From 5975c53ba0f69ffb7bfa040a7fee6584eecb26fa Mon Sep 17 00:00:00 2001
From: zdevito <zdevito@meta.com>
Date: Tue, 21 Oct 2025 11:59:06 -0700
Subject: [PATCH] order async messages, provide workaround flag

Differential Revision: [D85172238](https://our.internmc.facebook.com/intern/diff/D85172238/)

**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D85172238/)!

[ghstack-poisoned]
---
 monarch_hyperactor/src/actor.rs         | 33 ++++++++++++++++---------
 monarch_hyperactor/src/config.rs        |  3 +++
 python/monarch/_src/actor/actor_mesh.py | 11 +++++++--
 3 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/monarch_hyperactor/src/actor.rs b/monarch_hyperactor/src/actor.rs
index ab07af6a7..3b1534482 100644
--- a/monarch_hyperactor/src/actor.rs
+++ b/monarch_hyperactor/src/actor.rs
@@ -54,6 +54,7 @@ use tokio::sync::oneshot;
 use tracing::Instrument;
 
 use crate::buffers::FrozenBuffer;
+use crate::config::MONARCH_OLD_ASYNC_WORKAROUND;
 use crate::config::SHARED_ASYNCIO_RUNTIME;
 use crate::context::PyInstance;
 use crate::local_state_broker::BrokerId;
@@ -484,6 +485,9 @@ pub struct PythonActor {
     /// so that we can store information from the Init (spawn rank, controller controller)
     /// and provide it to other calls
     instance: Option<Py<crate::context::PyInstance>>,
+
+    /// Cached config value for old async workaround
+    old_async_workaround: bool,
 }
 
 impl PythonActor {
@@ -559,6 +563,7 @@ impl Actor for PythonActor {
                 panic_watcher: UnhandledErrorObserver::ForwardTo(rx),
                 panic_sender: tx,
                 instance: None,
+                old_async_workaround: hyperactor::config::global::get(MONARCH_OLD_ASYNC_WORKAROUND),
             })
         })?)
     }
@@ -809,19 +814,23 @@ impl Handler<PythonMessage> for PythonActor {
             .map_err(|err| err.into())
         })?;
 
-        // Spawn a child actor to await the Python handler method.
-        tokio::spawn(
-            handle_async_endpoint_panic(
-                self.panic_sender.clone(),
-                PythonTask::new(future),
-                receiver,
-            )
-            .instrument(
-                tracing::info_span!("py_panic_handler")
-                    .follows_from(tracing::Span::current().id())
-                    .clone(),
-            ),
+        let future = handle_async_endpoint_panic(
+            self.panic_sender.clone(),
+            PythonTask::new(future),
+            receiver,
+        )
+        .instrument(
+            tracing::info_span!("py_panic_handler")
+                .follows_from(tracing::Span::current().id())
+                .clone(),
         );
+        if self.old_async_workaround {
+            // Spawn a child actor to await the Python handler method.
+            tokio::spawn(future);
+        } else {
+            // things happen in order
+            future.await;
+        }
         Ok(())
     }
 }
diff --git a/monarch_hyperactor/src/config.rs b/monarch_hyperactor/src/config.rs
index f3215c35f..def224a39 100644
--- a/monarch_hyperactor/src/config.rs
+++ b/monarch_hyperactor/src/config.rs
@@ -35,6 +35,9 @@ use crate::channel::PyChannelTransport;
 declare_attrs! {
     /// Use a single asyncio runtime for all Python actors, rather than one per actor
     pub attr SHARED_ASYNCIO_RUNTIME: bool = false;
+
+    /// Use old async workaround that spawns Python handler methods in tokio tasks
+    pub attr MONARCH_OLD_ASYNC_WORKAROUND: bool = false;
 }
 
 /// Python API for configuration management
diff --git a/python/monarch/_src/actor/actor_mesh.py b/python/monarch/_src/actor/actor_mesh.py
index 43c3465ba..8bdef3ce8 100644
--- a/python/monarch/_src/actor/actor_mesh.py
+++ b/python/monarch/_src/actor/actor_mesh.py
@@ -13,10 +13,12 @@
 import inspect
 import itertools
 import logging
+
+import os
 import threading
 from abc import abstractproperty
-
 from dataclasses import dataclass
+from functools import cache
 from pprint import pformat
 from textwrap import indent
 from traceback import TracebackException
@@ -213,6 +215,11 @@ def _root_client_context() -> "Context": ...
 T = TypeVar("T")
 
 
+@cache
+def _old_async_workaround():
+    return os.environ.get("MONARCH_OLD_ASYNC_WORKAROUND", "0") != "0"
+
+
 class _Lazy(Generic[T]):
     def __init__(self, init: Callable[[], T]) -> None:
         self._lock = threading.Lock()
@@ -1141,7 +1148,7 @@ def __init__(
                 else:
                     sync_endpoints.append(attr_name)
 
-        if sync_endpoints and async_endpoints:
+        if (sync_endpoints and async_endpoints) or _old_async_workaround():
             raise ValueError(
                 f"{self._class} mixes both async and sync endpoints."
                 "Synchronous endpoints cannot be mixed with async endpoints because they can cause the asyncio loop to deadlock if they wait."