Breaks under concurrency when using gevent

[ddorian]

When running concurrent requests to the flask app I get this error often. It goes away if I comment this line:
app.teardown_appcontext(_close_async_sessions).

Debugging middleware caught exception in streamed response at a point where response headers were already sent.
Traceback (most recent call last):
  File "/home/user/proj/.venv/lib/python3.12/site-packages/flask/app.py", line 1498, in __call__
    return self.wsgi_app(environ, start_response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/proj/.venv/lib/python3.12/site-packages/whitenoise/base.py", line 81, in __call__
    return self.application(environ, start_response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/proj/.venv/lib/python3.12/site-packages/werkzeug/middleware/proxy_fix.py", line 183, in __call__
    return self.app(environ, start_response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/proj/.venv/lib/python3.12/site-packages/opentelemetry/instrumentation/flask/__init__.py", line 390, in _wrapped_app
    result = wsgi_app(wrapped_app_environ, _start_response)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/proj/.venv/lib/python3.12/site-packages/flask/app.py", line 1489, in wsgi_app
    ctx.pop(error)
  File "/home/user/proj/.venv/lib/python3.12/site-packages/flask/ctx.py", line 426, in pop
    app_ctx.pop(exc)
  File "/home/user/proj/.venv/lib/python3.12/site-packages/flask/ctx.py", line 262, in pop
    self.app.do_teardown_appcontext(exc)
  File "/home/user/proj/.venv/lib/python3.12/site-packages/flask/app.py", line 1344, in do_teardown_appcontext
    self.ensure_sync(func)(exc)
RuntimeError: You cannot use AsyncToSync in the same thread as an async event loop - just await the async function directly.