Add User service.

service/user/context.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 user
+
+import (
+        "golang.org/x/net/context"
+)
+
+type key int
+
+var serviceKey key
+
+// Factory is the function signature for factory methods compatible with
+// SetFactory.
+type Factory func(context.Context) Interface
+
+// Get pulls the user service implementation from context or nil if it
+// wasn't set.
+func Get(c context.Context) Interface {
+        if f, ok := c.Value(serviceKey).(Factory); ok && f != nil {
+                return f(c)
+        }
+        return nil
+}
+
+// SetFactory sets the function to produce user.Interface instances,
+// as returned by the Get method.
+func SetFactory(c context.Context, f Factory) context.Context {
+        return context.WithValue(c, serviceKey, f)
+}
+
+// Set sets the user service in this context. Useful for testing with a quick
+// mock. This is just a shorthand SetFactory invocation to set a factory which
+// always returns the same object.
+func Set(c context.Context, u Interface) context.Context {
+        return SetFactory(c, func(context.Context) Interface { return u })
+}