[issue33196] SEGV in mp.synchronize.Lock.__repr__ in spawn'ed proc if ctx mismatched

[STINNER Victor]

STINNER Victor <vstinner at redhat.com> added the comment:

> At a minimum I think there should be a check trying to reduce arguments via incompatible context's process to prevent a SEGV.

I'm not sure that I understand the bug. The reproducer script pass a multiprocessing.Queue to a child process and then the child crash when attempting to call multiprocessing.synchronize.Lock.__repr__().

Does the child reuse a copy of the lock of the parent process? Or does the child create a new SemLock?


I reproduced the bug on Fedora 26. I attached the child process in gdb. The crash occurs on sem_getvalue() in the child process.

Program received signal SIGSEGV, Segmentation fault.
0x00007f29a5156610 in sem_getvalue@@GLIBC_2.2.5 () from /lib64/libpthread.so.0
(gdb) where
#0  0x00007f29a5156610 in sem_getvalue@@GLIBC_2.2.5 () from /lib64/libpthread.so.0
#1  0x00007f299c60e7bb in semlock_getvalue (self=0x7f299a95e2b0, _unused_ignored=0x0)
    at /home/haypo/prog/python/master/Modules/_multiprocessing/semaphore.c:541
#2  0x0000000000434537 in _PyMethodDef_RawFastCallKeywords (method=0x7f299c8102e0 <semlock_methods+192>, 
    self=<_multiprocessing.SemLock at remote 0x7f299a95e2b0>, args=0x7f299c5f47e8, nargs=0, kwnames=0x0) at Objects/call.c:629
#3  0x0000000000607aff in _PyMethodDescr_FastCallKeywords (descrobj=<method_descriptor at remote 0x7f299ca42520>, args=0x7f299c5f47e0, nargs=1, 
    kwnames=0x0) at Objects/descrobject.c:288
#4  0x0000000000512f92 in call_function (pp_stack=0x7ffd3591f730, oparg=1, kwnames=0x0) at Python/ceval.c:4595
(...)

(gdb) py-bt
Traceback (most recent call first):
  File "/home/haypo/prog/python/master/Lib/multiprocessing/synchronize.py", line 170, in __repr__
    elif self._semlock._get_value() == 1:
  File "/home/haypo/prog/python/master/test_lock_sigsegv.py", line 20, in child
    print("Child r_q: %r, %r, %r" % (r_q._rlock, r_q._wlock, r_q._sem), flush=True)
  File "/home/haypo/prog/python/master/Lib/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/home/haypo/prog/python/master/Lib/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/home/haypo/prog/python/master/Lib/multiprocessing/spawn.py", line 130, in _main
    return self._bootstrap()
  File "/home/haypo/prog/python/master/Lib/multiprocessing/spawn.py", line 629, in spawn_main
  File "<string>", line 1, in <module>

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33196>
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