Add meta package to datastore.

service/datastore/meta/namespaces.go

+// 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 (
+        "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. We ignore this.

[iannucci, 2016/04/25 08:05:27]

Out of curiosity... why? Why not call cb("")?

[dnj, 2016/04/25 15:24:17]

The thought was that someone enumerating namespaces would not find the default
namespace to be useful. I'll update the comment and have it return "".

+//      - Other namespaces will have 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 {
+                ns := k.StringID()
+                if ns == "" {
+                        return nil
+                }
+
+                return cb(ns)
+        })
+}
+
+// NamespacesWithPrefix runs Namespaces, returning only namespaces beginning
+// with the supplied prefix string.
+func NamespacesWithPrefix(c context.Context, p string, cb NamespacesCallback) error {

[iannucci, 2016/04/25 08:05:27]

TODO: https://github.com/luci/gae/issues/49 : Actually support this in the
underlying query.

[dnj, 2016/04/25 15:24:17]

Done.

+        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
+}