blobserver/localdisk: be sure to implement SubFetcher for blobpacked

My fault for not running the (slow) integration tests before I broke
things in a4d0cc6ab7.

Fixes #1136

Change-Id: Ia30051da02974d0c3e79e0b220ff86dcab5771e4

pkg/blobserver/localdisk/localdisk.go

@@ -48,10 +48,13 @@ import (
 	"go4.org/syncutil"
 )
 
+// TODO: rename DiskStorage to Storage.
+
 // DiskStorage implements the blobserver.Storage interface using the
 // local filesystem.
 type DiskStorage struct {
 	blobserver.Storage
+	blob.SubFetcher
 
 	root string
 
@@ -59,6 +62,12 @@ type DiskStorage struct {
 	gen *local.Generationer
 }
 
+// Validate we implement expected interfaces.
+var (
+	_ blobserver.Storage = (*DiskStorage)(nil)
+	_ blob.SubFetcher    = (*DiskStorage)(nil) // for blobpacked; Issue 1136
+)
+
 func (ds *DiskStorage) String() string {
 	return fmt.Sprintf("\"filesystem\" file-per-blob at %s", ds.root)
 }
@@ -106,9 +115,10 @@ func New(root string) (*DiskStorage, error) {
 	}
 	fileSto := files.NewStorage(files.OSFS(), root)
 	ds := &DiskStorage{
-		Storage: fileSto,
-		root:    root,
-		gen:     local.NewGenerationer(root),
+		Storage:    fileSto,
+		SubFetcher: fileSto,
+		root:       root,
+		gen:        local.NewGenerationer(root),
 	}
 	if _, _, err := ds.StorageGeneration(); err != nil {
 		return nil, fmt.Errorf("Error initialization generation for %q: %v", root, err)