Make setting a Memcache Item with a nil Value OK

impl/prod/memcache.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 prod
 
 import (
         "time"
 
         mc "github.com/luci/gae/service/memcache"
@@ skipping 56 matching lines @@
 // and passes along an error.
 func mcR2FErr(i *memcache.Item, err error) (mc.Item, error) {
         if err != nil {
                 return nil, err
         }
         return mcItem{i}, nil
 }
 
 // mcF2R (MC fake-to-real) converts a mc.Item. i must originate from inside
 // this package for this function to work (see the panic message for why).
+//
+// If the item's Value == nil, it will be copied and replaced with []byte{}.
 func mcF2R(i mc.Item) *memcache.Item {
         if mci, ok := i.(mcItem); ok {
+                if mci.i.Value == nil {
+                        ret := *mci.i
+                        ret.Value = []byte{}
+                        return &ret
+                }
                 return mci.i
         }
         panic(
                 "you may not use other mc.Item implementations with this " +
                         "implementation of gae.Memcache, since it will cause all CompareAndSwap " +
                         "operations to fail. Please use the NewItem api instead.")
 }
 
 // mcMF2R (MC multi-fake-to-real) converts a slice of mc.Item to a slice of
 // *memcache.Item.
@@ skipping 62 matching lines @@
         return memcache.Flush(m.aeCtx)
 }
 
 func (m mcImpl) Stats() (*mc.Statistics, error) {
         stats, err := memcache.Stats(m.aeCtx)
         if err != nil {
                 return nil, err
         }
         return (*mc.Statistics)(stats), nil
 }