Support waking up headless window event loops while they are sleeping.

ports/glutin/events_loop.rs

@@ -7,27 +7,36 @@
 
 use glutin;
 use servo::embedder_traits::EventLoopWaker;
-use std::sync::Arc;
+use std::sync::{Arc, Condvar, Mutex};
 use std::rc::Rc;
 use std::cell::RefCell;
-use std::thread;
 use std::time;
 
-pub struct EventsLoop(Option<glutin::EventsLoop>);
+#[allow(dead_code)]
+enum EventLoop {
+    /// A real Glutin windowing event loop.
+    Glutin(Option<glutin::EventsLoop>),
+    /// A fake event loop which contains a signalling flag used to ensure
+    /// that pending events get processed in a timely fashion, and a condition
+    /// variable to allow waiting on that flag changing state.
+    Headless(Arc<(Mutex<bool>, Condvar)>),
+}
+
+pub struct EventsLoop(EventLoop);
 
 impl EventsLoop {
     // Ideally, we could use the winit event loop in both modes,
     // but on Linux, the event loop requires a X11 server.
     #[cfg(not(target_os = "linux"))]
     pub fn new(_headless: bool) -> Rc<RefCell<EventsLoop>> {
-        Rc::new(RefCell::new(EventsLoop(Some(glutin::EventsLoop::new()))))
+        Rc::new(RefCell::new(EventsLoop(EventLoop::Glutin(Some(glutin::EventsLoop::new())))))
     }
     #[cfg(target_os = "linux")]
     pub fn new(headless: bool) -> Rc<RefCell<EventsLoop>> {
         let events_loop = if headless {
-            None
+            EventLoop::Headless(Arc::new((Mutex::new(false), Condvar::new())))
         } else {
-            Some(glutin::EventsLoop::new())
+            EventLoop::Glutin(Some(glutin::EventsLoop::new()))
         };
         Rc::new(RefCell::new(EventsLoop(events_loop)))
     }
@@ -35,39 +44,79 @@ impl EventsLoop {
 
 impl EventsLoop {
     pub fn create_event_loop_waker(&self) -> Box<dyn EventLoopWaker> {
-        if let Some(ref events_loop) = self.0 {
-            Box::new(HeadedEventLoopWaker::new(&events_loop))
-        } else {
-            Box::new(HeadlessEventLoopWaker)
+        match self.0 {
+            EventLoop::Glutin(ref events_loop) => {
+                let events_loop = events_loop
+                    .as_ref()
+                    .expect("Can't create waker for unavailable event loop.");
+                Box::new(HeadedEventLoopWaker::new(&events_loop))
+            },
+            EventLoop::Headless(ref data) =>
+                Box::new(HeadlessEventLoopWaker(data.clone())),
         }
     }
     pub fn as_winit(&self) -> &glutin::EventsLoop {
-        &self.0.as_ref().expect("Can't access winit event loop while using the fake headless event loop")
+        match self.0 {
+            EventLoop::Glutin(Some(ref event_loop)) => event_loop,
+            EventLoop::Glutin(None) | EventLoop::Headless(..) =>
+                panic!("Can't access winit event loop while using the fake headless event loop"),
+        }
     }
     pub fn take(&mut self) -> Option<glutin::EventsLoop> {
-        self.0.take()
+        match self.0 {
+            EventLoop::Glutin(ref mut event_loop) => event_loop.take(),
+            EventLoop::Headless(..) => None,
+        }
     }
     pub fn poll_events<F>(&mut self, callback: F) where F: FnMut(glutin::Event) {
-        if let Some(ref mut events_loop) = self.0 {
-            events_loop.poll_events(callback);
-        } else {
-            self.sleep();
+        match self.0 {
+            EventLoop::Glutin(Some(ref mut events_loop)) => events_loop.poll_events(callback),
+            EventLoop::Glutin(None) => (),
+            EventLoop::Headless(ref data) => {
+                // This is subtle - the use of the event loop in App::run_loop
+                // optionally calls run_forever, then always calls poll_events.
+                // If our signalling flag is true before we call run_forever,
+                // we don't want to reset it before poll_events is called or
+                // we'll end up sleeping even though there are events waiting
+                // to be handled. We compromise by only resetting the flag here
+                // in poll_events, so that both poll_events and run_forever can
+                // check it first and avoid sleeping unnecessarily.
+                self.sleep(&data.0, &data.1);
+                *data.0.lock().unwrap() = false;
+            }
         }
     }
     pub fn run_forever<F>(&mut self, mut callback: F) where F: FnMut(glutin::Event) -> glutin::ControlFlow {
-        if let Some(ref mut events_loop) = self.0 {
-            events_loop.run_forever(callback);
-        } else {
-            loop {
-                self.sleep();
-                if callback(glutin::Event::Awakened) == glutin::ControlFlow::Break {
-                    break;
+        match self.0 {
+            EventLoop::Glutin(ref mut events_loop) => {
+                let events_loop = events_loop
+                    .as_mut()
+                    .expect("Can't run an unavailable event loop.");
+                events_loop.run_forever(callback);
+            }
+            EventLoop::Headless(ref data) => {
+                let &(ref flag, ref condvar) = &**data;
+                while { !*flag.lock().unwrap() } {
+                    self.sleep(flag, condvar);
+                    if callback(glutin::Event::Awakened) == glutin::ControlFlow::Break {
+                        break;
+                    }
                 }
             }
         }
     }
-    fn sleep(&self) {
-        thread::sleep(time::Duration::from_millis(5));
+    fn sleep(&self, lock: &Mutex<bool>, condvar: &Condvar) {
+        // To avoid sleeping when we should be processing events, do two things:
+        // * before sleeping, check whether our signalling flag has been set
+        // * wait on a condition variable with a maximum timeout, to allow
+        //   being woken up by any signals that occur while sleeping.
+        let guard = lock.lock().unwrap();
+        if *guard {
+            return;
+        }
+        let _ = condvar.wait_timeout(
+            guard, time::Duration::from_millis(5)
+        ).unwrap();
     }
 }
 
@@ -94,8 +143,19 @@ impl EventLoopWaker for HeadedEventLoopWaker {
     }
 }
 
-struct HeadlessEventLoopWaker;
+struct HeadlessEventLoopWaker(Arc<(Mutex<bool>, Condvar)>);
 impl EventLoopWaker for HeadlessEventLoopWaker {
-    fn wake(&self) {}
-    fn clone_box(&self) -> Box<dyn EventLoopWaker> { Box::new(HeadlessEventLoopWaker) }
+    fn wake(&self) {
+        // Set the signalling flag and notify the condition variable.
+        // This ensures that any sleep operation is interrupted,
+        // and any non-sleeping operation will have a change to check
+        // the flag before going to sleep.
+        let (ref flag, ref condvar) = *self.0;
+        let mut flag = flag.lock().unwrap();
+        *flag = true;
+        condvar.notify_all();
+    }
+    fn clone_box(&self) -> Box<dyn EventLoopWaker> {
+        Box::new(HeadlessEventLoopWaker(self.0.clone()))
+    }
 }