On Wayland, reduce amount of spurious wakeups

Mark it as breaking, since some clients relied on that behavior, simply
because dispatching clients queue always woke up a winit, meaning that
they won't be able to use user events for this sake.

CHANGELOG.md

@@ -8,6 +8,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 
 # Unreleased
 
+- **Breaking:** on Wayland, dispatching user created wayland queue won't wake up the loop unless winit has event to send back.
 - Removed platform-specific extensions that should be retrieved through `raw-window-handle` trait implementations instead:
   - `platform::windows::HINSTANCE`.
   - `WindowExtWindows::hinstance`.

src/platform_impl/linux/wayland/event_loop/mod.rs

@@ -90,8 +90,15 @@ impl<T: 'static> EventLoop<T> {
         // Register Wayland source.
         let wayland_source = WaylandSource::new(event_queue)?;
         let wayland_dispatcher =
-            calloop::Dispatcher::new(wayland_source, |_, queue, winit_state| {
-                queue.dispatch_pending(winit_state)
+            calloop::Dispatcher::new(wayland_source, |_, queue, winit_state: &mut WinitState| {
+                let result = queue.dispatch_pending(winit_state);
+                if result.is_ok()
+                    && (!winit_state.events_sink.is_empty()
+                        || !winit_state.window_compositor_updates.is_empty())
+                {
+                    winit_state.dispatched_events = true;
+                }
+                result
             });
 
         event_loop
@@ -102,21 +109,25 @@ impl<T: 'static> EventLoop<T> {
         let pending_user_events = Rc::new(RefCell::new(Vec::new()));
         let pending_user_events_clone = pending_user_events.clone();
         let (user_events_sender, user_events_channel) = calloop::channel::channel();
-        event_loop
-            .handle()
-            .insert_source(user_events_channel, move |event, _, _| {
+        event_loop.handle().insert_source(
+            user_events_channel,
+            move |event, _, winit_state: &mut WinitState| {
                 if let calloop::channel::Event::Msg(msg) = event {
+                    winit_state.dispatched_events = true;
                     pending_user_events_clone.borrow_mut().push(msg);
                 }
-            })?;
+            },
+        )?;
 
         // An event's loop awakener to wake up for window events from winit's windows.
         let (event_loop_awakener, event_loop_awakener_source) = calloop::ping::make_ping()?;
-        event_loop
-            .handle()
-            .insert_source(event_loop_awakener_source, move |_, _, _| {
+        event_loop.handle().insert_source(
+            event_loop_awakener_source,
+            move |_, _, winit_state: &mut WinitState| {
                 // No extra handling is required, we just need to wake-up.
-            })?;
+                winit_state.dispatched_events = true;
+            },
+        )?;
 
         let window_target = EventLoopWindowTarget {
             connection: connection.clone(),
@@ -220,6 +231,7 @@ impl<T: 'static> EventLoop<T> {
     where
         F: FnMut(Event<T>, &RootEventLoopWindowTarget<T>, &mut ControlFlow),
     {
+        let cause = loop {
             let start = Instant::now();
 
             // TODO(rib): remove this workaround and instead make sure that the calloop
@@ -246,7 +258,11 @@ impl<T: 'static> EventLoop<T> {
                 };
 
                 match queue.dispatch_pending(state) {
-                Ok(dispatched) => dispatched > 0,
+                    Ok(dispatched) => {
+                        state.dispatched_events |= !state.events_sink.is_empty()
+                            || !state.window_compositor_updates.is_empty();
+                        dispatched > 0
+                    }
                     Err(error) => {
                         error!("Error dispatching wayland queue: {}", error);
                         self.control_flow = ControlFlow::ExitWithCode(1);
@@ -318,6 +334,15 @@ impl<T: 'static> EventLoop<T> {
                 ControlFlow::ExitWithCode(_code) => unreachable!(),
             };
 
+            // Reduce spurious wake-ups.
+            let dispatched_events = self.with_state(|state| state.dispatched_events);
+            if matches!(cause, StartCause::WaitCancelled { .. }) && !dispatched_events {
+                continue;
+            }
+
+            break cause;
+        };
+
         self.single_iteration(&mut callback, cause);
     }
 
@@ -531,6 +556,11 @@ impl<T: 'static> EventLoop<T> {
             }
         }
 
+        // Reset the hint that we've dispatched events.
+        self.with_state(|state| {
+            state.dispatched_events = false;
+        });
+
         // This is always the last event we dispatch before poll again
         sticky_exit_callback(
             Event::AboutToWait,

src/platform_impl/linux/wayland/event_loop/sink.rs

@@ -20,6 +20,12 @@ impl EventSink {
         Default::default()
     }
 
+    /// Return `true` if there're pending events.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.window_events.is_empty()
+    }
+
     /// Add new device event to a queue.
     #[inline]
     pub fn push_device_event(&mut self, event: DeviceEvent, device_id: DeviceId) {

src/platform_impl/linux/wayland/state.rs

@@ -1,5 +1,6 @@
 use std::cell::RefCell;
 use std::error::Error;
+use std::sync::atomic::Ordering;
 use std::sync::{Arc, Mutex};
 
 use fnv::FnvHashMap;
@@ -104,6 +105,10 @@ pub struct WinitState {
 
     /// Loop handle to re-register event sources, such as keyboard repeat.
     pub loop_handle: LoopHandle<'static, Self>,
+
+    /// Whether we have dispatched events to the user thus we want to
+    /// send `AboutToWait` and normally wakeup the user.
+    pub dispatched_events: bool,
 }
 
 impl WinitState {
@@ -167,6 +172,8 @@ impl WinitState {
             monitors: Arc::new(Mutex::new(monitors)),
             events_sink: EventSink::new(),
             loop_handle,
+            // Make it true by default.
+            dispatched_events: true,
         })
     }
 
@@ -328,6 +335,18 @@ impl CompositorHandler for WinitState {
             None => return,
         };
 
+        // In case we have a redraw requested we must indicate the wake up.
+        if self
+            .window_requests
+            .get_mut()
+            .get(&window_id)
+            .unwrap()
+            .redraw_requested
+            .load(Ordering::Relaxed)
+        {
+            self.dispatched_events = true;
+        }
+
         window.lock().unwrap().frame_callback_received();
     }
 }

src/platform_impl/linux/wayland/window/mod.rs

@@ -287,11 +287,15 @@ impl Window {
 
     #[inline]
     pub fn request_redraw(&self) {
-        self.window_requests
+        if self
+            .window_requests
             .redraw_requested
-            .store(true, Ordering::Relaxed);
+            .compare_exchange(false, true, Ordering::Relaxed, Ordering::Relaxed)
+            .is_ok()
+        {
             self.event_loop_awakener.ping();
         }
+    }
 
     #[inline]
     pub fn pre_present_notify(&self) {