sync: handle panic during mpsc drop (tokio-rs#7094)

Author: mox692

Diff for: tokio/src/sync/mpsc/chan.rs

@@ -490,10 +490,34 @@ impl<T, S: Semaphore> Drop for Rx<T, S> {
 
         self.inner.rx_fields.with_mut(|rx_fields_ptr| {
             let rx_fields = unsafe { &mut *rx_fields_ptr };
+            struct Guard<'a, T, S: Semaphore> {
+                list: &'a mut list::Rx<T>,
+                tx: &'a list::Tx<T>,
+                sem: &'a S,
+            }
+
+            impl<'a, T, S: Semaphore> Guard<'a, T, S> {
+                fn drain(&mut self) {
+                    // call T's destructor.
+                    while let Some(Value(_)) = self.list.pop(self.tx) {
+                        self.sem.add_permit();
+                    }
+                }
+            }
 
-            while let Some(Value(_)) = rx_fields.list.pop(&self.inner.tx) {
-                self.inner.semaphore.add_permit();
+            impl<'a, T, S: Semaphore> Drop for Guard<'a, T, S> {
+                fn drop(&mut self) {
+                    self.drain();
+                }
             }
+
+            let mut guard = Guard {
+                list: &mut rx_fields.list,
+                tx: &self.inner.tx,
+                sem: &self.inner.semaphore,
+            };
+
+            guard.drain();
         });
     }
 }

Diff for: tokio/tests/sync_mpsc.rs

@@ -1454,4 +1454,50 @@ async fn test_is_empty_32_msgs() {
     }
 }
 
+#[test]
+#[cfg(not(panic = "abort"))]
+fn drop_all_elements_during_panic() {
+    use std::sync::atomic::AtomicUsize;
+    use std::sync::atomic::Ordering::Relaxed;
+    use tokio::sync::mpsc::UnboundedReceiver;
+    use tokio::sync::mpsc::UnboundedSender;
+
+    static COUNTER: AtomicUsize = AtomicUsize::new(0);
+
+    struct A(bool);
+    impl Drop for A {
+        // cause a panic when inner value is `true`.
+        fn drop(&mut self) {
+            COUNTER.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+            if self.0 {
+                panic!("panic!")
+            }
+        }
+    }
+
+    fn func(tx: UnboundedSender<A>, rx: UnboundedReceiver<A>) {
+        tx.send(A(true)).unwrap();
+        tx.send(A(false)).unwrap();
+        tx.send(A(false)).unwrap();
+
+        drop(rx);
+
+        // `mpsc::Rx`'s drop is called and gets panicked while dropping the first value,
+        // but will keep dropping following elements.
+    }
+
+    let (tx, rx) = mpsc::unbounded_channel();
+
+    let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+        func(tx.clone(), rx);
+    }));
+
+    // all A's destructor should be called at this point, even before `mpsc::Chan`'s
+    // drop gets called.
+    assert_eq!(COUNTER.load(Relaxed), 3);
+
+    drop(tx);
+    // `mpsc::Chan`'s drop is called, freeing the `Block` memory allocation.
+}
+
 fn is_debug<T: fmt::Debug>(_: &T) {}