Add datastore iterating query filter.

filter/dsQueryBatch/context.go

Index: filter/dsQueryBatch/context.go
diff --git a/filter/dsQueryBatch/context.go b/filter/dsQueryBatch/context.go
new file mode 100644
index 0000000000000000000000000000000000000000..ec31ed2e0ac4e0efb833cc175e603861cb3d5706
--- /dev/null
+++ b/filter/dsQueryBatch/context.go
@@ -0,0 +1,28 @@
+// 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 dsQueryBatch
+
+import (
+	ds "github.com/luci/gae/service/datastore"
+	"golang.org/x/net/context"
+)
+
+// BatchQueries installs a datastore filter that causes all queries to be broken
+// into a series of iterative fixed-size queries. The batching uses cursors to
+// chain the iterations together.
+//
+// This helps accommodate query size or time limits enforced by the backing
+// datastore implementation.
+//
+// Note that this expands a single query into a series of queries, which may
+// lose additional single-query consistency guarantees.
+func BatchQueries(c context.Context, batchSize int32) context.Context {
+	return ds.AddRawFilters(c, func(ic context.Context, ri ds.RawInterface) ds.RawInterface {
+		return &iterQueryFilter{
+			RawInterface: ri,
+			batchSize:    batchSize,
+		}
+	})
+}