Move XCB event handling into xcb_prepare_cb.

Previously, we used ev_check watchers, which are executed at the beginning of an
event loop iteration.

This was problematic if one of the handlers happened to fill the XCB event
queue, e.g. by reading a reply from X11 and an event happened in the meantime.

In that situation, we would hand control to the event loop, entirely ignoring
the pending event. This would manifest itself as a 1-minute hang,
reproducible (sometimes) in the i3 testsuite.

issue #2790 describes an instance of this issue in i3bar, and we fixed that by
changing the watcher priority to run last. Handling events in xcb_prepare_cb has
the same effect, as ev_prepare watchers are run just before the event loop goes
to sleep.

i3bar/src/xcb.c

@@ -83,7 +83,6 @@ int mod_pressed = 0;
 
 /* Event watchers, to interact with the user */
 ev_prepare *xcb_prep;
-ev_check *xcb_chk;
 ev_io *xcb_io;
 ev_io *xkb_io;
 
@@ -1075,21 +1074,11 @@ static void handle_resize_request(xcb_resize_request_event_t *event) {
 }
 
 /*
- * This function is called immediately before the main loop locks. We flush xcb
- * then (and only then)
+ * This function is called immediately before the main loop locks. We check for
+ * events from X11, handle them, then flush our outgoing queue.
  *
  */
 void xcb_prep_cb(struct ev_loop *loop, ev_prepare *watcher, int revents) {
-    xcb_flush(xcb_connection);
-}
-
-/*
- * This function is called immediately after the main loop locks, so when one
- * of the watchers registered an event.
- * We check whether an X-Event arrived and handle it.
- *
- */
-void xcb_chk_cb(struct ev_loop *loop, ev_check *watcher, int revents) {
     xcb_generic_event_t *event;
 
     if (xcb_connection_has_error(xcb_connection)) {
@@ -1210,6 +1199,8 @@ void xcb_chk_cb(struct ev_loop *loop, ev_check *watcher, int revents) {
         }
         free(event);
     }
+
+    xcb_flush(xcb_connection);
 }
 
 /*
@@ -1267,21 +1258,12 @@ char *init_xcb_early() {
     /* The various watchers to communicate with xcb */
     xcb_io = smalloc(sizeof(ev_io));
     xcb_prep = smalloc(sizeof(ev_prepare));
-    xcb_chk = smalloc(sizeof(ev_check));
 
     ev_io_init(xcb_io, &xcb_io_cb, xcb_get_file_descriptor(xcb_connection), EV_READ);
     ev_prepare_init(xcb_prep, &xcb_prep_cb);
-    ev_check_init(xcb_chk, &xcb_chk_cb);
-
-    /* Within an event loop iteration, run the xcb_chk watcher last: other
-     * watchers might call xcb_flush(), which, unexpectedly, can also read
-     * events into the queue (see _xcb_conn_wait). Hence, we need to drain xcb’s
-     * queue last, otherwise we risk dead-locking. */
-    ev_set_priority(xcb_chk, EV_MINPRI);
 
     ev_io_start(main_loop, xcb_io);
     ev_prepare_start(main_loop, xcb_prep);
-    ev_check_start(main_loop, xcb_chk);
 
     /* Now we get the atoms and save them in a nice data structure */
     get_atoms();
@@ -1535,11 +1517,9 @@ void clean_xcb(void) {
     xcb_aux_sync(xcb_connection);
     xcb_disconnect(xcb_connection);
 
-    ev_check_stop(main_loop, xcb_chk);
     ev_prepare_stop(main_loop, xcb_prep);
     ev_io_stop(main_loop, xcb_io);
 
-    FREE(xcb_chk);
     FREE(xcb_prep);
     FREE(xcb_io);
 }