openal: Retry deleting a buffer until success

The rationale for this change is Apple's OpenAL implementation,
which needs a little time after the
`alSourcei(source, AL_BUFFER, 0);`
call before any buffers used by that source are free for deletion.

The defaults in the config variables are such that the OpenAL manager
will attempt to delete a buffer up to 6 times (that is, the original
attempt plus 5 reattempts), with delays of 1ms, 2ms, 4ms, 8ms, and 16ms
before each reattempt - which means it'll wait a grand total of 31ms for
a buffer to be free before assuming that some even greater problem must
be happening and giving up.

panda/src/audiotraits/config_openalAudio.cxx

@@ -30,6 +30,23 @@ ConfigVariableString openal_device
  PRC_DESC("Specify the OpenAL device string for audio playback (no quotes).  If this "
           "is not specified, the OpenAL default device is used."));
 
+ConfigVariableInt openal_buffer_delete_reattempts
+("openal-buffer-delete-reattempts", 5,
+ PRC_DESC("If deleting a buffer fails due to still being in use, the OpenAL "
+          "sound plugin will wait a moment and reattempt deletion, with an "
+          "exponentially-increasing delay for each attempt.  This number "
+          "specifies how many repeat attempts (not counting the initial attempt) "
+          "should be made before giving up and raising an error."));
+
+ConfigVariableDouble openal_buffer_delete_delay
+("openal-buffer-delete-delay", 0.001,
+ PRC_DESC("If deleting a buffer fails due to still being in use, the OpenAL "
+          "sound plugin will wait a moment and reattempt deletion, with an "
+          "exponentially-increasing delay for each attempt.  This number "
+          "specifies how long, in seconds, the OpenAL plugin will wait after "
+          "its first failed attempt.  The second attempt will be double this "
+          "delay, the third quadruple, and so on."));
+
 
 /**
  * Initializes the library.  This must be called at least once before any of

panda/src/audiotraits/config_openalAudio.h

@@ -26,5 +26,7 @@ extern "C" EXPCL_OPENAL_AUDIO void init_libOpenALAudio();
 extern "C" EXPCL_OPENAL_AUDIO Create_AudioManager_proc *get_audio_manager_func_openal_audio();
 
 extern ConfigVariableString openal_device;
+extern ConfigVariableInt openal_buffer_delete_reattempts;
+extern ConfigVariableDouble openal_buffer_delete_delay;
 
 #endif // CONFIG_OPENALAUDIO_H

panda/src/audiotraits/openalAudioManager.cxx

@@ -1052,7 +1052,7 @@ OpenALAudioManager::SoundData::
   if (_sample != 0) {
     if (_manager->_is_valid) {
       _manager->make_current();
-      alDeleteBuffers(1,&_sample);
+      _manager->delete_buffer(_sample);
     }
     _sample = 0;
   }
@@ -1128,3 +1128,42 @@ discard_excess_cache(int sample_limit) {
     delete sd;
   }
 }
+
+/**
+ * Deletes an OpenAL buffer.  This is a special function because some
+ * implementations of OpenAL (e.g. Apple's) don't unlock the buffers
+ * immediately, due to needing to coordinate with another thread.  If this is
+ * the case, the alDeleteBuffers call will error back with AL_INVALID_OPERATION
+ * as if trying to delete an actively-used buffer, which will tell us to wait a
+ * bit and try again.
+ */
+void OpenALAudioManager::
+delete_buffer(ALuint buffer) {
+  ReMutexHolder holder(_lock);
+  int attempt = 0;
+  ALuint error;
+
+  // Keep trying until we succeed (or give up).
+  while (true) {
+    alDeleteBuffers(1, &buffer);
+    error = alGetError();
+
+    if (error == AL_NO_ERROR) {
+      // Success!  This will happen right away 99% of the time.
+      return;
+    } else if (error != AL_INVALID_OPERATION) {
+      // We weren't expecting that.  This should be reported.
+      break;
+    } else if (attempt >= openal_buffer_delete_reattempts.get_value()) {
+      // We ran out of reattempts.  Give up.
+      break;
+    } else {
+      // Make another attempt after (delay * 2^n) seconds.
+      Thread::sleep(openal_buffer_delete_delay.get_value() * (1 << attempt));
+      attempt++;
+    }
+  }
+
+  // If we got here, one of the breaks above happened, indicating an error.
+  audio_error("failed to delete a buffer: " << alGetString(error) );
+}

panda/src/audiotraits/openalAudioManager.h

@@ -129,6 +129,8 @@ private:
   void decrement_client_count(SoundData *sd);
   void discard_excess_cache(int limit);
 
+  void delete_buffer(ALuint buffer);
+
   void starting_sound(OpenALAudioSound* audio);
   void stopping_sound(OpenALAudioSound* audio);
 

panda/src/audiotraits/openalAudioSound.cxx

@@ -208,8 +208,7 @@ stop() {
     for (int i=0; i<((int)(_stream_queued.size())); i++) {
       ALuint buffer = _stream_queued[i]._buffer;
       if (buffer != _sd->_sample) {
-        alDeleteBuffers(1, &buffer);
-        al_audio_errcheck("deleting a buffer");
+        _manager->delete_buffer(buffer);
       }
     }
     _stream_queued.resize(0);
@@ -472,7 +471,7 @@ pull_used_buffers() {
           correct_calibrated_clock(rtc, al);
         }
         if (buffer != _sd->_sample) {
-          alDeleteBuffers(1,&buffer);
+          _manager->delete_buffer(buffer);
         }
       }
     } else {