osxaudiosrc: iOS resampling causes stuttering

Fixes stuttering audio when iOS AU is resampling. To make AU resample,
one has to request a rate that differs from AVAudioSession's
sampleRate. The resampling itself is not the culprit, but rather our
API misuse.

AudioUnitRender modifies the mDataByteSize members with the
actual read bytes count. Therefore, they must be reinitialized
before each AudioUnitRender. (The buffers themselves can be
preallocated.)

The "stutter" was caused by one AudioUnitRender making the buffer
too small for other AudioUnitRender invocations, making them fail
with -50 (paramErr). By way of luck, when AU didn't resample, all
AudioUnitRender invocations read the same number of bytes.

(This patch addresses some non-interleaved audio concerns, but
at this moment the elements do not support non-interleaved audio
and non-interleaved is untested.)

https://bugzilla.gnome.org/show_bug.cgi?id=744922

sys/osxaudio/gstosxaudiosrc.c

@@ -384,8 +384,16 @@ gst_osx_audio_src_io_proc (GstOsxAudioRingBuffer * buf,
   gint writeseg;
   gint len;
   gint remaining;
+  UInt32 n;
   gint offset = 0;
 
+  /* Previous invoke of AudioUnitRender changed mDataByteSize into
+   * number of bytes actually read. Reset the members. */
+  for (n = 0; n < buf->core_audio->recBufferList->mNumberBuffers; ++n) {
+    buf->core_audio->recBufferList->mBuffers[n].mDataByteSize =
+        buf->core_audio->recBufferSize;
+  }
+
   status = AudioUnitRender (buf->core_audio->audiounit, ioActionFlags,
       inTimeStamp, inBusNumber, inNumberFrames, buf->core_audio->recBufferList);
 
@@ -394,6 +402,9 @@ gst_osx_audio_src_io_proc (GstOsxAudioRingBuffer * buf,
     return status;
   }
 
+  /* TODO: To support non-interleaved audio, go over all mBuffers,
+   *       not just the first one. */
+
   remaining = buf->core_audio->recBufferList->mBuffers[0].mDataByteSize;
 
   while (remaining) {

sys/osxaudio/gstosxcoreaudio.c

@@ -126,9 +126,11 @@ gst_core_audio_initialize (GstCoreAudio * core_audio,
 
   if (core_audio->is_src) {
     /* create AudioBufferList needed for recording */
+    core_audio->recBufferSize = frame_size * format.mBytesPerFrame;
     core_audio->recBufferList =
-        buffer_list_alloc (format.mChannelsPerFrame,
-        frame_size * format.mBytesPerFrame);
+        buffer_list_alloc (format.mChannelsPerFrame, core_audio->recBufferSize,
+        /* Currently always TRUE (i.e. interleaved) */
+        !(format.mFormatFlags & kAudioFormatFlagIsNonInterleaved));
   }
 
   /* Initialize the AudioUnit */
@@ -141,10 +143,8 @@ gst_core_audio_initialize (GstCoreAudio * core_audio,
   return TRUE;
 
 error:
-  if (core_audio->is_src && core_audio->recBufferList) {
-    buffer_list_free (core_audio->recBufferList);
-    core_audio->recBufferList = NULL;
-  }
+  buffer_list_free (core_audio->recBufferList);
+  core_audio->recBufferList = NULL;
   return FALSE;
 }
 
@@ -153,10 +153,8 @@ gst_core_audio_unitialize (GstCoreAudio * core_audio)
 {
   AudioUnitUninitialize (core_audio->audiounit);
 
-  if (core_audio->recBufferList) {
-    buffer_list_free (core_audio->recBufferList);
-    core_audio->recBufferList = NULL;
-  }
+  buffer_list_free (core_audio->recBufferList);
+  core_audio->recBufferList = NULL;
 }
 
 void

sys/osxaudio/gstosxcoreaudio.h

@@ -90,6 +90,7 @@ struct _GstCoreAudio
 
   /* For LPCM in/out */
   AudioUnit audiounit;
+  UInt32 recBufferSize; /* AudioUnitRender clobbers mDataByteSize */
   AudioBufferList *recBufferList;
 
 #ifndef HAVE_IOS

sys/osxaudio/gstosxcoreaudiocommon.c

@@ -150,18 +150,21 @@ gst_core_audio_io_proc_stop (GstCoreAudio * core_audio)
 }
 
 AudioBufferList *
-buffer_list_alloc (int channels, int size)
+buffer_list_alloc (UInt32 channels, UInt32 size, gboolean interleaved)
 {
   AudioBufferList *list;
-  int total_size;
-  int n;
+  gsize list_size;
+  UInt32 num_buffers, n;
 
-  total_size = sizeof (AudioBufferList) + 1 * sizeof (AudioBuffer);
-  list = (AudioBufferList *) g_malloc (total_size);
+  num_buffers = interleaved ? 1 : channels;
+  list_size = G_STRUCT_OFFSET (AudioBufferList, mBuffers[num_buffers]);
+  list = (AudioBufferList *) g_malloc (list_size);
 
-  list->mNumberBuffers = 1;
-  for (n = 0; n < (int) list->mNumberBuffers; ++n) {
-    list->mBuffers[n].mNumberChannels = channels;
+  list->mNumberBuffers = num_buffers;
+  for (n = 0; n < num_buffers; ++n) {
+    /* See http://lists.apple.com/archives/coreaudio-api/2015/Feb/msg00027.html */
+    list->mBuffers[n].mNumberChannels = interleaved ? channels : 1;
+    /* AudioUnitRender will keep overwriting mDataByteSize */
     list->mBuffers[n].mDataByteSize = size;
     list->mBuffers[n].mData = g_malloc (size);
   }
@@ -172,11 +175,13 @@ buffer_list_alloc (int channels, int size)
 void
 buffer_list_free (AudioBufferList * list)
 {
-  int n;
+  UInt32 n;
 
-  for (n = 0; n < (int) list->mNumberBuffers; ++n) {
-    if (list->mBuffers[n].mData)
-      g_free (list->mBuffers[n].mData);
+  if (list == NULL)
+    return;
+
+  for (n = 0; n < list->mNumberBuffers; ++n) {
+    g_free (list->mBuffers[n].mData);
   }
 
   g_free (list);

sys/osxaudio/gstosxcoreaudiocommon.h

@@ -40,7 +40,7 @@ gboolean gst_core_audio_io_proc_start                     (GstCoreAudio * core_a
 
 gboolean gst_core_audio_io_proc_stop                      (GstCoreAudio * core_audio);
 
-AudioBufferList * buffer_list_alloc                       (int channels, int size);
+AudioBufferList * buffer_list_alloc                       (UInt32 channels, UInt32 size, gboolean interleaved);
 
 void buffer_list_free                                     (AudioBufferList * list);