Add Context cancellation to clock.

common/clock/clock.go

Index: common/clock/clock.go
diff --git a/common/clock/clock.go b/common/clock/clock.go
index 5fe5dc24b01600f5ce5f233af912b9416d2be92c..c07b46f6ecdec7c925e9764bda2ac232d21eb1e9 100644
--- a/common/clock/clock.go
+++ b/common/clock/clock.go
@@ -6,6 +6,8 @@ package clock
 
 import (
 	"time"
+
+	"golang.org/x/net/context"
 )
 
 // Clock is an interface to system time.
@@ -16,10 +18,21 @@ import (
 type Clock interface {
 	// Returns the current time (see time.Now).
 	Now() time.Time
-	// Sleeps the current goroutine (see time.Sleep)
-	Sleep(time.Duration)
+
+	// Sleeps the current goroutine (see time.Sleep).
+	//
+	// If the supplied Context is canceled prior to the specified duration,
+	// Sleep will return the Context's error. If the sleep completes naturally,
+	// it will return nil.
+	Sleep(context.Context, time.Duration) error

[iannucci, 2016/02/11 02:15:33]

Vadim suggests (and I agree) that this should probably be called something now,
to call it out from time.Sleep, which doesn't return an error. Suggestions:
  * SleepErr
  * CancelableSleep
  * ???

Probably the same for After as well.

The concern is code like:

for <condition> {
  clock.Sleep(time.Second) # <- doesn't check err
  // hot loop! hot loop!
}

[iannucci, 2016/02/11 02:16:35]

Which, I realize, sounds silly considering that you already had it named
CancelSleep :). But the difference is that it would be the only Sleep-like call.

[dnj (Google), 2016/02/11 17:22:37]

My initial reaction is that I don't like the idea of changing the name, (a)
because SleepErr and the like is ugly and (b) because the error generally does
not need to be checked.

Something that cancels Contexts and also loops should be responding to the
Context's cancellation somewhere in the loop, else we're in a different sort of
trouble. Code that doesn't cancel Contexts won't be affected. I can't really
think of a case where a loop over Sleep in code that cancels its Contexts isn't
a bug.

Just my initial thoughts, am happy to discuss.

+
 	// Creates a new Timer instance, bound to this Clock.
-	NewTimer() Timer
+	//
+	// If the supplied Context is canceled, the timer will expire immediately.
+	NewTimer(c context.Context) Timer
+
 	// Waits a duration, then sends the current time over the returned channel.
-	After(time.Duration) <-chan time.Time
+	//
+	// If the supplied Context is canceled, the timer will expire immediately.
+	After(context.Context, time.Duration) <-chan TimerResult
 }