core: don't flush X connection before go to sleep

See the added comments for details.

Fixes #1145
Fixes #1166
Fixes #1040?

(cherry picked from commit 75d0b7ba1e)
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/event.c

@@ -48,8 +48,14 @@
 /// When top half finished, we enter the render stage, where no server state should be
 /// queried. All rendering should be done with our internal knowledge of the server state.
 ///
+/// P.S. There is another reason to avoid sending any request to the server as much as
+/// possible. To make sure requests are sent, flushes are needed. And `xcb_flush`/`XFlush`
+/// functions may read more events from the server into their queues. This is
+/// undesirable, see the comments on `handle_queued_x_events` in picom.c for more details.
 
-// TODO(yshui) the things described above
+// TODO(yshui) the things described above. This is mostly done, maybe some of
+//             the functions here is still making unnecessary queries, we need
+//             to do some auditing to be sure.
 
 /**
  * Get a window's name from window ID.
@@ -590,7 +596,7 @@ static inline void repair_win(session_t *ps, struct managed_win *w) {
 	// to make sure the X server receives the DamageSubtract request, hence the
 	// `xcb_request_check` here.
 	// Otherwise, we fetch the damage regions. That means we will receive a reply
-	// from the X server, which implies it has received our request.
+	// from the X server, which implies it has received our DamageSubtract request.
 	if (!w->ever_damaged) {
 		auto e = xcb_request_check(
 		    ps->c.c, xcb_damage_subtract(ps->c.c, w->damage, XCB_NONE, XCB_NONE));

src/picom.c

@@ -1609,9 +1609,32 @@ static void unredirect(session_t *ps) {
 	log_debug("Screen unredirected.");
 }
 
-// Handle queued events before we go to sleep
+/// Handle queued events before we go to sleep.
+///
+/// This function is called by ev_prepare watcher, which is called just before
+/// the event loop goes to sleep. X damage events are incremental, which means
+/// if we don't handle the ones X server already sent us, we won't get new ones.
+/// And if we don't get new ones, we won't render, i.e. we would freeze. libxcb
+/// keeps an internal queue of events, so we have to be 100% sure no events are
+/// left in that queue before we go to sleep.
 static void handle_queued_x_events(EV_P attr_unused, ev_prepare *w, int revents attr_unused) {
 	session_t *ps = session_ptr(w, event_check);
+	// Flush because if we go into sleep when there is still requests in the
+	// outgoing buffer, they will not be sent for an indefinite amount of
+	// time. Use XFlush here too, we might still use some Xlib functions
+	// because OpenGL.
+	//
+	// Also note, after we have flushed here, we won't flush again in this
+	// function before going into sleep. This is because `xcb_flush`/`XFlush`
+	// may _read_ more events from the server (yes, this is ridiculous, I
+	// know). And we can't have that, see the comments above this function.
+	//
+	// This means if functions called ev_handle need to send some events,
+	// they need to carefully make sure those events are flushed, one way or
+	// another.
+	XFlush(ps->c.dpy);
+	xcb_flush(ps->c.c);
+
 	if (ps->vblank_scheduler) {
 		vblank_handle_x_events(ps->vblank_scheduler);
 	}
@@ -1621,13 +1644,6 @@ static void handle_queued_x_events(EV_P attr_unused, ev_prepare *w, int revents
 		ev_handle(ps, ev);
 		free(ev);
 	};
-	// Flush because if we go into sleep when there is still
-	// requests in the outgoing buffer, they will not be sent
-	// for an indefinite amount of time.
-	// Use XFlush here too, we might still use some Xlib functions
-	// because OpenGL.
-	XFlush(ps->c.dpy);
-	xcb_flush(ps->c.c);
 	int err = xcb_connection_has_error(ps->c.c);
 	if (err) {
 		log_fatal("X11 server connection broke (error %d)", err);