datastore: Add DecodeCursor to [Raw]Interface.

service/datastore/raw_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 (
         "fmt"
 
         "golang.org/x/net/context"
@@ skipping 147 matching lines @@
         NewQuery(kind string) Query
 
         // RunInTransaction runs f in a transaction.
         //
         // opts may be nil.
         //
         // NOTE: Implementations and filters are guaranteed that:
         //   - f is not nil
         RunInTransaction(f func(c context.Context) error, opts *TransactionOptions) 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(s string) (Cursor, error)
+
         // Run executes the given query, and calls `cb` for each successfully item.
         //
         // NOTE: Implementations and filters are guaranteed that:
         //   - query is not nil
         //   - cb is not nil
         Run(q Query, cb RawRunCB) error
 
         // GetMulti retrieves items from the datastore.
         //
         // Callback execues once per key, in the order of keys. Callback may not
@@ skipping 29 matching lines @@
         //   - len(keys) > 0
         //   - all keys are Valid, !Incomplete, and in the current namespace
         //   - none keys of the keys are 'special' (use a kind prefixed with '__')
         //   - cb is not nil
         DeleteMulti(keys []Key, cb DeleteMultiCB) error
 
         // Testable returns the Testable interface for the implementation, or nil if
         // there is none.
         Testable() Testable
 }