Add Context cancellation to clock.

common/parallel/semaphore.go

 // Copyright 2015 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 parallel
 
 // Semaphore is a sync.Locker that implements a n-semaphore.
 //
 // Lock the semaphore acquires a semaphore token, possibly blocking until one
 // is available.
 //
 // Unlock releases an owned token, returning it to the semaphore.
 type Semaphore chan struct{}
 
 // Lock acquires a semaphore resource, blocking until one is available.
 func (s Semaphore) Lock() {
         if cap(s) > 0 {
                 s <- struct{}{}
         }
 }
 
 // Unlock releases a single semaphore resource.
 func (s Semaphore) Unlock() {
         if cap(s) > 0 {
                 <-s
         }
 }
+
+// TakeAll blocks until it holds all available semaphore resources. When it
+// returns, the caller owns all of the resources in the semaphore.
+func (s Semaphore) TakeAll() {
+        for i := 0; i < cap(s); i++ {
+                s.Lock()
+        }
+}