id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32815	Failed to reset ContextVars in sync/async middlewares	Michael Manganiello	nobody	"When using a middleware that can process both sync and async requests, and trying to set and reset a ContextVar (in different methods of its request lifecycle), Python fails with error:
`ValueError: <Token var=<ContextVar name='current_context' at 0x7f9a8b9ad900> at 0x7f9a68575180> was created in a different Context`

This is a simple middleware example to reproduce the mentioned issue:

{{{
import contextvars

current_context = contextvars.ContextVar('current_context')

@sync_and_async_middleware
class TemplateResponseMiddleware(BaseMiddleware):
    def process_view(self, request, view_func, view_args, view_kwargs):
        request.META['_CONTEXT_RESET_TOKEN'] = current_context.set(id(request))

    def process_template_response(self, request, response):
        current_context.reset(request.META['_CONTEXT_RESET_TOKEN'])
        return response
}}}

This use case is what the OpenTelemetry integration uses for spans to be traced in Django: https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/middleware.py

* In `process_request`, a `ContextVar` is set, and the generated token is persisted in the `request.META` object.
* In `process_response`, the `ContextVar` is reset, by using the persisted token.

This approach works correctly for synchronous requests. However, as part of adding ASGI support to the Django integration for OpenTelemetry (in https://github.com/open-telemetry/opentelemetry-python-contrib/pull/391), we found that the `ContextVar` triggers the mentioned error when we want to reset it to its previous value. OpenTelemetry inherits from `MiddlewareMixin`, but I'm attaching a diff for a simple test scenario that reproduces the issue, using the new Middleware format.

The main suspects here are the calls to `sync_to_async`, which adapt the middleware methods to the async flow. However, both those calls explicitly set `thread_sensitive=True`.


Traceback for the attached test scenario:

{{{
$ ./runtests.py -k MiddlewareSyncAsyncTests.test_async_process_template_response
# ...
ERROR: test_async_process_template_response (middleware_exceptions.tests.MiddlewareSyncAsyncTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ""/home/mike/.virtualenvs/django/lib/python3.9/site-packages/asgiref/sync.py"", line 222, in __call__
    return call_result.result()
  File ""/usr/lib/python3.9/concurrent/futures/_base.py"", line 438, in result
    return self.__get_result()
  File ""/usr/lib/python3.9/concurrent/futures/_base.py"", line 390, in __get_result
    raise self._exception
  File ""/home/mike/.virtualenvs/django/lib/python3.9/site-packages/asgiref/sync.py"", line 287, in main_wrap
    result = await self.awaitable(*args, **kwargs)
  File ""/mnt/data/Proyectos/third_party/django/django/test/utils.py"", line 423, in inner
    return await func(*args, **kwargs)
  File ""/mnt/data/Proyectos/third_party/django/tests/middleware_exceptions/tests.py"", line 319, in test_async_process_template_response
    response = await self.async_client.get(
  File ""/mnt/data/Proyectos/third_party/django/django/test/client.py"", line 911, in request
    self.check_exception(response)
  File ""/mnt/data/Proyectos/third_party/django/django/test/client.py"", line 580, in check_exception
    raise exc_value
  File ""/home/mike/.virtualenvs/django/lib/python3.9/site-packages/asgiref/sync.py"", line 458, in thread_handler
    raise exc_info[1]
  File ""/mnt/data/Proyectos/third_party/django/django/core/handlers/exception.py"", line 38, in inner
    response = await get_response(request)
  File ""/mnt/data/Proyectos/third_party/django/django/core/handlers/base.py"", line 249, in _get_response_async
    response = await middleware_method(request, response)
  File ""/home/mike/.virtualenvs/django/lib/python3.9/site-packages/asgiref/sync.py"", line 423, in __call__
    ret = await asyncio.wait_for(future, timeout=None)
  File ""/usr/lib/python3.9/asyncio/tasks.py"", line 442, in wait_for
    return await fut
  File ""/home/mike/.virtualenvs/django/lib/python3.9/site-packages/asgiref/current_thread_executor.py"", line 22, in run
    result = self.fn(*self.args, **self.kwargs)
  File ""/home/mike/.virtualenvs/django/lib/python3.9/site-packages/asgiref/sync.py"", line 462, in thread_handler
    return func(*args, **kwargs)
  File ""/mnt/data/Proyectos/third_party/django/tests/middleware_exceptions/middleware.py"", line 135, in process_template_response
    current_context.reset(request.META['_CONTEXT_RESET_TOKEN'])
ValueError: <Token var=<ContextVar name='current_context' at 0x7f1dd4ec2720> at 0x7f1db129a880> was created in a different Context
}}}"	Uncategorized	closed	Uncategorized	3.2	Normal	invalid			Unreviewed	0	0	0	0	0	0