Add context-aware "time" library wrapper.

go/src/infra/gae/libs/wrapper/time.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 wrapper
-
-import (
-        "time"
-
-        "golang.org/x/net/context"
-)
-
-// TimeNowFactory is the function signature for factory methods compatible with
-// SetTimeNowFactory.
-type TimeNowFactory func(context.Context) time.Time
-
-// GetTimeNow gets the current time.Time from the context. If the context has no
-// time factory set with SetTimeNowFactory, it returns time.Now().
-func GetTimeNow(c context.Context) time.Time {
-        if f, ok := c.Value(timeNowKey).(TimeNowFactory); ok && f != nil {
-                return f(c)
-        }
-        return time.Now()
-}
-
-// SetTimeNowFactory sets the function to produce time.Time instances, as
-// returned by the GetTimeNow method.
-func SetTimeNowFactory(c context.Context, tnf TimeNowFactory) context.Context {
-        return context.WithValue(c, timeNowKey, tnf)
-}
-
-// SetTimeNow sets a pointer to the current time.Time object in the context.
-// Useful for testing with a quick mock. This is just a shorthand
-// SetTimeNowFactory invocation to set a factory which always returns the same
-// object.
-//
-// Note the pointer asymmetry with GetTimeNow. This is done intentionally to
-// allow you to modify the pointed-to-time to be able to manually monkey with
-// the clock in a test. Otherwise this would essentially freeze time in place.
-// If you pass 'nil', this will set the context back to returning time.Now().
-func SetTimeNow(c context.Context, t *time.Time) context.Context {
-        if t == nil {
-                return SetTimeNowFactory(c, nil)
-        }
-        return SetTimeNowFactory(c, func(context.Context) time.Time { return *t })
-}