Add Info Testable interface, implement for memory.

impl/memory/info_test.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 memory
 
 import (
         "testing"
 
         "github.com/luci/gae/service/info"
         "golang.org/x/net/context"
 
         . "github.com/smartystreets/goconvey/convey"
 )
 
 func TestMustNamespace(t *testing.T) {
         Convey("MustNamespace panics", t, func() {
                 c := Use(context.Background())
                 i := info.Get(c)
 
                 So(func() {
                         i.MustNamespace("valid_namespace_name")
                 }, ShouldNotPanic)
                 So(func() {
                         i.MustNamespace("invalid namespace name")
                 }, ShouldPanic)
         })
+
+        Convey("Testable interface works", t, func() {
+                c := Use(context.Background())
+                c = useGID(c, func(mod *globalInfoData) {
+                        mod.appid = "app-id"
+                })
+
+                // Default value.
+                So(info.Get(c).AppID(), ShouldEqual, "app-id")
+                So(info.Get(c).RequestID(), ShouldEqual, "test-request-id")
+
+                // Setting to "override" applies to initial context.
+                c = info.Get(c).Testable().SetRequestID("override")
+                So(info.Get(c).RequestID(), ShouldEqual, "override")
+
+                // Derive inner context, "override" applies.
+                c = info.Get(c).MustNamespace("valid_namespace_name")
+                So(info.Get(c).RequestID(), ShouldEqual, "override")
+        })
 }