reactor: properly reset registered wakers state when needed (async-rs#13)

If the list of pending wakers grew too big and we woke everything,
also drop the bit from wakers_registered

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

src/reactor.rs

@@ -473,7 +473,10 @@ impl Source {
             // Register the current task's waker if not present already.
             if w.readers.iter().all(|w| !w.will_wake(cx.waker())) {
                 w.readers.push(cx.waker().clone());
-                limit_waker_list(&mut w.readers);
+                if limit_waker_list(&mut w.readers) {
+                    self.wakers_registered
+                        .fetch_and(!Self::READERS_REGISTERED, Ordering::SeqCst);
+                }
             }
 
             // Remember the current ticks.
@@ -527,7 +530,10 @@ impl Source {
             // Register the current task's waker if not present already.
             if w.writers.iter().all(|w| !w.will_wake(cx.waker())) {
                 w.writers.push(cx.waker().clone());
-                limit_waker_list(&mut w.writers);
+                if limit_waker_list(&mut w.writers) {
+                    self.wakers_registered
+                        .fetch_and(!Self::WRITERS_REGISTERED, Ordering::SeqCst);
+                }
             }
 
             // Remember the current ticks.
@@ -549,7 +555,7 @@ impl Source {
     }
 }
 
-/// Wakes up all wakers in the list if it grew too big.
+/// Wakes up all wakers in the list if it grew too big and returns whether it did.
 ///
 /// The waker list keeps growing in pathological cases where a single async I/O handle has lots of
 /// different reader or writer tasks. If the number of interested wakers crosses some threshold, we
@@ -562,11 +568,14 @@ impl Source {
 /// However, we don't worry about such scenarios because it's very unlikely to have more than two
 /// actually concurrent tasks operating on a single async I/O handle. If we happen to cross the
 /// aforementioned threshold, we have bigger problems to worry about.
-fn limit_waker_list(wakers: &mut Vec<Waker>) {
+fn limit_waker_list(wakers: &mut Vec<Waker>) -> bool {
     if wakers.len() > 50 {
         for waker in wakers.drain(..) {
             // Don't let a panicking waker blow everything up.
             let _ = panic::catch_unwind(|| waker.wake());
         }
+        true
+    } else {
+        false
     }
 }