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

[Arcadiy Ivanov]

New submission from Arcadiy Ivanov <arcadiy at ivanov.biz>:

While working on GH gevent/gevent#993 I've encountered a stall trying to read from an mp.Queue passed to mp.Process's target as an argument. Trying to print out the lock state in child process I encountered as SEGV in Lock's __repr__. I originally thought it was due to gevent/greenlet stack magic, but it wasn't. 

This happens when `fork` context Queue (default) is used with `spawn` context Process (obvious stupidity on my part, alas shouldn't crash).

Python 3.6.4 from PyEnv
Fedora 27

```
$ python test_lock_sigsegv.py 
Parent r_q: <Lock(owner=None)>, <Lock(owner=None)>, <BoundedSemaphore(value=2147483647, maxvalue=2147483647)>
-11
```

```
Program terminated with signal SIGSEGV, Segmentation fault.
#0  __new_sem_getvalue (sem=0x7fc877f54000, sval=sval at entry=0x7fffb130db9c) at sem_getvalue.c:38
38        *sval = atomic_load_relaxed (&isem->data) & SEM_VALUE_MASK;
...
#0  __new_sem_getvalue (sem=0x7fc877f54000, sval=sval at entry=0x7fffb130db9c) at sem_getvalue.c:38
#1  0x00007f1116aeb202 in semlock_getvalue (self=<optimized out>) at /tmp/python-build.20171219170845.6548/Python-3.6.4/Modules/_multiprocessing/semaphore.c:531
```

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

Test attached.

----------
components: Library (Lib)
files: test_lock_sigsegv.py
messages: 314762
nosy: arcivanov
priority: normal
severity: normal
status: open
title: SEGV in mp.synchronize.Lock.__repr__ in spawn'ed proc if ctx mismatched
type: crash
versions: Python 3.6
Added file: https://bugs.python.org/file47510/test_lock_sigsegv.py

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