provide a means for cores to coordinate with the frontend when shutting down multiple threads gracefully

dynamic.c

@@ -1042,6 +1042,16 @@ static void core_performance_counter_stop(struct retro_perf_counter *perf)
       perf->total += cpu_features_get_perf_counter() - perf->start;
 }
 
+bool rarch_clear_all_thread_waits(unsigned clear_threads, void *data)
+{
+   if ( clear_threads > 0)
+      audio_driver_start(false) ;
+   else
+      audio_driver_stop() ;
+
+   return true ;
+}
+
 /**
  * rarch_environment_cb:
  * @cmd                          : Identifier of command.
@@ -1833,8 +1843,8 @@ bool rarch_environment_cb(unsigned cmd, void *data)
             int* result_p = (int*)data;
             *result_p = result;
          }
-      }
          break;
+      }
 
       case RETRO_ENVIRONMENT_GET_MIDI_INTERFACE:
       {
@@ -1849,16 +1859,21 @@ bool rarch_environment_cb(unsigned cmd, void *data)
             midi_interface->write = midi_driver_write;
             midi_interface->flush = midi_driver_flush;
          }
-      }
          break;
+      }
 
       case RETRO_ENVIRONMENT_GET_FASTFORWARDING:
       {
          extern bool runloop_fastmotion;
          *(bool *)data = runloop_fastmotion;
-      }
          break;
+      }
 
+      case RETRO_ENVIRONMENT_GET_CLEAR_ALL_THREAD_WAITS_CB:
+      {
+         *(retro_environment_t *)data = rarch_clear_all_thread_waits;
+         break;
+      }
       
       default:
          RARCH_LOG("Environ UNSUPPORTED (#%u).\n", cmd);

libretro-common/include/libretro.h

@@ -1195,6 +1195,20 @@ struct retro_led_interface
                                             * fastforwarding mode.
                                             */
 
+#define RETRO_ENVIRONMENT_SET_SAVE_STATE_IN_BACKGROUND (50 | RETRO_ENVIRONMENT_EXPERIMENTAL)
+                                            /* bool * --
+                                            * Boolean value that tells the front end to save states in the
+                                            * background or not.
+                                            */
+
+#define RETRO_ENVIRONMENT_GET_CLEAR_ALL_THREAD_WAITS_CB (51 | RETRO_ENVIRONMENT_EXPERIMENTAL)
+                                            /* retro_environment_t * --
+                                            * Provides the callback to the frontend method which will cancel
+                                            * all currently waiting threads.  Used when coordination is needed
+                                            * between the core and the frontend to gracefully stop all threads.
+                                            */
+
+
 /* Retrieves the current state of the MIDI input.
  * Returns true if it's enabled, false otherwise. */
 typedef bool (RETRO_CALLCONV *retro_midi_input_enabled_t)(void);

retroarch.h

@@ -33,8 +33,6 @@
 
 RETRO_BEGIN_DECLS
 
-#define RETRO_ENVIRONMENT_SET_SAVE_STATE_IN_BACKGROUND RETRO_ENVIRONMENT_PRIVATE+1
-
 enum rarch_ctl_state
 {
    RARCH_CTL_NONE = 0,