Test some more mpsc behavior with loom (#2246)

tokio/src/sync/tests/loom_mpsc.rs

@@ -21,3 +21,57 @@ fn closing_tx() {
         assert!(v.is_none());
     });
 }
+
+#[test]
+fn closing_unbounded_tx() {
+    loom::model(|| {
+        let (tx, mut rx) = mpsc::unbounded_channel();
+
+        thread::spawn(move || {
+            tx.send(()).unwrap();
+            drop(tx);
+        });
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_some());
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}
+
+#[test]
+fn dropping_tx() {
+    loom::model(|| {
+        let (tx, mut rx) = mpsc::channel::<()>(16);
+
+        for _ in 0..2 {
+            let tx = tx.clone();
+            thread::spawn(move || {
+                drop(tx);
+            });
+        }
+        drop(tx);
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}
+
+#[test]
+fn dropping_unbounded_tx() {
+    loom::model(|| {
+        let (tx, mut rx) = mpsc::unbounded_channel::<()>();
+
+        for _ in 0..2 {
+            let tx = tx.clone();
+            thread::spawn(move || {
+                drop(tx);
+            });
+        }
+        drop(tx);
+
+        let v = block_on(poll_fn(|cx| rx.poll_recv(cx)));
+        assert!(v.is_none());
+    });
+}