client API: add mpv_wait_async_requests()

This does what it's documented to do.

The implementation reuses the code in mpv_detach_destroy(). Due to the
way async requests currently work, just sending a synchronous dummy
request (like a "ignore" command) would be enough to ensure
synchronization, but this code will continue to work even if this
changes.

The line "ctx->event_mask = 0;" is removed, but it shouldn't be needed.
(If a client is somehow very slow to terminate, this could silence an
annoying queue overflow message, but all in all it does nothing.)

Calling mpv_wait_async_requests() and mpv_wait_event() concurrently is
in theory allowed, so change pthread_cond_signal() to
pthread_cond_broadcast() to avoid missed wakeups.

As requested in issue #1542.

libmpv/client.h

@@ -167,7 +167,7 @@ extern "C" {
  * relational operators (<, >, <=, >=).
  */
 #define MPV_MAKE_VERSION(major, minor) (((major) << 16) | (minor) | 0UL)
-#define MPV_CLIENT_API_VERSION MPV_MAKE_VERSION(1, 13)
+#define MPV_CLIENT_API_VERSION MPV_MAKE_VERSION(1, 14)
 
 /**
  * Return the MPV_CLIENT_API_VERSION the mpv source has been compiled with.
@@ -1478,6 +1478,20 @@ void mpv_set_wakeup_callback(mpv_handle *ctx, void (*cb)(void *d), void *d);
  */
 int mpv_get_wakeup_pipe(mpv_handle *ctx);
 
+/**
+ * Block until all asynchronous requests are done. This affects functions like
+ * mpv_command_async(), which return immediately and return their result as
+ * events.
+ *
+ * This is a helper, and somewhat equivalent to calling mpv_wait_event() in a
+ * loop until all known asynchronous requests have sent their reply as event,
+ * except that the event queue is not emptied.
+ *
+ * In case you called mpv_suspend() before, this will also forcibly reset the
+ * suspend counter of the given handle.
+ */
+void mpv_wait_async_requests(mpv_handle *ctx);
+
 typedef enum mpv_sub_api {
     /**
      * For using mpv's OpenGL renderer on an external OpenGL context.

libmpv/mpv.def

@@ -38,5 +38,6 @@ mpv_set_wakeup_callback
 mpv_suspend
 mpv_terminate_destroy
 mpv_unobserve_property
+mpv_wait_async_requests
 mpv_wait_event
 mpv_wakeup