Add transaction buffer filter.

filter/txnBuf/ds.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"
+)
+
+// 10MB is the current initial limit
+const transactionSizeLimit = 10 * 1000 * 1000
+
+type dsBuf struct {
+        ds.RawInterface
+
+        ns string
+}
+
+var _ ds.RawInterface = (*dsBuf)(nil)
+
+func (d *dsBuf) RunInTransaction(f func(context.Context) error, opts *ds.TransactionOptions) error {

[iannucci, 2015/09/29 03:32:22]

So you can see here that in not-a-transaction, literally all methods pass
straight through without any interference (except for this one).

+        return doRunInTransaction(d.RawInterface, f, opts)
+}
+
+func doRunInTransaction(base ds.RawInterface, f func(context.Context) error, opts *ds.TransactionOptions) error {
+        return base.RunInTransaction(func(ctx context.Context) error {
+                return withTxnBuf(ctx, f, opts)
+        }, opts)
+}