already required
behavior of the services.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/content.go

@@ -19,23 +19,16 @@ import (
 
 type contentStore struct {
 	content.Store
-	db *DB
+	db transactor
 }
 
-// NewContentStore returns a namespaced content store using an existing
+// newContentStore returns a namespaced content store using an existing
 // content store interface.
-func NewContentStore(db *DB, cs content.Store) content.Store {
-	db.storeL.Lock()
-	defer db.storeL.Unlock()
-
-	if db.cs == nil {
-		db.cs = &contentStore{
-			Store: cs,
-			db:    db,
-		}
+func newContentStore(db transactor, cs content.Store) content.Store {
+	return &contentStore{
+		Store: cs,
+		db:    db,
 	}
-	return db.cs
-
 }
 
 func (cs *contentStore) Info(ctx context.Context, dgst digest.Digest) (content.Info, error) {
@@ -360,7 +353,7 @@ type namespacedWriter struct {
 	content.Writer
 	ref       string
 	namespace string
-	db        *DB
+	db        transactor
 }
 
 func (nw *namespacedWriter) Commit(ctx context.Context, size int64, expected digest.Digest, opts ...content.Opt) error {