fix race between block initialization and receiver disconnection

library/std/src/sync/mpmc/list.rs

@@ -547,7 +547,7 @@ fn discard_all_messages(&self) {
         }
 
         let mut head = self.head.index.load(Ordering::Acquire);
-        let mut block = self.head.block.load(Ordering::Acquire);
+        let mut block = self.head.block.swap(ptr::null_mut(), Ordering::AcqRel);
 
         // If we're going to be dropping messages we need to synchronize with initialization
         if head >> SHIFT != tail >> SHIFT {
@@ -588,8 +588,8 @@ fn discard_all_messages(&self) {
                 drop(Box::from_raw(block));
             }
         }
+
         head &= !MARK_BIT;
-        self.head.block.store(ptr::null_mut(), Ordering::Release);
         self.head.index.store(head, Ordering::Release);
     }