pulseaudio: Be more aggressive with hotplug thread synchronization.

(Borrowed from the SDL2 branch.)

src/audio/pulseaudio/SDL_pulseaudio.c

@@ -847,28 +847,47 @@ static int SDLCALL HotplugThread(void *data)
 {
     uint32_t prev_default_sink_index = default_sink_index;
     uint32_t prev_default_source_index = default_source_index;
+    pa_operation *op;
 
     SDL_SetThreadPriority(SDL_THREAD_PRIORITY_LOW);
     PULSEAUDIO_pa_threaded_mainloop_lock(pulseaudio_threaded_mainloop);
     PULSEAUDIO_pa_context_set_subscribe_callback(pulseaudio_context, HotplugCallback, NULL);
-    PULSEAUDIO_pa_operation_unref(PULSEAUDIO_pa_context_subscribe(pulseaudio_context, PA_SUBSCRIPTION_MASK_SINK | PA_SUBSCRIPTION_MASK_SOURCE | PA_SUBSCRIPTION_MASK_SERVER, NULL, NULL));
+
+    /* don't WaitForPulseOperation on the subscription; when it's done we'll be able to get hotplug events, but waiting doesn't changing anything. */
+    op = PULSEAUDIO_pa_context_subscribe(pulseaudio_context, PA_SUBSCRIPTION_MASK_SINK | PA_SUBSCRIPTION_MASK_SOURCE | PA_SUBSCRIPTION_MASK_SERVER, NULL, NULL);
+
+    SDL_PostSemaphore((SDL_Semaphore *) data);
+
     while (SDL_AtomicGet(&pulseaudio_hotplug_thread_active)) {
         PULSEAUDIO_pa_threaded_mainloop_wait(pulseaudio_threaded_mainloop);
+        if (op && PULSEAUDIO_pa_operation_get_state(op) != PA_OPERATION_RUNNING) {
+            PULSEAUDIO_pa_operation_unref(op);
+            op = NULL;
+        }
 
         // Update default devices; don't hold the pulse lock during this, since it could deadlock vs a playing device that we're about to lock here.
         PULSEAUDIO_pa_threaded_mainloop_unlock(pulseaudio_threaded_mainloop);
         CheckDefaultDevice(&prev_default_sink_index, default_sink_index);
         CheckDefaultDevice(&prev_default_source_index, default_source_index);
         PULSEAUDIO_pa_threaded_mainloop_lock(pulseaudio_threaded_mainloop);
+
     }
 
-    PULSEAUDIO_pa_context_set_subscribe_callback(pulseaudio_context, NULL, NULL);  // Don't fire HotplugCallback again.
+    if (op) {
+        PULSEAUDIO_pa_operation_unref(op);
+    }
+
+    PULSEAUDIO_pa_context_set_subscribe_callback(pulseaudio_context, NULL, NULL);
     PULSEAUDIO_pa_threaded_mainloop_unlock(pulseaudio_threaded_mainloop);
     return 0;
+
+
 }
 
 static void PULSEAUDIO_DetectDevices(SDL_AudioDevice **default_output, SDL_AudioDevice **default_capture)
 {
+    SDL_Semaphore *ready_sem = SDL_CreateSemaphore(0);
+
     PULSEAUDIO_pa_threaded_mainloop_lock(pulseaudio_threaded_mainloop);
     WaitForPulseOperation(PULSEAUDIO_pa_context_get_server_info(pulseaudio_context, ServerInfoCallback, NULL));
     WaitForPulseOperation(PULSEAUDIO_pa_context_get_sink_info_list(pulseaudio_context, SinkInfoCallback, (void *)((intptr_t)SDL_TRUE)));
@@ -888,7 +907,9 @@ static void PULSEAUDIO_DetectDevices(SDL_AudioDevice **default_output, SDL_Audio
 
     /* ok, we have a sane list, let's set up hotplug notifications now... */
     SDL_AtomicSet(&pulseaudio_hotplug_thread_active, 1);
-    pulseaudio_hotplug_thread = SDL_CreateThreadInternal(HotplugThread, "PulseHotplug", 256 * 1024, NULL);  /* !!! FIXME: this can probably survive in significantly less stack space. */
+    pulseaudio_hotplug_thread = SDL_CreateThreadInternal(HotplugThread, "PulseHotplug", 256 * 1024, ready_sem);  // !!! FIXME: this can probably survive in significantly less stack space.
+    SDL_WaitSemaphore(ready_sem);
+    SDL_DestroySemaphore(ready_sem);
 }
 
 static void PULSEAUDIO_Deinitialize(void)