sync: return TryRecvError::Disconnected from Receiver::try_recv after Receiver::close (#7686)

Author: KR-bluejay

tokio/src/sync/mpsc/chan.rs

@@ -439,6 +439,10 @@ impl<T, S: Semaphore> Rx<T, S> {
                             return Ok(value);
                         }
                         TryPopResult::Closed => return Err(TryRecvError::Disconnected),
+                        // If close() was called, an empty queue should report Disconnected.
+                        TryPopResult::Empty if rx_fields.rx_closed => {
+                            return Err(TryRecvError::Disconnected)
+                        }
                         TryPopResult::Empty => return Err(TryRecvError::Empty),
                         TryPopResult::Busy => {} // fall through
                     }

tokio/src/sync/mpsc/list.rs

@@ -35,8 +35,14 @@ pub(crate) enum TryPopResult<T> {
     /// Successfully popped a value.
     Ok(T),
     /// The channel is empty.
+    ///
+    /// Note that `list.rs` only tracks the close state set by senders. If the
+    /// channel is closed by `Rx::close()`, then `TryPopResult::Empty` is still
+    /// returned, and the close state needs to be handled by `chan.rs`.
     Empty,
     /// The channel is empty and closed.
+    ///
+    /// Returned when the send half is closed (all senders dropped).
     Closed,
     /// The channel is not empty, but the first value is being written.
     Busy,

tokio/tests/sync_mpsc.rs

@@ -966,6 +966,15 @@ fn try_recv_unbounded() {
     }
 }
 
+#[test]
+fn try_recv_after_receiver_close() {
+    let (_tx, mut rx) = mpsc::channel::<()>(5);
+
+    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
+    rx.close();
+    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
+}
+
 #[test]
 fn try_recv_close_while_empty_bounded() {
     let (tx, mut rx) = mpsc::channel::<()>(5);