Add meta package to datastore.

service/datastore/meta/eg.go

Index: service/datastore/meta/eg.go
diff --git a/service/datastore/meta/eg.go b/service/datastore/meta/eg.go
new file mode 100644
index 0000000000000000000000000000000000000000..11c31024473e7ef84f8d816e72a1727a0e3e5a3b
--- /dev/null
+++ b/service/datastore/meta/eg.go
@@ -0,0 +1,37 @@
+// 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 meta
+
+import (
+	"github.com/luci/gae/service/datastore"
+	"golang.org/x/net/context"
+)
+
+// EntityGroupMeta is the model corresponding to the __entity_group__ model in
+// appengine. You shouldn't need to use this struct directly, but instead should
+// use GetEntityGroupVersion.
+type EntityGroupMeta struct {
+	kind   string         `gae:"$kind,__entity_group__"`
+	id     int64          `gae:"$id,1"`
+	Parent *datastore.Key `gae:"$parent"`
+
+	Version int64 `gae:"__version__"`
+}
+
+// GetEntityGroupVersion returns the entity group version for the entity group
+// containing root. If the entity group doesn't exist, this function will return
+// zero and a nil error.
+func GetEntityGroupVersion(c context.Context, key *datastore.Key) (int64, error) {
+	ds := datastore.Get(c)
+	egm := &EntityGroupMeta{Parent: key.Root()}
+	err := ds.Get(egm)
+	ret := egm.Version
+	if err == datastore.ErrNoSuchEntity {
+		// this is OK for callers. The version of the entity group is effectively 0
+		// in this case.
+		err = nil
+	}
+	return ret, err
+}