Rework SET_AUDIO_CALLBACK.

When disabling audio and/or muting, the core has no way
of knowing this, causing potential issues in a threaded mixer.

This environ is not used by any public core to my knowledge so the old
API has been removed and replaced with a change to environment number
(20 -> 22).

audio/thread_wrapper.c

@@ -57,7 +57,7 @@ static void audio_thread_loop(void *data)
       }
 
       slock_unlock(thr->lock);
-      g_extern.system.audio_callback();
+      g_extern.system.audio_callback.callback();
    }
 }
 
@@ -98,12 +98,14 @@ static bool audio_thread_stop(void *data)
 {
    audio_thread_t *thr = (audio_thread_t*)data;
    audio_thread_block(thr);
+   g_extern.system.audio_callback.set_state(false);
    return true;
 }
 
 static bool audio_thread_start(void *data)
 {
    audio_thread_t *thr = (audio_thread_t*)data;
+   g_extern.system.audio_callback.set_state(true);
    audio_thread_unblock(thr);
    return true;
 }

driver.c

@@ -512,7 +512,7 @@ void init_audio(void)
 
 #ifdef HAVE_THREADS
    find_audio_driver();
-   if (g_extern.system.audio_callback)
+   if (g_extern.system.audio_callback.callback)
    {
       RARCH_LOG("Starting threaded audio driver ...\n");
       if (!rarch_threaded_audio_init(&driver.audio, &driver.audio_data,
@@ -566,7 +566,7 @@ void init_audio(void)
    rarch_assert(g_extern.audio_data.outsamples = (float*)malloc(outsamples_max * sizeof(float)));
 
    g_extern.audio_data.rate_control = false;
-   if (!g_extern.system.audio_callback && g_extern.audio_active && g_settings.audio.rate_control)
+   if (!g_extern.system.audio_callback.callback && g_extern.audio_active && g_settings.audio.rate_control)
    {
       if (driver.audio->buffer_size && driver.audio->write_avail)
       {
@@ -586,7 +586,7 @@ void init_audio(void)
 
    g_extern.measure_data.buffer_free_samples_count = 0;
 
-   if (g_extern.audio_active && !g_extern.audio_data.mute && g_extern.system.audio_callback) // Threaded driver is initially stopped.
+   if (g_extern.audio_active && !g_extern.audio_data.mute && g_extern.system.audio_callback.callback) // Threaded driver is initially stopped.
       audio_start_func();
 }
 

dynamic.c

@@ -737,7 +737,7 @@ bool rarch_environment_cb(unsigned cmd, void *data)
             return false;
 #endif
 
-         g_extern.system.audio_callback = info->callback;
+         g_extern.system.audio_callback = *info;
          break;
       }
 #endif

general.h

@@ -396,7 +396,8 @@ struct global
       char valid_extensions[PATH_MAX];
       
       retro_keyboard_event_t key_event;
-      retro_audio_callback_t audio_callback;
+
+      struct retro_audio_callback audio_callback;
 
       struct retro_disk_control_callback disk_control; 
       struct retro_hw_render_callback hw_render_callback;

libretro.h

@@ -489,7 +489,10 @@ enum retro_mod
                                            // NULL is returned if the libretro was loaded statically (i.e. linked statically to frontend), or if the path cannot be determined.
                                            // Mostly useful in cooperation with SET_SUPPORT_NO_GAME as assets can be loaded without ugly hacks.
                                            //
-#define RETRO_ENVIRONMENT_SET_AUDIO_CALLBACK 20
+                                           //
+// Environment 20 was an obsolete version of SET_AUDIO_CALLBACK. It was not used by any known core at the time,
+// and was removed from the API.
+#define RETRO_ENVIRONMENT_SET_AUDIO_CALLBACK 22
                                            // const struct retro_audio_callback * --
                                            // Sets an interface which is used to notify a libretro core about audio being available for writing.
                                            // The callback can be called from any thread, so a core using this must have a thread safe audio implementation.
@@ -515,9 +518,15 @@ enum retro_mod
 
 // Notifies libretro that audio data should be written.
 typedef void (*retro_audio_callback_t)(void);
+
+// True: Audio driver in frontend is active, and callback is expected to be called regularily.
+// False: Audio driver in frontend is paused or inactive. Audio callback will not be called until set_state has been called with true.
+// Initial state is false (inactive).
+typedef void (*retro_audio_set_state_callback_t)(bool enabled);
 struct retro_audio_callback
 {
    retro_audio_callback_t callback;
+   retro_audio_set_state_callback_t set_state;
 };
 
 // Notifies a libretro core of time spent since last invocation of retro_run() in microseconds.

retroarch.c

@@ -1434,7 +1434,7 @@ void rarch_init_rewind(void)
    if (!g_settings.rewind_enable || g_extern.state_manager)
       return;
 
-   if (g_extern.system.audio_callback)
+   if (g_extern.system.audio_callback.callback)
    {
       RARCH_ERR("Implementation uses threaded audio. Cannot use rewind.\n");
       return;