datastore: Add DecodeCursor to [Raw]Interface.

impl/memory/datastore_query.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 memory
 
 import (
+        "encoding/base64"
         "errors"
         "fmt"
         "math"
         "strings"
 
         ds "github.com/luci/gae/service/datastore"
         "github.com/luci/gae/service/datastore/serialize"
 )
 
 // MaxQueryComponents was lifted from a hard-coded constant in dev_appserver.
@@ skipping 38 matching lines @@
                         err = fmt.Errorf("datastore: invalid operator %q in filter %q", toks[1], f)
                 } else {
                         prop = toks[0]
                 }
         }
         return
 }
 
 type queryCursor string
 
-func (q queryCursor) String() string { return string(q) }
+func (q queryCursor) String() string { return base64.URLEncoding.EncodeToString([]byte(q)) }
 func (q queryCursor) Valid() bool    { return q != "" }
 
 type queryIneqFilter struct {
         prop string
 
         low  *string
         high *string
 }
 
+func decodeCursor(s string) (ds.Cursor, error) {
+        d, err := base64.URLEncoding.DecodeString(s)
+        if err != nil {
+                return nil, fmt.Errorf("Failed to Base64-decode cursor: %s", err)
+        }
+        c := queryCursor(string(d))
+        if !c.Valid() {
+                return nil, errors.New("Decoded cursor is not valid.")
+        }
+        return c, nil
+}
+
 func increment(bstr string, positive bool) string {
         lastIdx := len(bstr) - 1
         last := bstr[lastIdx]
         if positive {
                 if last == 0xFF {
                         return bstr + "\x00"
                 }
                 return bstr[:lastIdx-1] + string(last+1)
         } else {
                 if last == 0 {
@@ skipping 431 matching lines @@
                         q.end = curs
                 })
 }
 
 func (q *queryImpl) EventualConsistency() ds.Query {
         return q.checkMutateClone(
                 nil, func(q *queryImpl) {
                         q.eventualConsistency = true
                 })
 }