Fix error message from KeyForObj when passing an invalid struct.

filter/dscache/support.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 dscache
 
 import (
         "fmt"
         "math/rand"
         "time"
@@ skipping 24 matching lines @@
         }
         if ret > MaxShards {
                 ret = MaxShards
         }
         return ret
 }
 
 func (s *supportContext) mkRandKeys(keys []*ds.Key, metas ds.MultiMetaGetter) []string {
         ret := []string(nil)
         for i, key := range keys {
-                if !metas.GetMetaDefault(i, CacheEnableMeta, true).(bool) {
+                mg := metas.GetSingle(i)
+                if !ds.GetMetaDefault(mg, CacheEnableMeta, true).(bool) {
                         continue
                 }
                 shards := s.numShards(key)
                 if shards == 0 {
                         continue
                 }
                 if ret == nil {
                         ret = make([]string, len(keys))
                 }
                 ret[i] = MakeMemcacheKey(s.mr.Intn(shards), key)
@@ skipping 93 matching lines @@
                 return nil, nil
         }
         ret := make([]memcache.Item, len(mcKeys))
         for i := range ret {
                 ret[i] = (s.mc.NewItem(mcKeys[i]).
                         SetFlags(uint32(ItemHasLock)).
                         SetExpiration(time.Second * time.Duration(LockTimeSeconds)))
         }
         return ret, mcKeys
 }