#10470: Core handlers are not threadsafe
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Reporter: tdterry | Owner: nobody
Status: new | Milestone:
Component: Core framework | Version: 1.0
Keywords: wsgi, basehandler, mod_python, threadsafe | Stage:
Unreviewed
Has_patch: 1 |
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I have found an incomplete-initialization bug the `WSGIHandler` when
running under Apache mpm_worker with mod_wsgi. Code review shows this is
a potential problem with all of the handlers.
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 attached patch changes `BaseHandler.load_middleware()` to do an atomic
set of all four middleware lists. `self._request_middleware` is set last
to signal the completion of middleware loading.
--
Ticket URL: <http://code.djangoproject.com/ticket/10470>
Django <http://code.djangoproject.com/>
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