pulseaudio: fix WaitForPulseOperation blocking longer than necessary.

Now the operation state change we're waiting on will signal the
threaded mainloop, so this doesn't wait longer than necessary.

This requires PulseAudio 4.0 or later, so don't merge this into SDL2,
which requires PulseAudio 0.9.15.

Fixes #7971.

src/audio/pulseaudio/SDL_pulseaudio.c

@@ -70,6 +70,7 @@ static void (*PULSEAUDIO_pa_threaded_mainloop_free)(pa_threaded_mainloop *);
 
 static pa_operation_state_t (*PULSEAUDIO_pa_operation_get_state)(
     const pa_operation *);
+static void (*PULSEAUDIO_pa_operation_set_state_callback)(pa_operation *, pa_operation_notify_cb_t, void *);
 static void (*PULSEAUDIO_pa_operation_cancel)(pa_operation *);
 static void (*PULSEAUDIO_pa_operation_unref)(pa_operation *);
 
@@ -191,6 +192,7 @@ static int load_pulseaudio_syms(void)
     SDL_PULSEAUDIO_SYM(pa_threaded_mainloop_free);
     SDL_PULSEAUDIO_SYM(pa_operation_get_state);
     SDL_PULSEAUDIO_SYM(pa_operation_cancel);
+    SDL_PULSEAUDIO_SYM(pa_operation_set_state_callback);
     SDL_PULSEAUDIO_SYM(pa_operation_unref);
     SDL_PULSEAUDIO_SYM(pa_context_new);
     SDL_PULSEAUDIO_SYM(pa_context_set_state_callback);
@@ -266,14 +268,19 @@ static const char *getAppName(void)
     return retval;
 }
 
-/* This function assume you are holding `mainloop`'s lock and that `o` has a callback that will signal pulseaudio_threaded_mainloop.
-   The caller may optionally call pa_threaded_mainloop_accept() if the signal is blocking. The operation is
-   unref'd in here, assuming you did the work in the callback and just want to know it's done, though. */
+static void OperationStateChangeCallback(pa_operation *o, void *userdata)
+{
+    PULSEAUDIO_pa_threaded_mainloop_signal(pulseaudio_threaded_mainloop, 0);  // just signal any waiting code, it can look up the details.
+}
+
+/* This function assume you are holding `mainloop`'s lock. The operation is unref'd in here, assuming
+   you did the work in the callback and just want to know it's done, though. */
 static void WaitForPulseOperation(pa_operation *o)
 {
     /* This checks for NO errors currently. Either fix that, check results elsewhere, or do things you don't care about. */
     SDL_assert(pulseaudio_threaded_mainloop != NULL);
     if (o) {
+        PULSEAUDIO_pa_operation_set_state_callback(o, OperationStateChangeCallback, NULL);
         while (PULSEAUDIO_pa_operation_get_state(o) == PA_OPERATION_RUNNING) {
             PULSEAUDIO_pa_threaded_mainloop_wait(pulseaudio_threaded_mainloop);  /* this releases the lock and blocks on an internal condition variable. */
         }