Fix memory corruption bug in impl/memory

impl/memory/gkvlite_iter.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 (
         "bytes"
         "sync"
 
         "github.com/luci/gae/service/datastore/serialize"
         "github.com/luci/gkvlite"
 )
 
 type iterDefinition struct {
         // The collection to iterate over
-        c *memCollection
+        c memCollection
 
         // The prefix to always assert for every row. A nil prefix matches every row.
         prefix []byte
 
         // prefixLen is the number of prefix bytes that the caller cares about. It
         // may be <= len(prefix). When doing a multiIterator, this number will be used
         // to determine the amount of suffix to transfer accross iterators. This is
         // used specifically when using builtin indexes to service ancestor queries.
         // The builtin index represents the ancestor key with prefix bytes, but in a
         // multiIterator context, it wants the entire key to be included in the
@@ skipping 84 matching lines @@
 }
 
 type iterator struct {
         stopper sync.Once
 
         stopped bool
         ch      chan<- *cmd
 }
 
 func (def *iterDefinition) mkIter() *iterator {
+        if !def.c.IsReadOnly() {
+                panic("attempting to make an iterator with r/w collection")
+        }
+
         cmdChan := make(chan *cmd)
         ret := &iterator{
                 ch: cmdChan,
         }
 
         prefix := def.prefix
         collection := def.c
 
         // convert the suffixes from the iterDefinition into full rows for the
         // underlying storage.
@@ skipping 75 matching lines @@
         t.ch <- &cmd{targ, func(i *gkvlite.Item) {
                 defer close(waiter)
 
                 if i == nil {
                         t.stop()
                 }
                 cb(i)
         }}
         <-waiter
 }