Add context-aware "time" library wrapper.

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

Index: go/src/infra/libs/clock/clock.go
diff --git a/go/src/infra/libs/clock/clock.go b/go/src/infra/libs/clock/clock.go
new file mode 100644
index 0000000000000000000000000000000000000000..c666f23763428d86e381745abdc8262bb05cd73a
--- /dev/null
+++ b/go/src/infra/libs/clock/clock.go
@@ -0,0 +1,52 @@
+// 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"
+)
+
+// A Clock is an interface to system time.
+//
+// The standard clock is SystemClock, which falls through to the system time library.
+// Another clock, FakeClock, is available to simulate time facilities for testing.
+type Clock interface {
+	Now() time.Time                       // Returns the current time (see time.Now).
+	Sleep(time.Duration)                  // Sleeps the current goroutine (see time.Sleep)
+	NewTimer() Timer                      // Creates a new Timer instance.
+	After(time.Duration) <-chan time.Time // Waits a duration, then sends the current time.
+}
+
+// Implementation of Clock that uses Go's standard library.

[iannucci, 2015/06/02 06:15:33]

implementations should go in subpackages I think. maybe:


clock/
  // Clock interface
  // Testable interface
  .SetFactory
  .Set
  .Get
  realclock/
    .Use(context) context  // adds a real clock implementation to context
  testclock/
    .Use(context) context  // adds a testing clock implementation to context
    // Also implements clock.Testable, so clock.Get(c).(clock.Testable) works.
    // or have Use return a clock.Testable for pulling the strings on the clock.

+type systemClock struct{}
+
+// System clock instance.
+var systemClockInstance systemClock
+
+// SystemClock returns an instance of a Clock whose method calls directly use Go's "time"
+// library.
+func SystemClock() Clock {
+	return systemClockInstance
+}
+
+// Implements Clock.
+func (systemClock) Now() time.Time {
+	return time.Now()
+}
+
+// Implements Clock.
+func (systemClock) Sleep(d time.Duration) {
+	time.Sleep(d)
+}
+
+// Implements Clock.
+func (systemClock) NewTimer() Timer {
+	return new(systemTimer)
+}
+
+// Implements Clock.
+func (systemClock) After(d time.Duration) <-chan time.Time {
+	return time.After(d)
+}