Add meta package to datastore.

service/datastore/meta/namespaces.go

Index: service/datastore/meta/namespaces.go
diff --git a/service/datastore/meta/namespaces.go b/service/datastore/meta/namespaces.go
new file mode 100644
index 0000000000000000000000000000000000000000..a3fdea7b87e2f64f3309b1ff3a2172b5501fcdb3
--- /dev/null
+++ b/service/datastore/meta/namespaces.go
@@ -0,0 +1,73 @@
+// 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 (
+	"fmt"
+	"strings"
+
+	"github.com/luci/gae/service/datastore"
+	"golang.org/x/net/context"
+)
+
+// NamespacesCallback is the callback type used with Namespaces. The callback
+// will be invoked with each identified namespace.
+//
+// If the callback returns an error, iteration will stop. If the error is
+// datastore.Stop, Namespaces will stop iterating and return nil. Otherwise,
+// the error will be forwarded.
+type NamespacesCallback func(string) error
+
+// Namespaces returns a list of all of the namespaces in the datastore.
+//
+// This is done by issuing a datastore query for kind "__namespace__". The
+// resulting keys will have IDs for the namespaces, namely:
+//	- The empty namespace will have integer ID 1. This will be forwarded to the
+//	  callback as "".
+//	- Other namespaces will have non-zero string IDs.
+func Namespaces(c context.Context, cb NamespacesCallback) error {
+	q := datastore.NewQuery("__namespace__").KeysOnly(true)
+
+	// Query our datastore for the full set of namespaces.
+	return datastore.Get(c).Run(q, func(k *datastore.Key) error {
+		switch {
+		case k.IntID() == 1:
+			return cb("")
+		case k.IntID() != 0:
+			return fmt.Errorf("unexpected namepsace integer key (%d)", k.IntID())
+		default:
+			return cb(k.StringID())
+		}
+	})
+}
+
+// NamespacesWithPrefix runs Namespaces, returning only namespaces beginning
+// with the supplied prefix string.
+func NamespacesWithPrefix(c context.Context, p string, cb NamespacesCallback) error {
+	// TODO: https://github.com/luci/gae/issues/49 : When inequality filters are
+	// supported, implement this using a "Gte" filter.
+	any := false
+	return Namespaces(c, func(ns string) error {
+		if !strings.HasPrefix(ns, p) {
+			if any {
+				return datastore.Stop
+			}
+			return nil
+		}
+
+		any = true
+		return cb(ns)
+	})
+}
+
+// NamespacesCollector exposes a NamespacesCallback function that aggregates
+// resulting namespaces into the collector slice.
+type NamespacesCollector []string
+
+// Callback is a NamespacesCallback which adds each namespace to the collector.
+func (c *NamespacesCollector) Callback(v string) error {
+	*c = append(*c, v)
+	return nil
+}