Failed to reset ContextVars in sync/async middlewares

[Michael Manganiello]

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'])
        response.context_data['mw'].append(self.__class__.__name__)
        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

[Michael Manganiello]

Test scenario (diff over main commit f10c52afab)