Support target snapshot references on prepare

Allows backend snapshots to bring existing snapshots into
a namespace without requiring clients to fully snapshots
when the target reference is known. Backend snapshots must
explicitly implement this functionality, it is equivalent
to sharing across namespaces and is up to the backend to
use the label when it is given or ignore it.

This enables remote snapshot functionality for a backend to
query for a target snapshot before a client has performed
any work to create that snapshot.

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

metadata/db_test.go

@@ -48,9 +48,31 @@ import (
 	bolt "go.etcd.io/bbolt"
 )
 
-func testDB(t *testing.T) (context.Context, *DB, func()) {
+type testOptions struct {
+	extraSnapshots map[string]func(string) (snapshots.Snapshotter, error)
+}
+
+type testOpt func(*testOptions)
+
+func withSnapshotter(name string, fn func(string) (snapshots.Snapshotter, error)) testOpt {
+	return func(to *testOptions) {
+		if to.extraSnapshots == nil {
+			to.extraSnapshots = map[string]func(string) (snapshots.Snapshotter, error){}
+		}
+		to.extraSnapshots[name] = fn
+	}
+}
+
+func testDB(t *testing.T, opt ...testOpt) (context.Context, *DB, func()) {
 	ctx, cancel := context.WithCancel(context.Background())
 	ctx = namespaces.WithNamespace(ctx, "testing")
+	ctx = logtest.WithT(ctx, t)
+
+	var topts testOptions
+
+	for _, o := range opt {
+		o(&topts)
+	}
 
 	dirname, err := ioutil.TempDir("", strings.Replace(t.Name(), "/", "_", -1)+"-")
 	if err != nil {
@@ -62,6 +84,18 @@ func testDB(t *testing.T) (context.Context, *DB, func()) {
 		t.Fatal(err)
 	}
 
+	snapshotters := map[string]snapshots.Snapshotter{
+		"native": snapshotter,
+	}
+
+	for name, fn := range topts.extraSnapshots {
+		snapshotter, err := fn(filepath.Join(dirname, name))
+		if err != nil {
+			t.Fatal(err)
+		}
+		snapshotters[name] = snapshotter
+	}
+
 	cs, err := local.NewStore(filepath.Join(dirname, "content"))
 	if err != nil {
 		t.Fatal(err)
@@ -72,7 +106,7 @@ func testDB(t *testing.T) (context.Context, *DB, func()) {
 		t.Fatal(err)
 	}
 
-	db := NewDB(bdb, cs, map[string]snapshots.Snapshotter{"native": snapshotter})
+	db := NewDB(bdb, cs, snapshotters)
 	if err := db.Init(ctx); err != nil {
 		t.Fatal(err)
 	}