sync: don't panic in oneshot::try_recv (#3674)

tokio/src/sync/oneshot.rs

@@ -672,7 +672,7 @@ impl<T> Receiver<T> {
                 return Err(TryRecvError::Empty);
             }
         } else {
-            panic!("called after complete");
+            Err(TryRecvError::Closed)
         };
 
         self.inner = None;

tokio/tests/sync_oneshot.rs

@@ -2,6 +2,7 @@
 #![cfg(feature = "full")]
 
 use tokio::sync::oneshot;
+use tokio::sync::oneshot::error::TryRecvError;
 use tokio_test::*;
 
 use std::future::Future;
@@ -190,6 +191,17 @@ fn close_after_recv() {
     rx.close();
 }
 
+#[test]
+fn try_recv_after_completion() {
+    let (tx, mut rx) = oneshot::channel::<i32>();
+
+    tx.send(17).unwrap();
+
+    assert_eq!(17, rx.try_recv().unwrap());
+    assert_eq!(Err(TryRecvError::Closed), rx.try_recv());
+    rx.close();
+}
+
 #[test]
 fn drops_tasks() {
     let (mut tx, mut rx) = oneshot::channel::<i32>();