[dsreadonly] Add read-only datastore filter.

filter/readonly/filter.go

Index: filter/readonly/filter.go
diff --git a/filter/readonly/filter.go b/filter/readonly/filter.go
new file mode 100644
index 0000000000000000000000000000000000000000..d3e77f66c2f851cd0c320531d7605b4c05e5b7a7
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+++ b/filter/readonly/filter.go
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+// Copyright 2017 The LUCI Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package readonly implements a filter that enforces read-only accesses to
+// services.
+//
+// This can be useful in hybrid environments where one cluster wants to read
+// from a cache-backed datastore, but cannot modify the cache, so reads are safe
+// and direct, but writes would create a state where the cached values are
+// invalid.
+//
+// This happens when mixing AppEngine datastore/memcache with Cloud Datastore
+// readers.
+package readonly