Add missing Count api

service/datastore/interface.go

 // 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
 
 import (
         "golang.org/x/net/context"
 )
 
@@ skipping 82 matching lines @@
         //   func(obj TYPE, getCursor CursorCB) bool
         //
         // Where TYPE is one of:
         //   - S or *S where S is a struct
         //   - P or *P where *P is a concrete type implementing PropertyLoadSaver
         //   - *Key (implies a keys-only query)
         //
         // Run stops on the first error encountered.
         Run(q *Query, cb interface{}) error
 
+        // Count executes the given query and returns the number of entries which
+        // match it.
+        Count(q *Query) (int64, error)
+
         // DecodeCursor converts a string returned by a Cursor into a Cursor instance.
         // It will return an error if the supplied string is not valid, or could not
         // be decoded by the implementation.
         DecodeCursor(string) (Cursor, error)
 
         // GetAll retrieves all of the Query results into dst.
         //
         // dst must be one of:
         //   - *[]S or *[]*S where S is a struct
         //   - *[]P or *[]*P where *P is a concrete type implementing
@@ skipping 48 matching lines @@
 
         // Testable returns the Testable interface for the implementation, or nil if
         // there is none.
         Testable() Testable
 
         // Raw returns the underlying RawInterface. The Interface and RawInterface may
         // be used interchangably; there's no danger of interleaving access to the
         // datastore via the two.
         Raw() RawInterface
 }