fix crash in AUAudioOutputStream::Render()

media/audio/mac/audio_low_latency_output_mac.cc

 // Copyright (c) 2012 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.
 
 #include "media/audio/mac/audio_low_latency_output_mac.h"
 
 #include <CoreServices/CoreServices.h>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
@@ skipping 32 matching lines @@
 // 6) The same thread that called stop will call Close() where we cleanup
 // and notify the audio manager, which likely will destroy this object.
 
 AUAudioOutputStream::AUAudioOutputStream(
     AudioManagerMac* manager, const AudioParameters& params)
     : manager_(manager),
       source_(NULL),
       output_unit_(0),
       output_device_id_(kAudioObjectUnknown),
       volume_(1),
-      hardware_latency_frames_(0) {
+      hardware_latency_frames_(0),
+      stopped_(false) {
   // We must have a manager.
   DCHECK(manager_);
   // A frame is one sample across all channels. In interleaved audio the per
   // frame fields identify the set of n |channels|. In uncompressed audio, a
   // packet is always one frame.
   format_.mSampleRate = params.sample_rate();
   format_.mFormatID = kAudioFormatLinearPCM;
   format_.mFormatFlags = kLinearPCMFormatFlagIsPacked |
                          kLinearPCMFormatFlagIsSignedInteger;
   format_.mBitsPerChannel = params.bits_per_sample();
@@ skipping 107 matching lines @@
   // destruction.
   manager_->ReleaseOutputStream(this);
 }
 
 void AUAudioOutputStream::Start(AudioSourceCallback* callback) {
   DCHECK(callback);
   DLOG_IF(ERROR, !output_unit_) << "Open() has not been called successfully";
   if (!output_unit_)
     return;
 
+  stopped_ = false;
   source_ = callback;
 
   AudioOutputUnitStart(output_unit_);
 }
 
 void AUAudioOutputStream::Stop() {
   // We request a synchronous stop, so the next call can take some time. In
   // the windows implementation we block here as well.
-  source_ = NULL;
+  if (stopped_)
+    return;
 
   AudioOutputUnitStop(output_unit_);
+
+  source_ = NULL;
+  stopped_ = true;
 }
 
 void AUAudioOutputStream::SetVolume(double volume) {
   if (!output_unit_)
     return;
   volume_ = static_cast<float>(volume);
 
   // TODO(crogers): set volume property
 }
 
@@ skipping 135 matching lines @@
   // Get the delay between the moment getting the callback and the scheduled
   // time stamp that tells when the data is going to be played out.
   UInt64 output_time_ns = AudioConvertHostTimeToNanos(
       output_time_stamp->mHostTime);
   UInt64 now_ns = AudioConvertHostTimeToNanos(AudioGetCurrentHostTime());
   double delay_frames = static_cast<double>
       (1e-9 * (output_time_ns - now_ns) * format_.mSampleRate);
 
   return (delay_frames + hardware_latency_frames_);
 }