Add context-aware "time" library wrapper.

go/src/infra/libs/clock/systemclock/systemclock.go

Index: go/src/infra/libs/clock/systemclock/systemclock.go
diff --git a/go/src/infra/libs/clock/systemclock/systemclock.go b/go/src/infra/libs/clock/systemclock/systemclock.go
new file mode 100644
index 0000000000000000000000000000000000000000..532a0f253051dde1e9b8d874888f2e9375852250
--- /dev/null
+++ b/go/src/infra/libs/clock/systemclock/systemclock.go
@@ -0,0 +1,48 @@
+// Copyright (c) 2014 The Chromium Authors. All rights reserved.

[iannucci, 2015/06/02 22:19:02]

(copyrights)

Also, as discussed, I think it makes sense to have the system clock impl go in
the base package (just so that the default implementation can be the system
clock).

[dnj, 2015/06/03 00:25:28]

Done.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package systemclock
+
+import (
+	"time"
+
+	"golang.org/x/net/context"
+	"infra/libs/clock"
+)
+
+// Implementation of Clock that uses Go's standard library.
+type systemClock struct{}
+
+// System clock instance.
+var systemClockInstance systemClock
+
+// systemClock implements clock.Clock.
+var _ clock.Clock = (*systemClock)(nil)

[iannucci, 2015/06/02 22:19:03]

I think this would be even better as

  var _ clock.Clock = systemClock{}

since you don't actually use the pointer receiver for anything.

[dnj, 2015/06/03 00:25:28]

Done.

+
+// Get returns an instance of a Clock whose method calls directly use Go's "time"
+// library.
+func Get() clock.Clock {
+	return systemClockInstance
+}
+
+func (systemClock) Now() time.Time {
+	return time.Now()
+}
+
+func (systemClock) Sleep(d time.Duration) {
+	time.Sleep(d)
+}
+
+func (systemClock) NewTimer() clock.Timer {
+	return new(systemTimer)
+}
+
+func (systemClock) After(d time.Duration) <-chan time.Time {
+	return time.After(d)
+}
+
+// Set creates a new Context using the supplied TestClock instance.
+func Set(ctx context.Context) context.Context {
+	return clock.SetClock(ctx, Get())
+}