Auto merge of rust-lang#36893 - apasel422:issue-32114, r=alexcrichton

Restore `DISCONNECTED` state in `oneshot::Packet::send`

Closes rust-lang#32114

I'm not sure if this is the best approach, but the current action of swapping `DISCONNECTED` with `DATA` seems wrong. Additionally, it is strange that the `send` method (and others in the `oneshot` module) takes `&mut self` despite performing atomic operations, as this requires extra discipline to avoid data races and lets us use methods like `AtomicUsize::get_mut` instead of methods that require a memory ordering.

Author: bors

Diff for: src/libstd/sync/mpsc/mod.rs

@@ -1940,6 +1940,13 @@ mod tests {
         // wait for the child thread to exit before we exit
         rx2.recv().unwrap();
     }
+
+    #[test]
+    fn issue_32114() {
+        let (tx, _) = channel();
+        let _ = tx.send(123);
+        assert_eq!(tx.send(123), Err(SendError(123)));
+    }
 }
 
 #[cfg(all(test, not(target_os = "emscripten")))]

Diff for: src/libstd/sync/mpsc/oneshot.rs

@@ -113,6 +113,8 @@ impl<T> Packet<T> {
             // Couldn't send the data, the port hung up first. Return the data
             // back up the stack.
             DISCONNECTED => {
+                self.state.swap(DISCONNECTED, Ordering::SeqCst);
+                self.upgrade = NothingSent;
                 Err(self.data.take().unwrap())
             }