| msg309402 - (view) |
Author: André Neto (André Neto) |
Date: 2018-01-03 09:44 |
| I'm working on a project where I need to share state between several processes (forked using gunicorn). I'm using dictionaries obtained from a multiprocessing SyncManager to achieve this. The issue is that if I have multiple forked processes concurrently accessing to different dictionaries created by the same SyncManager, the code will randomly crash while accessing (read-only and read-write) to any of the dictionaries. This behaviour is independent of the way I access the dictionary (i.e. it happens both using the dictionary functions (e.g. has_key) or the built-in keywords (e.g. key in dict). The attached snippet demonstrates the issue (tested only in python2.7): the function test_manyForkedProcessesSingleThreaded will crash. The issue is also being discussed here: https://stackoverflow.com/questions/48052148/python-multiprocessing-dict-sharing-between-processes |
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| msg309403 - (view) |
Author: Christian Heimes (christian.heimes) *  |
Date: 2018-01-03 09:55 |
| Does it it actually crash (segfault) or are you merely getting an exception? I'm just getting bunch of exception: Traceback (most recent call last): File "multiprocessing_crash.py", line 155, in test_manyForkedProcessesSingleThreaded(inst1, inst2, nRuns, nProcesses) File "multiprocessing_crash.py", line 76, in test_manyForkedProcessesSingleThreaded run(inst1, nRuns) File "multiprocessing_crash.py", line 28, in run inst.run() File "multiprocessing_crash.py", line 23, in run self.d[self.key] = 0 File "", line 2, in __setitem__ Traceback (most recent call last): File "/usr/lib64/python2.7/multiprocessing/managers.py", line 759, in _callmethod File "multiprocessing_crash.py", line 155, in test_manyForkedProcessesSingleThreaded(inst1, inst2, nRuns, nProcesses) File "multiprocessing_crash.py", line 82, in test_manyForkedProcessesSingleThreaded run(inst2, nRuns) File "multiprocessing_crash.py", line 28, in run inst.run() File "multiprocessing_crash.py", line 19, in run item = self.d.get(self.key) File "", line 2, in get File "/usr/lib64/python2.7/multiprocessing/managers.py", line 759, in _callmethod kind, result = conn.recv() kind, result = conn.recv() cPickle.UnpicklingErrorMemoryError : invalid load key, '#'. Traceback (most recent call last): File "/usr/lib64/python2.7/multiprocessing/util.py", line 268, in _run_finalizers Traceback (most recent call last): File "/usr/lib64/python2.7/multiprocessing/util.py", line 268, in _run_finalizers finalizer() finalizer() File "/usr/lib64/python2.7/multiprocessing/util.py", line 201, in __call__ File "/usr/lib64/python2.7/multiprocessing/util.py", line 201, in __call__ res = self._callback(*self._args, **self._kwargs) res = self._callback(*self._args, **self._kwargs) File "/usr/lib64/python2.7/multiprocessing/managers.py", line 609, in _finalize_manager File "/usr/lib64/python2.7/multiprocessing/managers.py", line 609, in _finalize_manager if process.is_alive(): if process.is_alive(): File "/usr/lib64/python2.7/multiprocessing/process.py", line 155, in is_alive File "/usr/lib64/python2.7/multiprocessing/process.py", line 155, in is_alive assert self._parent_pid == os.getpid(), 'can only test a child process' AssertionError: can only test a child process |
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| msg309405 - (view) |
Author: André Neto (André Neto) |
Date: 2018-01-03 10:12 |
| You are right, it does not segfault (sorry for the abuse of language). It raises an exception while accessing the shared dictionary. The exception varies but typically is: Traceback (most recent call last): File "multiprocessbug.py", line 156, in test_manyForkedProcessesSingleThreaded(inst1, inst2, nRuns, nProcesses) File "multiprocessbug.py", line 77, in test_manyForkedProcessesSingleThreaded run(inst1, nRuns) File "multiprocessbug.py", line 29, in run inst.run() File "multiprocessbug.py", line 18, in run if (self.d.has_key(self.key)): File "", line 2, in has_key File "/usr/local/lib/python2.7/multiprocessing/managers.py", line 759, in _callmethod kind, result = conn.recv() cPickle.UnpicklingError: invalid load key, '#'. |
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| msg309410 - (view) |
Author: Christian Heimes (christian.heimes) * |
Date: 2018-01-03 11:29 |
| Don't worry about your choice of words. We usually use the C definition of the term crash. An exception is just a bug, but a segfault may be a security issue. |
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| msg329452 - (view) |
Author: Augusto Goulart (augustogoulart) * |
Date: 2018-11-08 00:38 |
| This is probably a bug that was fixed sometime in Python 3x. Testing on 3.7.1, Python gracefully errors out: ``` ➜ cpython git:(bpo-32485) ✗ python issue_32485.py Same manager, same mux Starting test_sameProcessSameThread Traceback (most recent call last): File "issue_32485.py", line 130, in test_sameProcessSameThread(inst1, inst2, nRuns) File "issue_32485.py", line 48, in test_sameProcessSameThread run(inst1, nRuns) File "issue_32485.py", line 30, in run inst.run() File "issue_32485.py", line 19, in run if (self.d.has_key(self.key)): File "", line 2, in has_key File "/home/gus/.pyenv/versions/3.7.1/lib/python3.7/multiprocessing/managers.py", line 811, in _callmethod raise convert_to_error(kind, result) multiprocessing.managers.RemoteError: --------------------------------------------------------------------------- Traceback (most recent call last): File "/home/gus/.pyenv/versions/3.7.1/lib/python3.7/multiprocessing/managers.py", line 251, in serve_client function = getattr(obj, methodname) AttributeError: 'dict' object has no attribute 'has_key' During handling of the above exception, another exception occurred: Traceback (most recent call last): File "/home/gus/.pyenv/versions/3.7.1/lib/python3.7/multiprocessing/managers.py", line 271, in serve_client fallback_func = self.fallback_mapping[methodname] KeyError: 'has_key' --------------------------------------------------------------------------- ``` |
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| msg329479 - (view) |
Author: Tal Einat (taleinat) * |
Date: 2018-11-08 16:28 |
| Is this supposed to work at all? Does multiprocessing support sharing resources between processes forked outside of its own abstractions? Reading the docs[1], calling os.fork() directly doesn't seem to be the way that the multiprocessing module is supposed to be used. [1] https://docs.python.org/library/multiprocessing.html |
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| msg329552 - (view) |
Author: Antoine Pitrou (pitrou) * |
Date: 2018-11-09 20:10 |
| Gus, dict don't have a has_key() method in Python 3. multiprocessing objects are generally not fork-safe: multiprocessing has its own mechanisms to duplicate objects between processes, but you cannot assume that after calling fork() yourself, objects will still be usable in the child. I'm closing this issue as not a bug. |
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