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)

[Vadim Sh., 2015/03/06 03:01:48]

nit: 'mark' is too general. It is ok in such small file, but in larger file it
may get ambiguous...

How do you feel about standardizing on some name like "markError" or "pkgError",
and (maybe) standartaize on making it package global (not file global), e.g.

var makeError = errors.MakeMarkFn("infra/gae/libs/meta")

?

+	} else {
+		// this is OK for callers. The version of the entity group is effectively 0
+		// in this case.
+		err = nil
+	}
+	return egm.Version, err
+}