wayland: instantly wake up if events are pending (#1153)

Just before starting to poll/wait on calloop(mio), check if there
are already events pending in the internal buffer of our wayland
event queue. If so, dispatch them and force an instant wakeup from
the polling, in order to behave as if we were instantly woken up by
incoming wayland events.

When using OpenGL, mesa shares our wayland socket, and also reads
from it, especially if vsync is enabled as it'll do blocking reads.
When doing so, it may enqueue events in the internal buffer of our
event queue.

As the socket has been read, mio will thus not notify it to calloop
as read, and thus calloop will not know it needs to dispatch. In some
cases this can lead to some events being delivered much later than
they should. Combined with key repetition this can actually cause some
flooding of the event queue making this effect event worse.

Fixes #1148

src/platform_impl/linux/wayland/event_loop.rs

@@ -314,6 +314,25 @@ impl<T: 'static> EventLoop<T> {
             // send pending events to the server
             self.display.flush().expect("Wayland connection lost.");
 
+            // During the run of the user callback, some other code monitoring and reading the
+            // wayland socket may have been run (mesa for example does this with vsync), if that
+            // is the case, some events may have been enqueued in our event queue.
+            //
+            // If some messages are there, the event loop needs to behave as if it was instantly
+            // woken up by messages arriving from the wayland socket, to avoid getting stuck.
+            let instant_wakeup = {
+                let window_target = match self.window_target.p {
+                    crate::platform_impl::EventLoopWindowTarget::Wayland(ref wt) => wt,
+                    _ => unreachable!(),
+                };
+                let dispatched = window_target
+                    .evq
+                    .borrow_mut()
+                    .dispatch_pending()
+                    .expect("Wayland connection lost.");
+                dispatched > 0
+            };
+
             match control_flow {
                 ControlFlow::Exit => break,
                 ControlFlow::Poll => {
@@ -328,7 +347,12 @@ impl<T: 'static> EventLoop<T> {
                     );
                 }
                 ControlFlow::Wait => {
-                    self.inner_loop.dispatch(None, &mut ()).unwrap();
+                    let timeout = if instant_wakeup {
+                        Some(::std::time::Duration::from_millis(0))
+                    } else {
+                        None
+                    };
+                    self.inner_loop.dispatch(timeout, &mut ()).unwrap();
                     callback(
                         crate::event::Event::NewEvents(crate::event::StartCause::WaitCancelled {
                             start: Instant::now(),
@@ -341,7 +365,7 @@ impl<T: 'static> EventLoop<T> {
                 ControlFlow::WaitUntil(deadline) => {
                     let start = Instant::now();
                     // compute the blocking duration
-                    let duration = if deadline > start {
+                    let duration = if deadline > start && !instant_wakeup {
                         deadline - start
                     } else {
                         ::std::time::Duration::from_millis(0)