Add dscache.AlwaysFilterRDS function.

filter/dscache/context.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 (
         ds "github.com/luci/gae/service/datastore"
         "github.com/luci/gae/service/info"
         mc "github.com/luci/gae/service/memcache"
         "github.com/luci/luci-go/common/mathrand"
         "golang.org/x/net/context"
 )
 
 type key int
 
 var dsTxnCacheKey key
 
 // FilterRDS installs a caching RawDatastore filter in the context.
 //
+// It does nothing if IsGloballyEnabled returns false. That way it is possible
+// to disable the cache in runtime (e.g. in case memcache service is having
+// issues).
+//
 // shardsForKey is a user-controllable function which calculates the number of
 // shards to use for a certain datastore key. The provided key will always be
 // valid and complete.
 //
 // The # of shards returned may be between 1 and 256. Values above this range
 // will be clamped into that range. A return value of 0 means that NO cache
 // operations should be done for this key, regardless of the dscache.enable
 // setting.
 //
 // If shardsForKey is nil, the value of DefaultShards is used for all keys.
 func FilterRDS(c context.Context, shardsForKey func(*ds.Key) int) context.Context {
         if !IsGloballyEnabled(c) {
                 return c
         }
+        return AlwaysFilterRDS(c, shardsForKey)
+}
 
+// AlwaysFilterRDS installs a caching RawDatastore filter in the context.
+//
+// Unlike FilterRDS it doesn't check GlobalConfig via IsGloballyEnabled call,
+// assuming caller already knows whether filter should be applied or not.
+func AlwaysFilterRDS(c context.Context, shardsForKey func(*ds.Key) int) context.Context {
         return ds.AddRawFilters(c, func(c context.Context, ds ds.RawInterface) ds.RawInterface {
                 i := info.Get(c)
 
                 sc := &supportContext{
                         i.AppID(),
                         i.GetNamespace(),
                         c,
                         mc.Get(c),
                         mathrand.Get(c),
                         shardsForKey,
                 }
 
                 v := c.Value(dsTxnCacheKey)
                 if v == nil {
                         return &dsCache{ds, sc}
                 }
                 return &dsTxnCache{ds, v.(*dsTxnState), sc}
         })
 }