Thin library for interacting with the GAE datastore metadata entities.

go/src/infra/gae/libs/meta/eg.go

Index: go/src/infra/gae/libs/meta/eg.go
diff --git a/go/src/infra/gae/libs/meta/eg.go b/go/src/infra/gae/libs/meta/eg.go
new file mode 100644
index 0000000000000000000000000000000000000000..55ddce438685cff033561bdfbf30c12ccda0331c
--- /dev/null
+++ b/go/src/infra/gae/libs/meta/eg.go
@@ -0,0 +1,45 @@
+// 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 (
+	"appengine/datastore"
+
+	"infra/gae/libs/context"
+	"infra/libs/errors"
+)
+
+var mark = errors.MakeMarkFn("eg")
+
+// 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 `datastore:"-" goon:"kind,__entity_group__"`
+
+	ID     int64          `datastore:"-" goon:"id"`
+	Parent *datastore.Key `datastore:"-" goon:"parent"`
+
+	Version int64 `datastore:"__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.SingleReadWriter, root *datastore.Key) (int64, error) {
+	for root.Parent() != nil {
+		root = root.Parent()
+	}
+	egm := &EntityGroupMeta{ID: 1, Parent: root}
+	err := c.Get(egm)
+	if err != datastore.ErrNoSuchEntity {
+		err = mark(err)
+	} else {
+		// this is OK for callers. The version of the entity group is effectively 0
+		// in this case.
+		err = nil
+	}
+	return egm.Version, err
+}