From 525855d17331bf9351bd9fe9391c1519d7613c86 Mon Sep 17 00:00:00 2001
From: Helge Milde <helgefmi@gmail.com>
Date: Wed, 10 Jun 2026 17:05:44 +0200
Subject: [PATCH] Fixed #535: ThreadSensitiveContext.__aexit__ blocking the
 event loop.

executor.shutdown() is a blocking join, and the executor's worker thread
may itself be waiting on the event loop, deadlocking it. Join in a
separate thread instead, as asyncio's shutdown_default_executor() does.
---
 CHANGELOG.txt      |  7 +++++++
 asgiref/sync.py    |  8 +++++--
 tests/test_sync.py | 52 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index ee4c69c0..0ee4ea92 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -1,3 +1,10 @@
+UNRELEASED
+----------
+
+* Fixed a deadlock of the entire event loop when exiting
+  ``ThreadSensitiveContext`` while its executor thread was still blocked
+  waiting on the event loop. (#535)
+
 3.11.1 (2026-02-03)
 -------------------
 
diff --git a/asgiref/sync.py b/asgiref/sync.py
index e733c8e5..37afa133 100644
--- a/asgiref/sync.py
+++ b/asgiref/sync.py
@@ -144,9 +144,13 @@ async def __aexit__(self, exc, value, tb):
             return
 
         executor = SyncToAsync.context_to_thread_executor.pop(self, None)
-        if executor:
-            executor.shutdown()
         SyncToAsync.thread_sensitive_context.reset(self.token)
+        if executor:
+            # The executor's worker thread may itself be waiting for this
+            # event loop, so a blocking shutdown() here would deadlock it.
+            # Join in a separate thread instead.
+            loop = asyncio.get_running_loop()
+            await loop.run_in_executor(None, executor.shutdown)
 
 
 class AsyncToSync(Generic[_P, _R]):
diff --git a/tests/test_sync.py b/tests/test_sync.py
index 4199ce65..8effff50 100644
--- a/tests/test_sync.py
+++ b/tests/test_sync.py
@@ -690,6 +690,58 @@ async def test_thread_sensitive_context_without_sync_work():
         pass
 
 
+def cancel_inside_thread_sensitive_context():
+    """Cancels a thread-sensitive task parked in async_to_sync, then exits the context"""
+    worker_started = threading.Event()
+
+    async def inner():
+        await asyncio.sleep(0.2)
+
+    def sync_code():
+        worker_started.set()
+        async_to_sync(inner)()
+
+    async def main():
+        async with ThreadSensitiveContext():
+            task = asyncio.create_task(
+                sync_to_async(sync_code, thread_sensitive=True)()
+            )
+            # Let the executor pick up sync_code, then hold the event loop
+            # with sync sleeps so the call_soon_threadsafe callback enqueued
+            # by async_to_sync is still queued when the cancellation lands.
+            await asyncio.sleep(0)
+            assert worker_started.wait(5)
+            time.sleep(0.1)
+            task.cancel()
+            try:
+                await task
+            except asyncio.CancelledError:
+                pass
+
+    asyncio.run(main())
+
+
+def test_thread_sensitive_context_exit_does_not_block_event_loop():
+    """
+    Tests that exiting ThreadSensitiveContext does not deadlock the event
+    loop if its executor thread is still parked in AsyncToSync, as happens
+    when the task running the sync code is cancelled before the event loop
+    runs the create_task callback enqueued by async_to_sync. (#535)
+
+    Runs in a separate process as the parked executor thread would otherwise
+    hang the test suite at interpreter exit.
+    """
+    process = multiprocessing.Process(target=cancel_inside_thread_sensitive_context)
+    process.start()
+    process.join(30)
+    # Force cleanup in failed test case
+    if process.is_alive():
+        process.terminate()
+        process.join(5)
+        pytest.fail("event loop deadlocked exiting ThreadSensitiveContext")
+    assert process.exitcode == 0
+
+
 def test_thread_sensitive_double_nested_sync():
     """
     Tests that thread_sensitive SyncToAsync nests inside itself where the