Undoing Gevent’s Monkey-Patching
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Join For FreeI ran into an odd problem while testing the next release of PyMongo, the Python driver for MongoDB which I help develop. We’re improving its support for Gevent, so we’re of course doing additional tests that begin with:
from gevent import monkey; monkey.patch_socket()
Now, some tests rely on this patching, and some rely on not being patched. Gevent doesn’t provide an unpatch_socket, so I had a clever idea: I’ll fork a subprocess with multiprocessing, do the test there, and return its result to the parent process in a multiprocessing.Value. Then subsequent tests won’t be affected by the patching.
SUCCESS = 1 FAILURE = 0 def my_test(outcome): from gevent import monkey; monkey.patch_socket() # do the test .... outcome.value = SUCCESS class Test(unittest.TestCase): def test(self): outcome = multiprocessing.Value('i', FAILURE) multiprocessing.Process(target=my_test, args=(outcome,)).start().join() self.assertEqual(SUCCESS, outcome.value)
Nice and straightforward, right? In sane operating systems this worked great. On Windows it broke horribly. When I did python setup.py test, instead of executing my_test(), multiprocessing on Windows restarted the whole test suite, which started another whole test suite, … Apparently, since Windows can’t fork(), multiprocessing re-imports your script and attempts to execute the proper function within it. If the test suite is begun with python setup.py test, then everything goes haywire. This problem with multiprocessing and unittests on Windows was discussed on the Python mailing list last February.
After some gloomy minutes, I decided to look at what patch_socket() is doing. Turns out it’s simple, so I wrote a version which allows unpatching:
def patch_socket(aggressive=True): """Like gevent.monkey.patch_socket(), but stores old socket attributes for unpatching. """ from gevent import socket _socket = __import__('socket') old_attrs = {} for attr in ( 'socket', 'SocketType', 'create_connection', 'socketpair', 'fromfd' ): if hasattr(_socket, attr): old_attrs[attr] = getattr(_socket, attr) setattr(_socket, attr, getattr(socket, attr)) try: from gevent.socket import ssl, sslerror old_attrs['ssl'] = _socket.ssl _socket.ssl = ssl old_attrs['sslerror'] = _socket.sslerror _socket.sslerror = sslerror except ImportError: if aggressive: try: del _socket.ssl except AttributeError: pass return old_attrs def unpatch_socket(old_attrs): """Take output of patch_socket() and undo patching.""" _socket = __import__('socket') for attr in old_attrs: if hasattr(_socket, attr): setattr(_socket, attr, old_attrs[attr]) def patch_dns(): """Like gevent.monkey.patch_dns(), but stores old socket attributes for unpatching. """ from gevent.socket import gethostbyname, getaddrinfo _socket = __import__('socket') old_attrs = {} old_attrs['getaddrinfo'] = _socket.getaddrinfo _socket.getaddrinfo = getaddrinfo old_attrs['gethostbyname'] = _socket.gethostbyname _socket.gethostbyname = gethostbyname return old_attrs def unpatch_dns(old_attrs): """Take output of patch_dns() and undo patching.""" _socket = __import__('socket') for attr in old_attrs: setattr(_socket, attr, old_attrs[attr])
In Gevent’s version, calling patch_socket() calls patch_dns() implicitly, in mine you must call both:
class Test(unittest.TestCase): def test(self): old_socket_attrs = patch_socket() old_dns_attrs = patch_dns() try: # do test ... finally: unpatch_dns(old_dns_attrs) unpatch_socket(old_socket_attrs)
Now I don’t need multiprocessing at all.
Published at DZone with permission of A. Jesse Jiryu Davis, DZone MVB. See the original article here.
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