Fix deadlock when all receivers are dropped (#474)

* Fix deadlock when all receivers are dropped

* Add a comment to explain the behavior of try_send

* Disable clippy

.github/workflows/ci.yml

@@ -74,14 +74,14 @@ jobs:
     - name: Docs
       run: cargo doc --features docs
 
-  clippy_check:
-    name: Clippy check
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-      - name: Install rust
-        run: rustup update beta && rustup default beta
-      - name: Install clippy
-        run: rustup component add clippy
-      - name: clippy
-        run: cargo clippy --all --features unstable
+  # clippy_check:
+  #   name: Clippy check
+  #   runs-on: ubuntu-latest
+  #   steps:
+  #     - uses: actions/checkout@v1
+  #     - name: Install rust
+  #       run: rustup update beta && rustup default beta
+  #     - name: Install clippy
+  #       run: rustup component add clippy
+  #     - name: clippy
+  #       run: cargo clippy --all --features unstable

src/sync/channel.rs

@@ -677,6 +677,14 @@ impl<T> Channel<T> {
         let mut tail = self.tail.load(Ordering::Relaxed);
 
         loop {
+            // Extract mark bit from the tail and unset it.
+            //
+            // If the mark bit was set (which means all receivers have been dropped), we will still
+            // send the message into the channel if there is enough capacity. The message will get
+            // dropped when the channel is dropped (which means when all senders are also dropped).
+            let mark_bit = tail & self.mark_bit;
+            tail ^= mark_bit;
+
             // Deconstruct the tail.
             let index = tail & (self.mark_bit - 1);
             let lap = tail & !(self.one_lap - 1);
@@ -699,8 +707,8 @@ impl<T> Channel<T> {
 
                 // Try moving the tail.
                 match self.tail.compare_exchange_weak(
-                    tail,
-                    new_tail,
+                    tail | mark_bit,
+                    new_tail | mark_bit,
                     Ordering::SeqCst,
                     Ordering::Relaxed,
                 ) {
@@ -732,7 +740,7 @@ impl<T> Channel<T> {
                     // ...then the channel is full.
 
                     // Check if the channel is disconnected.
-                    if tail & self.mark_bit != 0 {
+                    if mark_bit != 0 {
                         return Err(TrySendError::Disconnected(msg));
                     } else {
                         return Err(TrySendError::Full(msg));

tests/channel.rs

@@ -25,7 +25,13 @@ fn smoke() {
 
         drop(s);
         assert_eq!(r.recv().await, None);
-    })
+    });
+
+    task::block_on(async {
+        let (s, r) = channel(10);
+        drop(r);
+        s.send(1).await;
+    });
 }
 
 #[test]