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Create an event source registered with wl_event_source_check, for instance an event source for a Wayland connection to a remote compositor (code taken from Weston):

static int dispatch_events(int fd, uint32_t mask, void *data) {
	int count = 0;
	if (mask & WL_EVENT_READABLE) {
		count = wl_display_dispatch(wl->remote_display);
	}
	if (mask & WL_EVENT_WRITABLE) {
		wl_display_flush(wl->remote_display);
	}
	if (mask == 0) {
		count = wl_display_dispatch_pending(wl->remote_display);
		wl_display_flush(wl->remote_display);
	}
	return count;
}

[…]

int fd = wl_display_get_fd(remote_display);
uint32_t events = WL_EVENT_READABLE;
struct wl_event_source *event_source = wl_event_loop_add_fd(loop, fd, events, dispatch_events, NULL);
wl_event_source_check(event_source);

Setup an idle source, and send a request to the remote compositor from the idle source:

static void handle_idle(void *data) {
	wl_display_sync(remote_display);
}

wl_event_loop_add_idle(loop, handle_idle, NULL);

The request gets queued to the Wayland internal buffer, because WAYLAND_DEBUG=client shows it. The request doesn't get flushed to the parent compositor, because WAYLAND_DEBUG=server doesn't show it. Adding wl_display_flush after sending the request fixes the bug.

It seems like wl_event_loop_dispatch dispatches idle sources twice since 5ec8062d. However, checked sources are only dispatched after the second idle source dispatch, not the first. Should we dispatch checked sources twice too?