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
33252	CacheMiddleware and FetchFromCacheMiddleware are not thread safe.	Iuri de Silvio	Iuri de Silvio	"CacheMiddleware persist `self.cache = caches[cache_alias]` on startup and it is not thread safe. https://github.com/django/django/blob/main/django/middleware/cache.py#L186

I found that after some production errors with pylibmc and uwsgi threaded. Created a small project to reproduce it. Nothing fancy, just pylibmc cache and a `@cache_page` cached view. It fails even with development server, with concurrent requests.

{{{
Traceback (most recent call last):
  File ""versions/pylibmcbug/lib/python3.9/site-packages/django/core/handlers/exception.py"", line 47, in inner
    response = get_response(request)
  File ""versions/pylibmcbug/lib/python3.9/site-packages/django/core/handlers/base.py"", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File ""versions/pylibmcbug/lib/python3.9/site-packages/django/utils/decorators.py"", line 122, in _wrapped_view
    result = middleware.process_request(request)
  File ""versions/pylibmcbug/lib/python3.9/site-packages/django/middleware/cache.py"", line 145, in process_request
    cache_key = get_cache_key(request, self.key_prefix, 'GET', cache=self.cache)
  File ""versions/pylibmcbug/lib/python3.9/site-packages/django/utils/cache.py"", line 362, in get_cache_key
    headerlist = cache.get(cache_key)
  File ""versions/pylibmcbug/lib/python3.9/site-packages/django/core/cache/backends/memcached.py"", line 77, in get
    return self._cache.get(key, default)
pylibmc.ConnectionError: error 3 from memcached_get(:1:views.decorators.cache.cache_): (0x7f290400bd60) FAILURE, poll() returned a value that was not dealt with,  host: localhost:11211 -> libmemcached/io.cc:254
}}}

Looking for git history, it is this way since 2010. https://github.com/django/django/commit/673e6fc7fb243ed44841b9969d26a161c25733b3
"	Bug	closed	Core (Cache system)	3.2	Normal	fixed		Nick Pope	Ready for checkin	1	0	0	0	0	0