sync: Ensure try_send() wakes up receivers

This branch of try_send() just forgot to wake up any receiver waiting for data.

Closes #15761

src/libsync/comm/mod.rs

@@ -2098,4 +2098,23 @@ mod sync_tests {
         });
         assert_eq!(rx.recv(), 1);
     } #[ignore(reason = "flaky on libnative")])
+
+    test!(fn issue_15761() {
+        fn repro() {
+            let (tx1, rx1) = sync_channel::<()>(3);
+            let (tx2, rx2) = sync_channel::<()>(3);
+
+            spawn(proc() {
+                rx1.recv();
+                tx2.try_send(()).unwrap();
+            });
+
+            tx1.try_send(()).unwrap();
+            rx2.recv();
+        }
+
+        for _ in range(0u, 100) {
+            repro()
+        }
+    })
 }

src/libsync/comm/sync.rs

@@ -218,9 +218,15 @@ impl<T: Send> Packet<T> {
             }
         } else {
             // If the buffer has some space and the capacity isn't 0, then we
-            // just enqueue the data for later retrieval.
+            // just enqueue the data for later retrieval, ensuring to wake up
+            // any blocked receiver if there is one.
             assert!(state.buf.size() < state.buf.cap());
             state.buf.enqueue(t);
+            match mem::replace(&mut state.blocker, NoneBlocked) {
+                BlockedReceiver(task) => wakeup(task, guard),
+                NoneBlocked => {}
+                BlockedSender(..) => unreachable!(),
+            }
             Ok(())
         }
     }