Add context-aware "time" library wrapper.

go/src/infra/libs/clock/testclock.go

Index: go/src/infra/libs/clock/testclock.go
diff --git a/go/src/infra/libs/clock/testclock.go b/go/src/infra/libs/clock/testclock.go
new file mode 100644
index 0000000000000000000000000000000000000000..a9b034a7692d7aab5edb22fa0a9c69fdcc33b2af
--- /dev/null
+++ b/go/src/infra/libs/clock/testclock.go
@@ -0,0 +1,48 @@
+// Copyright (c) 2014 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 clock
+
+import (
+	"time"
+)
+
+// TestClock is a test-oriented implementation of the 'Clock' interface.
+//
+// This implementation's Clock responses are configurable by modifying its member
+// variables. Time-based events are explicitly triggered by sending on a Timer
+// instance's channel.
+type TestClock struct {
+	T     time.Time      // The current clock time.
+	C     chan time.Time // Channel to unblock sleeping and After.
+	Timer func() Timer   // Timer generator function. Must be set to use NewTimer() and After().
+}
+

[iannucci, 2015/06/02 06:15:34]

Add a line here

  var _ Clock = (*TestClock)(nil)

Then you don't need the `X implements Y` comments.

[dnj, 2015/06/02 17:03:12]

Done.

+// Now implements Clock.
+func (c *TestClock) Now() time.Time {
+	return c.T
+}
+
+// Sleep implements Clock.
+func (c *TestClock) Sleep(time.Duration) {
+	if c.C != nil {
+		// Wait for sleep signal; update current time.
+		c.T = <-c.C
+	}
+}
+
+// NewTimer implements Clock.
+func (c *TestClock) NewTimer() Timer {
+	if c.Timer == nil {
+		panic("No test timer generator is configured.")
+	}
+	return c.Timer()
+}
+
+// After implements Clock.
+func (c *TestClock) After(d time.Duration) <-chan time.Time {
+	t := c.NewTimer()
+	t.Reset(d)
+	return t.GetC()
+}