Add context-aware "time" library wrapper.

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

Index: go/src/infra/libs/clock/testclock/testclock.go
diff --git a/go/src/infra/libs/clock/testclock/testclock.go b/go/src/infra/libs/clock/testclock/testclock.go
new file mode 100644
index 0000000000000000000000000000000000000000..10d17d65efe896cc1d543b420e5f559971a75c43
--- /dev/null
+++ b/go/src/infra/libs/clock/testclock/testclock.go
@@ -0,0 +1,178 @@
+// Copyright 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 testclock
+
+import (
+	"sync"
+	"time"
+
+	"infra/libs/clock"
+)
+
+// TestClock is a Clock interface with additional methods to help instrument it.
+type TestClock interface {
+	clock.Clock
+	Set(time.Time)
+	Add(time.Duration)
+	SetTimerCallback(TimerCallback)
+}
+
+// TimerCallback that can be invoked when a timer has been set. This is useful
+// for sychronizing state when testing.
+type TimerCallback func(clock.Timer)
+
+type cancelFunc func()
+
+// 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 {
+	sync.RWMutex
+
+	now       time.Time  // The current clock time.
+	timerCond *sync.Cond // Condition used to manage timer blocking.
+
+	timerCallback TimerCallback // Optional callback when a timer has been set.
+}
+
+var _ TestClock = (*testClock)(nil)
+
+// New returns a TestClock instance set at the specified time.
+func New(now time.Time) TestClock {
+	c := testClock{
+		now: now,
+	}
+	c.timerCond = sync.NewCond(&c)
+	return &c
+}
+
+func (c *testClock) Now() time.Time {
+	c.RLock()
+	defer c.RUnlock()
+
+	return c.now
+}
+
+func (c *testClock) Sleep(d time.Duration) {
+	<-c.After(d)
+}
+
+func (c *testClock) NewTimer() clock.Timer {
+	return newTimer(c)
+}
+
+func (c *testClock) After(d time.Duration) <-chan time.Time {
+	t := c.NewTimer()
+	t.Reset(d)
+	return t.GetC()
+}
+
+func (c *testClock) Set(t time.Time) {
+	c.Lock()
+	defer c.Unlock()
+
+	c.setTimeLocked(t)
+}
+
+func (c *testClock) Add(d time.Duration) {
+	c.Lock()
+	defer c.Unlock()
+
+	c.setTimeLocked(c.now.Add(d))
+}
+
+func (c *testClock) setTimeLocked(t time.Time) {
+	if t.Before(c.now) {
+		panic("Cannot go backwards in time. You're not Doc Brown.")
+	}
+	c.now = t
+
+	// Unblock any blocking timers that are waiting on our lock.
+	c.pokeTimers(true)
+}
+
+func (c *testClock) SetTimerCallback(callback TimerCallback) {
+	c.Lock()
+	defer c.Unlock()
+
+	c.timerCallback = callback
+}
+
+func (c *testClock) getTimerCallback() TimerCallback {
+	c.Lock()
+	defer c.Unlock()
+
+	return c.timerCallback
+}
+
+func (c *testClock) signalTimerSet(t clock.Timer) {
+	callback := c.getTimerCallback()
+	if callback != nil {
+		callback(t)
+	}
+}
+
+func (c *testClock) pokeTimers(locked bool) {
+	if !locked {
+		c.Lock()
+		defer c.Unlock()
+	}
+
+	c.timerCond.Broadcast()
+}
+
+// invokeAt invokes the specified callback when the Clock has advanced at
+// or after the specified threshold.
+//
+// The returned cancelFunc can be used to cancel the blocking. If the cancel
+// function is invoked before the callback, the callback will not be invoked.
+func (c *testClock) invokeAt(threshold time.Time, callback func(time.Time)) cancelFunc {
+	stopC := make(chan struct{})
+	finishedC := make(chan struct{})
+
+	// Our control goroutine will monitor both time and stop signals. It will only
+	// terminate when stopC has been closed.
+	//
+	// The lock that we take our here is owned by the following goroutine.
+	c.Lock()
+	go func() {
+		defer func() {
+			now := c.now
+			c.Unlock()
+			close(finishedC)
+			callback(now)
+		}()
+
+		for {
+			// Determine if we are past our signalling threshold. We can safely access
+			// our clock's time member directly, since we hold its lock from the
+			// condition firing.
+			if !c.now.Before(threshold) {
+				return
+			}
+
+			// Wait for a signal from our clock's condition.
+			c.timerCond.Wait()
+
+			// Have we been stopped?
+			select {
+			case <-stopC:
+				return
+
+			default:
+				// Nope.
+			}
+		}
+	}()
+
+	return func() {
+		// Close our stop channel and block pending goroutine termination.
+		close(stopC)
+		c.pokeTimers(false)
+		<-finishedC
+	}
+}