Bugfix: Properly integrate libxcb’s event loop into libev.

Fixes a race condition with GIMP (where it configured its window,
sent the map request and waited for the window to get mapped, but
i3 didn’t get the event until another one was sent (key binding for
example)).

The new solution is much better as it properly hands off all
the work to xcb_poll_for_event.

Inspired by awesome, which uses the same mechanism. Thanks.

diff --git a/src/mainx.c b/src/mainx.c
index ca1bc8ad7ce8bf042786042be80e0c5abc1ca1b4..a832f0985c622d594d696d0f0e4a07e3918b0f8b 100644 (file)
--- a/src/mainx.c
+++ b/src/mainx.c
@@ -72,31 +72,34 @@ xcb_atom_t atoms[NUM_ATOMS];
 int num_screens = 0;
 
 /*
- * Callback for activity on the connection to the X server
+ * This callback is only a dummy, see xcb_prepare_cb and xcb_check_cb.
+ * See also man libev(3): "ev_prepare" and "ev_check" - customise your event loop
  *
  */
 static void xcb_got_event(EV_P_ struct ev_io *w, int revents) {
+        /* empty, because xcb_prepare_cb and xcb_check_cb are used */
+}
+
+/*
+ * Flush before blocking (and waiting for new events)
+ *
+ */
+static void xcb_prepare_cb(EV_P_ ev_prepare *w, int revents) {
+        xcb_flush(evenths.c);
+}
+
+/*
+ * Instead of polling the X connection socket we leave this to
+ * xcb_poll_for_event() which knows better than we can ever know.
+ *
+ */
+static void xcb_check_cb(EV_P_ ev_check *w, int revents) {
         xcb_generic_event_t *event;
 
-        /* When an event is available… */
         while ((event = xcb_poll_for_event(evenths.c)) != NULL) {
-                /* …we handle all events in a row: */
-                do {
-                        xcb_event_handle(&evenths, event);
-                        xcb_aux_sync(evenths.c);
-                        free(event);
-                } while ((event = xcb_poll_for_event(evenths.c)));
-
-                /* Make sure all replies are handled/discarded */
-                xcb_aux_sync(evenths.c);
-
-                /* Afterwards, there may be new events available which would
-                 * not trigger the select() (libev) immediately, so we check
-                 * again (and don’t bail out of the loop). */
+                xcb_event_handle(&evenths, event);
+                free(event);
         }
-
-        /* Make sure all replies are handled/discarded */
-        xcb_aux_sync(evenths.c);
 }
 
 int main(int argc, char *argv[], char *env[]) {
@@ -190,6 +193,27 @@ int main(int argc, char *argv[], char *env[]) {
         }
         /* end of ugliness */
 
+        /* Initialize event loop using libev */
+        struct ev_loop *loop = ev_default_loop(0);
+        if (loop == NULL)
+                die("Could not initialize libev. Bad LIBEV_FLAGS?\n");
+
+        struct ev_io *xcb_watcher = scalloc(sizeof(struct ev_io));
+        struct ev_check *xcb_check = scalloc(sizeof(struct ev_check));
+        struct ev_prepare *xcb_prepare = scalloc(sizeof(struct ev_prepare));
+
+        ev_io_init(xcb_watcher, xcb_got_event, xcb_get_file_descriptor(conn), EV_READ);
+        ev_io_start(loop, xcb_watcher);
+
+        ev_check_init(xcb_check, xcb_check_cb);
+        ev_check_start(loop, xcb_check);
+
+        ev_prepare_init(xcb_prepare, xcb_prepare_cb);
+        ev_prepare_start(loop, xcb_prepare);
+
+        /* Grab the server to delay any events until we enter the eventloop */
+        xcb_grab_server(conn);
+
         xcb_event_handlers_init(conn, &evenths);
 
         /* DEBUG: Trap all events and print them */
@@ -351,20 +375,8 @@ int main(int argc, char *argv[], char *env[]) {
                 c_ws = &workspaces[screen->current_workspace];
         }
 
-
-        /* Initialize event loop using libev */
-        struct ev_loop *loop = ev_default_loop(0);
-        if (loop == NULL)
-                die("Could not initialize libev. Bad LIBEV_FLAGS?\n");
-
-        struct ev_io *xcb_watcher = scalloc(sizeof(struct ev_io));
-        ev_io_init(xcb_watcher, xcb_got_event, xcb_get_file_descriptor(conn), EV_READ);
-
-        /* Call the handler to work all events which arrived before the libev-stuff was set up */
-        xcb_got_event(NULL, xcb_watcher, 0);
-
-        /* Enter the libev eventloop */
-        ev_io_start(loop, xcb_watcher);
+        /* Ungrab the server to receive events and enter libev’s eventloop */
+        xcb_ungrab_server(conn);
         ev_loop(loop, 0);
 
         /* not reached */