Update the bound field within PortHandle.bind_default method (#1439)

Summary:
Pull Request resolved: #1439

Currently, `PortHandle.bind_default` does not save `port_index` in the handle's `bound` field after it binds the port. This diff changes that. In this way, we can know whether the handle object is bound, and bound to which port, by using `PortHandle::location` method.

This change is a behavior change to another method, `PortHandle::bind`. Previously, `bind` can still mutate the `bound` field aster `bind_default` was called. After this diff, it cannot. The new behavior is documented in the new test,  `test_bind_port_handle_to_default`.

Reviewed By: mariusae

Differential Revision: D83934172

fbshipit-source-id: e761c1f697dc26b219a01f45046a1421ca0bd065

Author: pzhan9

hyperactor/src/mailbox.rs

@@ -1644,6 +1644,16 @@ impl<M: RemoteMessage> PortHandle<M> {
     /// This is used by [`actor::Binder`] implementations to bind actor refs.
     /// This is not intended for general use.
     pub(crate) fn bind_actor_port(&self) {
+        let port_id = self.mailbox.actor_id().port_id(M::port());
+        self.bound
+            .set(port_id)
+            .map_err(|p| {
+                format!(
+                    "could not bind port handle {} as {p}: already bound",
+                    self.port_index
+                )
+            })
+            .unwrap();
         self.mailbox.bind_to_actor_port(self);
     }
 }
@@ -3763,4 +3773,41 @@ mod tests {
             .unwrap();
         assert_eq!(rx.recv().await.unwrap(), (1, "hello".to_string()));
     }
+
+    #[test]
+    #[should_panic(expected = "already bound")]
+    fn test_bind_port_handle_to_actor_port_twice() {
+        let mbox = Mailbox::new_detached(id!(test[0].test));
+        let (handle, _rx) = mbox.open_port::<String>();
+        handle.bind_actor_port();
+        handle.bind_actor_port();
+    }
+
+    #[test]
+    fn test_bind_port_handle_to_actor_port() {
+        let mbox = Mailbox::new_detached(id!(test[0].test));
+        let default_port = mbox.actor_id().port_id(String::port());
+        let (handle, _rx) = mbox.open_port::<String>();
+        // Handle's port index is allocated by mailbox, not the actor port.
+        assert_ne!(default_port.index(), handle.port_index);
+        // Bind the handle to the actor port.
+        handle.bind_actor_port();
+        assert_matches!(handle.location(), PortLocation::Bound(port) if port == default_port);
+        // bind() can still be used, just it will not change handle's state.
+        handle.bind();
+        handle.bind();
+        assert_matches!(handle.location(), PortLocation::Bound(port) if port == default_port);
+    }
+
+    #[test]
+    #[should_panic(expected = "already bound")]
+    fn test_bind_port_handle_to_actor_port_when_already_bound() {
+        let mbox = Mailbox::new_detached(id!(test[0].test));
+        let (handle, _rx) = mbox.open_port::<String>();
+        // Bound handle to the port allocated by mailbox.
+        handle.bind();
+        assert_matches!(handle.location(), PortLocation::Bound(port) if port.index() == handle.port_index);
+        // Since handle is already bound, call bind_to() on it will cause panic.
+        handle.bind_actor_port();
+    }
 }