fix very slow poll_writable()

src/reactor.rs

@@ -461,7 +461,9 @@ impl Source {
             panic::catch_unwind(|| w.wake()).ok();
         }
         state[dir].waker = Some(cx.waker().clone());
-        state[dir].ticks = Some((Reactor::get().ticker(), state[dir].tick));
+        if state[dir].ticks.is_none() {
+            state[dir].ticks = Some((Reactor::get().ticker(), state[dir].tick));
+        }
 
         // Update interest in this I/O handle.
         if was_empty {

tests/async.rs

@@ -8,6 +8,7 @@ use std::thread;
 use std::time::Duration;
 
 use async_io::{Async, Timer};
+use futures_lite::future::poll_fn;
 use futures_lite::{future, prelude::*};
 #[cfg(unix)]
 use tempfile::tempdir;
@@ -155,6 +156,25 @@ fn udp_send_recv() -> io::Result<()> {
     })
 }
 
+#[test]
+fn test_poll_writable_iterations() -> io::Result<()> {
+    future::block_on(async {
+        let socket = Async::<UdpSocket>::bind(([127, 0, 0, 1], 0))?;
+
+        let mut attempts = 0;
+
+        poll_fn(|cx| {
+            attempts += 1;
+            socket.poll_writable(cx)
+        })
+        .await?;
+
+        assert!(attempts < 5);
+
+        Ok(())
+    })
+}
+
 #[cfg(unix)]
 #[test]
 fn udp_connect() -> io::Result<()> {