io: allow clear_readiness after io driver shutdown (#6067)

tokio/src/runtime/io/scheduled_io.rs

@@ -210,8 +210,8 @@ impl ScheduledIo {
     pub(super) fn set_readiness(&self, tick: Tick, f: impl Fn(Ready) -> Ready) {
         let mut current = self.readiness.load(Acquire);
 
-        // The shutdown bit should not be set
-        debug_assert_eq!(0, SHUTDOWN.unpack(current));
+        // If the io driver is shut down, then you are only allowed to clear readiness.
+        debug_assert!(SHUTDOWN.unpack(current) == 0 || matches!(tick, Tick::Clear(_)));
 
         loop {
             // Mask out the tick bits so that the modifying function doesn't see

tokio/tests/io_async_fd.rs

@@ -578,6 +578,23 @@ fn driver_shutdown_wakes_poll() {
     assert_err!(futures::executor::block_on(poll_writable(&afd_a)));
 }
 
+#[test]
+fn driver_shutdown_then_clear_readiness() {
+    let rt = rt();
+
+    let (a, _b) = socketpair();
+    let afd_a = {
+        let _enter = rt.enter();
+        AsyncFd::new(a).unwrap()
+    };
+
+    let mut write_ready = rt.block_on(afd_a.writable()).unwrap();
+
+    std::mem::drop(rt);
+
+    write_ready.clear_ready();
+}
+
 #[test]
 fn driver_shutdown_wakes_poll_race() {
     // TODO: make this a loom test