[Python-Dev] Threading bug review + commit privs

[Reid Kleckner]

Hey all,

I'm porting some fixes for threading.py that we applied to unladen-swallow:
http://bugs.python.org/issue6643

We ran into these bizarre race conditions involving fork + threads
while running the test suite with a background JIT compilation thread.
 I really wish we weren't trying to support forking from a child
thread, but it's already in the test suite.  I've "solved" the problem
by throwing away radioactive locks that may have been held across a
fork.*

If I could get a reviewer to look at this, I would be very grateful,
since reviewing threading patches is somewhat tricky.  =/

I'm also expecting to be doing more work merging unladen-swallow into
the py3k-jit branch, so I was wondering if I could get commit
privileges for that.

Thanks,
Reid

* In general I wouldn't think this is safe, but we already do it for
_active_limbo_lock in threading.py.  One of the problems I've
encountered is that on OS X, releasing locks held by other threads
across a fork results in a crash.  Furthermore, when locks are
deallocated, the destructor does a non-blocking acquire and then
release, which I would think would crash.  However, we get lucky here,
because any thread that holds a lock across a fork usually holds a
reference to it on the stack.  Therefore the lock is leaked and the
destructor never run.  Moral: fork + threads is *crazy*, avoid it if
you can.