Add testable interface for datastore.

service/datastore/testable.go

Index: service/datastore/testable.go
diff --git a/service/datastore/testable.go b/service/datastore/testable.go
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+++ b/service/datastore/testable.go
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package datastore
+
+type TestingSnapshot interface{}
+
+// Testable is the testable interface for fake datastore implementations.
+type Testable interface {
+	// AddIndex adds the provided index.
+	// Blocks all datastore access while the index is built.
+	// Panics if any of the IndexDefinition objects are not Compound()
+	AddIndexes(...*IndexDefinition)
+
+	// TakeIndexSnapshot allows you to take a snapshot of the current index
+	// tables, which can be used later with SetIndexSnapshot.
+	TakeIndexSnapshot() TestingSnapshot
+
+	// SetIndexSnapshot allows you to set the state of the current index tables.
+	// Note that this would allow you to create 'non-lienarities' in the precieved
+	// index results (e.g. you could force the indexes to go back in time).
+	//
+	// SetIndexSnapshot takes a reference of the given TestingSnapshot. You're
+	// still responsible for closing the snapshot after this call.
+	SetIndexSnapshot(TestingSnapshot)
+
+	// CatchupIndexes catches the index table up to the current state of the
+	// datastore. This is equivalent to:
+	//   idxSnap := TakeIndexSnapshot()
+	//   SetIndexSnapshot(idxSnap)
+	//
+	// But depending on the implementation it may implemented with an atomic
+	// operation.
+	CatchupIndexes()
+}