coreaudio: Better handling of audio buffer queue management.

We don't fill buffers just to throw them away during shutdown now, we let the
AudioQueue free its own buffers during disposal (which fixes possible warnings
getting printed to stderr by CoreAudio), and we stop the queue after running
any queued audio during shutdown, which prevents dropping the end of the
audio playback if you opened the device with an enormous sample buffer.

Fixes Bugzilla #3555.

src/audio/coreaudio/SDL_coreaudio.m

@@ -400,8 +400,12 @@ static void
 outputCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer)
 {
     SDL_AudioDevice *this = (SDL_AudioDevice *) inUserData;
+    if (SDL_AtomicGet(&this->hidden->shutdown)) {
+        return;  /* don't do anything. */
+    }
+
     if (!SDL_AtomicGet(&this->enabled) || SDL_AtomicGet(&this->paused)) {
-        /* Supply silence if audio is enabled and not paused */
+        /* Supply silence if audio is not enabled or paused */
         SDL_memset(inBuffer->mAudioData, this->spec.silence, inBuffer->mAudioDataBytesCapacity);
     } else {
         UInt32 remaining = inBuffer->mAudioDataBytesCapacity;
@@ -430,9 +434,7 @@ outputCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffe
         }
     }
 
-    if (!SDL_AtomicGet(&this->hidden->shutdown)) {
-        AudioQueueEnqueueBuffer(this->hidden->audioQueue, inBuffer, 0, NULL);
-    }
+    AudioQueueEnqueueBuffer(this->hidden->audioQueue, inBuffer, 0, NULL);
 
     inBuffer->mAudioDataByteSize = inBuffer->mAudioDataBytesCapacity;
 }
@@ -443,7 +445,13 @@ inputCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer
               const AudioStreamPacketDescription *inPacketDescs )
 {
     SDL_AudioDevice *this = (SDL_AudioDevice *) inUserData;
-    if (SDL_AtomicGet(&this->enabled) && !SDL_AtomicGet(&this->paused)) {  /* ignore unless we're active. */
+
+    if (SDL_AtomicGet(&this->shutdown)) {
+        return;  /* don't do anything. */
+    }
+
+    /* ignore unless we're active. */
+    if (!SDL_AtomicGet(&this->paused) && SDL_AtomicGet(&this->enabled) && !SDL_AtomicGet(&this->paused)) {
         const Uint8 *ptr = (const Uint8 *) inBuffer->mAudioData;
         UInt32 remaining = inBuffer->mAudioDataByteSize;
         while (remaining > 0) {
@@ -466,9 +474,7 @@ inputCallback(void *inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer
         }
     }
 
-    if (!SDL_AtomicGet(&this->hidden->shutdown)) {
-        AudioQueueEnqueueBuffer(this->hidden->audioQueue, inBuffer, 0, NULL);
-    }
+    AudioQueueEnqueueBuffer(this->hidden->audioQueue, inBuffer, 0, NULL);
 }
 
 
@@ -514,7 +520,6 @@ static void
 COREAUDIO_CloseDevice(_THIS)
 {
     const SDL_bool iscapture = this->iscapture;
-    int i;
 
 /* !!! FIXME: what does iOS do when a bluetooth audio device vanishes? Headphones unplugged? */
 /* !!! FIXME: (we only do a "default" device on iOS right now...can we do more?) */
@@ -533,12 +538,6 @@ COREAUDIO_CloseDevice(_THIS)
     }
 
     if (this->hidden->audioQueue) {
-        for (i = 0; i < this->hidden->numAudioBuffers; i++) {
-            if (this->hidden->audioBuffer[i]) {
-                AudioQueueFreeBuffer(this->hidden->audioQueue, this->hidden->audioBuffer[i]);
-            }
-        }
-        SDL_free(this->hidden->audioBuffer);
         AudioQueueDispose(this->hidden->audioQueue, 1);
     }
 
@@ -546,6 +545,8 @@ COREAUDIO_CloseDevice(_THIS)
         SDL_DestroySemaphore(this->hidden->ready_semaphore);
     }
 
+    /* AudioQueueDispose() frees the actual buffer objects. */
+    SDL_free(this->hidden->audioBuffer);
     SDL_free(this->hidden->thread_error);
     SDL_free(this->hidden->buffer);
     SDL_free(this->hidden);
@@ -718,9 +719,9 @@ audioqueue_thread(void *arg)
     if (this->iscapture) {  /* just stop immediately for capture devices. */
         AudioQueueStop(this->hidden->audioQueue, 1);
     } else {  /* Drain off any pending playback. */
-        AudioQueueStop(this->hidden->audioQueue, 0);
         const CFTimeInterval secs = (((this->spec.size / (SDL_AUDIO_BITSIZE(this->spec.format) / 8)) / this->spec.channels) / ((CFTimeInterval) this->spec.freq)) * 2.0;
         CFRunLoopRunInMode(kCFRunLoopDefaultMode, secs, 0);
+        AudioQueueStop(this->hidden->audioQueue, 0);
     }
 
     return 0;