screencast: relax buffer flip

In case a client doesn't return a buffer early enough we can give it a
second chance by triggering on_process before we pass the buffer to
wlroots in the frame_buffer_done event.

include/screencast_common.h

@@ -131,6 +131,7 @@ struct xdpw_screencast_instance {
 	bool with_cursor;
 	int err;
 	bool quit;
+	bool need_buffer;
 
 	// fps limit
 	struct fps_limit_state fps_limit;

src/screencast/pipewire_screencast.c

@@ -44,6 +44,18 @@ static struct spa_pod *build_format(struct spa_pod_builder *b, enum spa_video_fo
 	return spa_pod_builder_pop(b, &f[0]);
 }
 
+static void pwr_handle_stream_on_process(void *data) {
+	logprint(TRACE, "pipewire: stream process");
+	struct xdpw_screencast_instance *cast = data;
+
+	if (cast->need_buffer) {
+		xdpw_pwr_dequeue_buffer(cast);
+		if (cast->current_frame.pw_buffer) {
+			cast->need_buffer = false;
+		}
+	}
+}
+
 static void pwr_handle_stream_state_changed(void *data,
 		enum pw_stream_state old, enum pw_stream_state state, const char *error) {
 	struct xdpw_screencast_instance *cast = data;
@@ -166,6 +178,7 @@ static const struct pw_stream_events pwr_stream_events = {
 	.param_changed = pwr_handle_stream_param_changed,
 	.add_buffer = pwr_handle_stream_add_buffer,
 	.remove_buffer = pwr_handle_stream_remove_buffer,
+	.process = pwr_handle_stream_on_process,
 };
 
 void xdpw_pwr_dequeue_buffer(struct xdpw_screencast_instance *cast) {
@@ -240,7 +253,11 @@ void xdpw_pwr_swap_buffer(struct xdpw_screencast_instance *cast) {
 
 dequeue_buffer:
 	assert(!cast->current_frame.pw_buffer);
+	cast->need_buffer = false;
 	xdpw_pwr_dequeue_buffer(cast);
+	if (!cast->current_frame.pw_buffer) {
+		cast->need_buffer = true;
+	}
 }
 
 void pwr_update_stream_param(struct xdpw_screencast_instance *cast) {

src/screencast/screencast.c

@@ -70,6 +70,7 @@ void xdpw_screencast_instance_init(struct xdpw_screencast_context *ctx,
 	cast->with_cursor = with_cursor;
 	cast->refcount = 1;
 	cast->node_id = SPA_ID_INVALID;
+	cast->need_buffer = false;
 	wl_list_init(&cast->buffer_list);
 	logprint(INFO, "xdpw: screencast instance %p has %d references", cast, cast->refcount);
 	wl_list_insert(&ctx->screencast_instances, &cast->link);

src/screencast/wlr_screencast.c

@@ -130,6 +130,7 @@ static void wlr_frame_buffer_done(void *data,
 		xdpw_pwr_dequeue_buffer(cast);
 	}
 
+	cast->need_buffer = false;
 	if (!cast->current_frame.xdpw_buffer) {
 		logprint(WARN, "wlroots: no current buffer");
 		xdpw_wlr_frame_finish(cast);