Add context-aware "time" library wrapper.

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

Index: go/src/infra/libs/clock/testclock/testtimer.go
diff --git a/go/src/infra/libs/clock/testclock/testtimer.go b/go/src/infra/libs/clock/testclock/testtimer.go
new file mode 100644
index 0000000000000000000000000000000000000000..3e9ab2515f5e41890ce6bb412eb8460882251833
--- /dev/null
+++ b/go/src/infra/libs/clock/testclock/testtimer.go
@@ -0,0 +1,77 @@
+// 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 testclock
+
+import (
+	"time"
+
+	"infra/libs/clock"
+)
+
+// timer is an implementation of clock.TestTimer that uses a channel
+// to signal the timer to fire.
+//
+// The channel is buffered so it can be used without requiring a separate signalling
+// goroutine.
+type timer struct {
+	clock   *testClock     // The underlying test clock instance.
+	signalC chan time.Time // Underlying signal channel.
+
+	// Cancels callback from clock.whenTimePasses. Being not nil implies that the timer
+	// is active.
+	cancelFunc cancelFunc
+}
+
+var _ clock.Timer = (*timer)(nil)
+
+// NewTimer returns a new, instantiated timer.
+func newTimer(clock *testClock) clock.Timer {
+	t := timer{
+		clock: clock,
+	}
+	return &t
+}
+
+func (t *timer) GetC() (c <-chan time.Time) {
+	if t.cancelFunc != nil {
+		c = t.signalC
+	}
+	return
+}
+
+func (t *timer) Reset(d time.Duration) (active bool) {
+	now := t.clock.Now()
+	triggerTime := now.Add(d)
+
+	// Signal our timerSet callback.
+	t.clock.signalTimerSet(t)
+
+	// Stop our current polling goroutine, if it's running.
+	active = t.Stop()
+
+	// Set timer properties.
+	t.signalC = make(chan time.Time, 1)
+	t.cancelFunc = t.clock.whenTimePasses(triggerTime, t.signal)

[iannucci, 2015/06/03 17:37:20]

why not `whenPastTime`, since it's taking an absolute time and not a relative
one?

[dnj, 2015/06/03 18:21:27]

"Past" implies ">", but I want to enforce ">=".

I went with "invokeAt".

+	return
+}
+
+func (t *timer) Stop() bool {
+	// If the timer is not running, we're done.
+	if t.cancelFunc == nil {
+		return false
+	}
+
+	// Clear our state.
+	t.cancelFunc()
+	t.cancelFunc = nil
+	return true
+}
+
+// Sends a single signal, clearing the channel afterwards.
+func (t *timer) signal(now time.Time) {
+	if t.signalC != nil {
+		t.signalC <- now
+	}
+}