Better attempt at an appengine 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 {
+        obj := c.Value(timeNowKey)
+        if obj == nil || obj.(TimeNowFactory) == nil {
+                return time.Now()
+        }
+        return obj.(TimeNowFactory)(c)
+}
+
+// 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 {
+        f := func(context.Context) time.Time { return *t }

[M-A Ruel, 2015/05/25 17:14:52]

I'd write the function as:

if t == nil {
  return SetTimeNowFactory(c, nil)
}
return SetTimeNowFactory(c, func(context.Context) time.Time { return *t })

otherwise I think you create a closure even when unnecessary and I think the
above wouldn't (not 100% sure). There's some dynamic compilation involved.

[iannucci, 2015/05/26 18:25:06]

cool done.

+        if t == nil {
+                f = nil
+        }
+        return SetTimeNowFactory(c, f)
+}