#33092: PyMemcacheCache backend fails when running as a wsgi application with
gevent worker class
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Reporter: Martijn van der Blom | Owner: nobody
Type: Bug | Status: new
Component: Core (Cache system) | Version: 3.2
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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In our application we were using the MemcachedCache backend to connect to
a memcached server. Since this backend will be removed in Django 4.1 we
thought we'd migrate to the alternative PyMemcacheCache backend as
suggested in the Django documentation at:
https://docs.djangoproject.com/en/3.2/topics/cache/
After upgrading we encountered several errors when running the application
in gunicorn with the gevent worker class.
Summary of errors:
- gevent._socketcommon.cancel_wait_ex: [Errno 9] File descriptor was
closed in another greenlet
- gevent.exceptions.ConcurrentObjectUseError: This socket is already used
by another greenlet: <bound method Waiter.switch of
<gevent._gevent_c_waiter.Waiter object at 0x7f8e77b00a40>>
- OSError: [Errno 9] Bad file descriptor
These errors seem to be related to either the Django backend
implementation or Pymemcache not handling multi-threading/thread-safety
properly.
There is a related bug for the Pymemcache library where a member of that
team states that is up the application using Pymemcache to handle thread-
safety
(https://github.com/pinterest/pymemcache/issues/195#issuecomment-452523524).
In this case the Django framework. This comment in Django's
BaseMemcachedCache implementation indicates that that was the original
intent:
https://github.com/django/django/blob/main/django/core/cache/backends/memcached.py#L38
so i think PyMemcacheCache should handle this.
Example project that reproduces the error:
https://github.com/mvanderblom/django-memcached-bugreport
For us, this error prevents us from using the PyMemcacheCache backend and
thus from upgrading to Django 4.1 when it gets released.
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Ticket URL: <https://code.djangoproject.com/ticket/33092>
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