Allow to call deliverRecordedData: on multiple threads

It is required to keep [RTCAudioDeviceModule deliverRecordedData:] calling
once an audio stream added. In such a case, you may invoke the method
with dummy samples on the background thread before real samples are ready,
and then invoke the method with real samples in threads handling audio.
This change covers the scenario.

Author: akihikodaki

README.pixiv.md

@@ -54,6 +54,16 @@ is [`CMSampleBuffer`](https://developer.apple.com/documentation/coremedia/cmsamp
 Provide [`CMSampleBuffer`](https://developer.apple.com/documentation/coremedia/cmsamplebuffer-u71)
 acquired with [`[RPBroadcastSampleHandler processSampleBuffer: with:]`](https://developer.apple.com/documentation/replaykit/rpbroadcastsamplehandler/2123045-processsamplebuffer).
 
+You must keep calling this method once you added an audio stream. However,
+broadcast extension does not provide audio buffer with
+`RPSampleBufferType.audioMic` while the microphone is disabled. Therefore,
+you have to provide dummy samples before the microphone is enabled if you
+are broadcasting the audio recorded with the microphone.
+
+Note that `NSTimer` and `DispatchSourceTimer` cannot be used to implement
+such a mechanism, at least on iOS 12.2. `DispatchQueue.asyncAfter` is
+a nice alternative.
+
 The audio unit must be uninitialized and disabled to use this method.
 
 ### `RTCAudioSession.useManualAudio`

sdk/objc/native/src/audio/audio_device_ios.h

@@ -208,8 +208,8 @@ class AudioDeviceIOS : public AudioDeviceGeneric,
   // created on.
   rtc::ThreadChecker thread_checker_;
 
-  // Native I/O audio thread checker.
-  rtc::ThreadChecker io_thread_checker_;
+  // Native audio I/O critical section.
+  rtc::CriticalSection io_lock_;
 
   // Thread that this object is created on.
   rtc::Thread* thread_;
@@ -282,7 +282,7 @@ class AudioDeviceIOS : public AudioDeviceGeneric,
 
   // Counts number of detected audio glitches on the playout side.
   int64_t num_detected_playout_glitches_ RTC_GUARDED_BY(thread_checker_);
-  int64_t last_playout_time_ RTC_GUARDED_BY(io_thread_checker_);
+  int64_t last_playout_time_ RTC_GUARDED_BY(io_lock_);
 
   // Counts number of playout callbacks per call.
   // The value isupdated on the native I/O thread and later read on the

sdk/objc/native/src/audio/audio_device_ios.mm

@@ -116,7 +116,6 @@ static void LogDeviceInfo() {
       num_playout_callbacks_(0),
       last_output_volume_change_time_(0) {
   LOGI() << "ctor" << ios::GetCurrentThreadDescription();
-  io_thread_checker_.Detach();
   thread_checker_.Detach();
   thread_ = rtc::Thread::Current();
 
@@ -139,7 +138,6 @@ static void LogDeviceInfo() {
 
 AudioDeviceGeneric::InitStatus AudioDeviceIOS::Init() {
   LOGI() << "Init";
-  io_thread_checker_.Detach();
   thread_checker_.Detach();
 
   RTC_DCHECK_RUN_ON(&thread_checker_);
@@ -359,7 +357,7 @@ static void LogDeviceInfo() {
 }
 
 void AudioDeviceIOS::OnDeliverRecordedExternalData(CMSampleBufferRef sample_buffer) {
-  RTC_DCHECK_RUN_ON(&io_thread_checker_);
+  rtc::CritScope scoped_lock(&io_lock_);
 
   if (audio_unit_ && audio_unit_->GetState() != VoiceProcessingAudioUnit::kUninitialized) {
     RTCLogError(@"External recorded data was provided while audio unit is enabled.");
@@ -421,7 +419,7 @@ static void LogDeviceInfo() {
                                                UInt32 bus_number,
                                                UInt32 num_frames,
                                                AudioBufferList* /* io_data */) {
-  RTC_DCHECK_RUN_ON(&io_thread_checker_);
+  rtc::CritScope scoped_lock(&io_lock_);
   OSStatus result = noErr;
   // Simply return if recording is not enabled.
   if (!rtc::AtomicOps::AcquireLoad(&recording_)) return result;
@@ -469,7 +467,7 @@ static void LogDeviceInfo() {
                                           UInt32 bus_number,
                                           UInt32 num_frames,
                                           AudioBufferList* io_data) {
-  RTC_DCHECK_RUN_ON(&io_thread_checker_);
+  rtc::CritScope scoped_lock(&io_lock_);
   // Verify 16-bit, noninterleaved mono PCM signal format.
   RTC_DCHECK_EQ(1, io_data->mNumberBuffers);
   AudioBuffer* audio_buffer = &io_data->mBuffers[0];
@@ -979,10 +977,6 @@ static void LogDeviceInfo() {
   // Close and delete the voice-processing I/O unit.
   audio_unit_.reset();
 
-  // Detach thread checker for the AURemoteIO::IOThread to ensure that the
-  // next session uses a fresh thread id.
-  io_thread_checker_.Detach();
-
   // Remove audio session notification observers.
   RTCAudioSession* session = [RTCAudioSession sharedInstance];
   [session removeDelegate:audio_session_observer_];
@@ -994,11 +988,6 @@ static void LogDeviceInfo() {
 
 void AudioDeviceIOS::PrepareForNewStart() {
   LOGI() << "PrepareForNewStart";
-  // The audio unit has been stopped and preparations are needed for an upcoming
-  // restart. It will result in audio callbacks from a new native I/O thread
-  // which means that we must detach thread checkers here to be prepared for an
-  // upcoming new audio stream.
-  io_thread_checker_.Detach();
 }
 
 bool AudioDeviceIOS::IsInterrupted() {