[dsreadonly] Add read-only datastore filter.

filter/readonly/rds.go

Index: filter/readonly/rds.go
diff --git a/filter/readonly/rds.go b/filter/readonly/rds.go
new file mode 100644
index 0000000000000000000000000000000000000000..d4521a19af79ab20cd3383885f461300d6758977
--- /dev/null
+++ b/filter/readonly/rds.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
+// datastore.
+//
+// This is 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
+
+import (
+	"golang.org/x/net/context"
+
+	ds "github.com/luci/gae/service/datastore"
+
+	"github.com/luci/luci-go/common/errors"
+)
+
+// ErrReadOnly is an error returned in response to mutating datastore
+// operations.
+var ErrReadOnly = errors.New("readonly: datastore is read-only")
+
+// readOnlyDatastore is a datastore.RawInterface implementation that returns
+// ErrReadOnly on mutating operations.
+type readOnlyDatastore struct {
+	ds.RawInterface
+}
+
+func (r *readOnlyDatastore) AllocateIDs(keys []*ds.Key, cb ds.NewKeyCB) error {
+	return ErrReadOnly
+}
+
+func (r *readOnlyDatastore) DeleteMulti(keys []*ds.Key, cb ds.DeleteMultiCB) error {
+	return ErrReadOnly
+}
+
+func (r *readOnlyDatastore) PutMulti(keys []*ds.Key, vals []ds.PropertyMap, cb ds.NewKeyCB) error {
+	return ErrReadOnly
+}
+
+// FilterRDS installs a read-only datastore filter in the context.
+//
+// This enforces that all datastore operations which could mutate the datastore
+// will return ErrReadOnly.
+func FilterRDS(c context.Context) context.Context {
+	return ds.AddRawFilters(c, func(ic context.Context, inner ds.RawInterface) ds.RawInterface {
+		return &readOnlyDatastore{inner}
+	})
+}