Add Context cancellation to clock.

common/clock/cancelSleep.go

Index: common/clock/cancelSleep.go
diff --git a/common/clock/cancelSleep.go b/common/clock/cancelSleep.go
new file mode 100644
index 0000000000000000000000000000000000000000..984b426edec05bb36e5279256d7d6d6c9b7340b8
--- /dev/null
+++ b/common/clock/cancelSleep.go
@@ -0,0 +1,33 @@
+// Copyright 2016 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"
+
+	"golang.org/x/net/context"
+)
+
+// CancelSleep sleeps the current goroutine (see time.Sleep).
+//
+// If the supplied Context is canceled prior to the specified duration,
+// CancelSleep will return the Context's error. If the sleep completes
+// naturally, it will return nil.
+func CancelSleep(c context.Context, d time.Duration) error {

[iannucci, 2016/02/09 20:41:10]

(discussed offline) let's just make normal Sleep (and After) take a context, on
the assumption it's never safe to sleep while ignoring the context.Done anyway
:)

[Vadim Sh., 2016/02/09 21:11:37]

I'd prefer to keep CancelSleep. clock package is mirroring time API pretty
closely, making Sleep cancelable will break this and introduce some WTF moments.

Also there's an issue for not paying (performance wise) for what you don't need.
I can imagine Sleep to be used in some simple lock-free busy loops (for
example). Plugging in context cancelling machinery there can be a big
performance impact.  

+	select {
+	case <-c.Done():
+		return c.Err()
+
+	case <-After(c, d):
+		// For determinism, prefer context cancellation over full sleep.
+		select {
+		case <-c.Done():
+			return c.Err()
+		default:
+			break
+		}
+		return nil
+	}
+}