I have found an incomplete-initialization bug the WSGIHandler when
running under Apache mpm_worker with mod_wsgi.
On the first request the WSGIHandler.__call__(...), we set up the
middleware. There is a lock wrapping the load of the middleware, but
the logic still allows incomplete initialization on other threads.
Below is the block of code in question from django/core/handlers/wsgi.py
(line 226):
if self._request_middleware is None:
self.initLock.acquire()
# Check that middleware is still uninitialised.
if self._request_middleware is None:
self.load_middleware()
self.initLock.release()
Example:
1. Initial start or restart of the Apache instance
2. Thread T1 - a request comes in. self._request_middleware is None
3. Thread T1 - acquires the self.initLock, calls self.load_middleware()
4. Thread T2 - a request comes in (T1 is part of the way through
self.load_middleware()
5. Thread T2 - self._request_middleware is not None, but it's not
completely loaded either, continues with the request
6. Thread T1 - completes self.load_middleware(), release the lock
The problem is the assignment of self._request_middleware is not atomic
(it is created as an empty list and populated with append() in a loop.
To fix this, we need an atomic assignment. I initially thought about
changing load_middleware() to build the _request_middleware array in a
local variable and then doing a single assignment to save it to
self._request_middleware. While this would solve the problem, I thought
that having the initialization test in WSGIHandler.__call__() test for
what is essentially a side effect of self.load_middleware() is error prone.
So, my proposed fix is to add a flag in WSGIHandler and then test for
that flag in __call__(). I implemented this in our custom WSGI handler
replacement and it worked perfectly. I will submit a ticket with patch
to Django if the group agrees. Here is my fix:
WSGIHandler.__init__(self):
BaseHandler.__init__(self)
self._middleware_loaded = False
WSGIHandler.__call__(...):
...
if not self._middleware_loaded:
self.initLock.acquire()
# Check that middleware is still uninitialised.
if not self._middleware_loaded:
self.load_middleware()
self._middleware_loaded = True
self.initLock.release()
...
Travis
Propeller.com
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Django developers" group.
To post to this group, send email to django-developers@googlegroups.com
To unsubscribe from this group, send email to
django-developers+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/django-developers?hl=en
-~----------~----~----~----~------~----~------~--~---
