pulseaudio: Some fixes that seem to avoid an assertion in libpulse.

- Make sure the hotplug thread has hit its main loop before letting
  DetectDevices continue.
- Don't unref the context subscription operation until it completes
  (or we are shutting down).

I'm not sure which change fixed the problem, but at least one of them
appears to have done so.

Reference Issue #7971.

src/audio/pulseaudio/SDL_pulseaudio.c

@@ -855,16 +855,29 @@ static void HotplugCallback(pa_context *c, pa_subscription_event_type_t t, uint3
 /* this runs as a thread while the Pulse target is initialized to catch hotplug events. */
 static int SDLCALL HotplugThread(void *data)
 {
+    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);
 
     /* don't WaitForPulseOperation on the subscription; when it's done we'll be able to get hotplug events, but waiting doesn't changing anything. */
-    PULSEAUDIO_pa_operation_unref(PULSEAUDIO_pa_context_subscribe(pulseaudio_context, PA_SUBSCRIPTION_MASK_SINK | PA_SUBSCRIPTION_MASK_SOURCE, NULL, NULL));
+    op = PULSEAUDIO_pa_context_subscribe(pulseaudio_context, PA_SUBSCRIPTION_MASK_SINK | PA_SUBSCRIPTION_MASK_SOURCE, NULL, NULL);
+
+    SDL_SemPost(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;
+        }
     }
+
+    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;
@@ -872,6 +885,8 @@ static int SDLCALL HotplugThread(void *data)
 
 static void PULSEAUDIO_DetectDevices()
 {
+    SDL_sem *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)));
@@ -880,7 +895,9 @@ static void PULSEAUDIO_DetectDevices()
 
     /* 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_SemWait(ready_sem);
+    SDL_DestroySemaphore(ready_sem);
 }
 
 static int PULSEAUDIO_GetDefaultAudioInfo(char **name, SDL_AudioSpec *spec, int iscapture)