fix race between block initialization and receiver disconnection

crossbeam-channel/src/flavors/list.rs

@@ -598,6 +598,7 @@ impl<T> Channel<T> {
                 block = self.head.block.load(Ordering::Acquire);
             }
         }
+
         unsafe {
             // Drop all messages between head and tail and deallocate the heap-allocated blocks.
             while head >> SHIFT != tail >> SHIFT {
@@ -624,8 +625,16 @@ impl<T> Channel<T> {
                 drop(Box::from_raw(block));
             }
         }
+
         head &= !MARK_BIT;
-        self.head.block.store(ptr::null_mut(), Ordering::Release);
+        let new_block = self.head.block.swap(ptr::null_mut(), Ordering::AcqRel);
+
+        // If the block was just initalized before the sender saw the MARK_BIT,
+        // there are no messages but we have to deallocate the block
+        if block.is_null() && tail >> SHIFT == 0 && !new_block.is_null() {
+            unsafe { drop(Box::from_raw(new_block)) };
+        }
+
         self.head.index.store(head, Ordering::Release);
     }