Add transaction buffer filter.

filter/txnBuf/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 txnBuf
+
+import (
+        "golang.org/x/net/context"
+
+        ds "github.com/luci/gae/service/datastore"
+        "github.com/luci/gae/service/info"
+)
+
+type key int
+
+var (
+        dsTxnBufParent key
+)
+
+// FilterRDS installs a transaction buffer datastore filter in the context.
+func FilterRDS(c context.Context) context.Context {
+        // TODO(riannucci): allow the specification of the set of roots to limit this
+        // transaction to, transitively.
+        return ds.AddRawFilters(c, func(c context.Context, rds ds.RawInterface) ds.RawInterface {
+                if par, _ := c.Value(dsTxnBufParent).(*txnBufState); par != nil {
+                        return &dsTxnBuf{c, par}
+                }
+                return &dsBuf{rds, info.Get(c).GetNamespace()}
+        })
+}
+
+// impossible is a marker function to indicate that the given error is an
+// impossible state, due to conditions outside of the function.
+func impossible(err error) {
+        if err != nil {
+                panic(err)
+        }
+}
+
+// memoryCorruption is a marker function to indicate that given error is
+// actually due to corrupted memory to make it easier to read the code.
+func memoryCorruption(err error) {
+        if err != nil {
+                panic(err)
+        }
+}