Issue 1878: class attribute cache failure (regression) (original) (raw)

Issue1878

Created on 2008-01-20 17:40 by _doublep, last changed 2022-04-11 14:56 by admin. This issue is now closed.

Messages (24)
msg61316 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 17:40
I have a regression from Python 2.5 to Python SVN (would-be-2.6). I believe this because of class attribute caching. The problem shown below happens because AbstractGCProtector is an extension class. So, Python interpreter doesn't have a chance to notice set_default() method below changes a class attribute. >>> from notify.all import * >>> original_protector = AbstractGCProtector.default >>> new_protector = FastGCProtector () >>> AbstractGCProtector.default is new_protector False >>> AbstractGCProtector.default is original_protector True Please note that this a regression. This code works as expected in 2.3 and 2.5.
msg61317 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 17:43
Eh, disregard that, I missed one line with set_default() call. Still, the unit test fails...
msg61318 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 17:45
OK, here it is: >>> from notify.all import * >>> original_protector = AbstractGCProtector.default >>> new_protector = FastGCProtector () >>> AbstractGCProtector.set_default (new_protector) >>> AbstractGCProtector.default is new_protector False >>> AbstractGCProtector.default is original_protector True It seems that this behaviour is somewhat random. Sometimes the False/True lines are reversed, as expected. I.e. it seems that attribute cache is sometimes recomputed...
msg61322 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2008-01-20 18:17
It would be very interesting to know what set_default() actually does. IOW, without the source code of the extension module we can't do anything about this.
msg61323 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 18:22
set_default() is a static method to set 'default'. Because of this: >>> AbstractGCProtector.default = 42 Traceback (most recent call last): File "", line 1, in TypeError: can't set attributes of built-in/extension type 'notify.gc.AbstractGCProtector' About source code: go to http://download.gna.org/py-notify/ download, unpack and do './run-tests.py' at top level. One test fails with Python SVN precisely because of this problem.
msg61324 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2008-01-20 18:44
I'm sorry, but I can't get this to run. With a clean 0.1.14 tarball, I get Building extension... running build_ext building 'notify.gc' extension creating build creating build/temp.linux-i686-2.5 creating build/temp.linux-i686-2.5/notify i686-pc-linux-gnu-gcc -pthread -fno-strict-aliasing -DNDEBUG -fPIC -I/usr/include/python2.5 -c notify/gc.c -o build/temp.linux-i686-2.5/notify/gc.o creating build/lib.linux-i686-2.5 creating build/lib.linux-i686-2.5/notify i686-pc-linux-gnu-gcc -pthread -shared build/temp.linux-i686-2.5/notify/gc.o -L/usr/lib -lpython2.5 -o build/lib.linux-i686-2.5/notify/gc.so [1] 28189 segmentation fault ~/devel/python/python run-tests.py when running with a trunk python (note the "2.5" in the paths...) When I build the extension manually and comment out the building command in run-tests.py, I get Note that most of the time is spent in gc.collect() calls, not in this package ..............................................................Fatal Python error: Objects/classobject.c:2311 object at 0x82dd2bc has negative ref count -606348326 [1] 28540 abort ~/devel/python/python run-tests.py
msg61325 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 18:50
Weird. Does it even run with a stable Python (not trunk)?
msg61327 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2008-01-20 18:51
Yes, runs fine with 2.5.
msg61328 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 18:54
Can you run the pasted script (from the third comment) manually then? The crash might be related to the bug in question.
msg61342 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2008-01-20 19:24
I've now built my trunk python without debugging enabled, and can reproduce your problem. Armin: the extension module directly modifies an extension type's tp_dict -- what should it do instead to make the cache happy?
msg61363 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 21:54
Even if there is an easy workaround for the extension (or even a fix, if modifying 'tp_dict' is not legal), I don't think it would be acceptable to make a backward-incompatible change in 2.6. I mean, sure Py-notify is by no means a widely-used library, but can you guarantee that no other extension will get broken?
msg61364 - (view)	Author: Amaury Forgeot d'Arc (amaury.forgeotdarc) *	Date: 2008-01-20 22:02
The issue seems similar to the one we had in ctypes when the method attribute cache was implemented. Ctypes was corrected in r59943. Maybe similar changes are needed for this extension. For example, PyDict_SetItemString (AbstractGCProtector_Type.tp_dict, "default", new_protector) should be modified like this: PyObject_SetAttr(AbstractGCProtector_Type, "default", new_protector)
msg61365 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-20 22:07
It doesn't help: ERROR: test_default_property (test._gc.AbstractGCProtectorTestCase) ---------------------------------------------------------------------- Traceback (most recent call last): File "/home/paul/notify/test/_gc.py", line 59, in test_default_property AbstractGCProtector.set_default (original_protector) TypeError: can't set attributes of built-in/extension type 'notify.gc.AbstractGCProtector' With this code: if (PyObject_SetAttrString ((PyObject *) &AbstractGCProtector_Type, "default", new_protector) == -1) return NULL;
msg61381 - (view)	Author: Armin Rigo (arigo) *	Date: 2008-01-21 10:16
I don't see in general how the patch can be kept compatible with extension modules that change the tp_dict of arbitrary types. All I can think of is a way to be safe against extension modules that only change the tp_dict of their own non-heap types (i.e. types defined in C). The method cache is disabled for types that don't have the Py_TPFLAGS_HAVE_VERSION_TAG flag; so if we would leave this flag out of Py_TPFLAGS_DEFAULT, non-heap types from extension modules would by default not use the cache. I'm not sure about the API for this. Would we then need to put Py_TPFLAGS_HAVE_VERSION_TAG explicitly on all core types? And about how to change tp_dict in a way that makes the cache happy - do we need to turn type_modified() into a public API? All in all, the lack of abstraction of the C API might kill the idea of this patch for CPython.
msg61415 - (view)	Author: Georg Brandl (georg.brandl) *	Date: 2008-01-21 16:58
We can of course add something like in #1229239, which allows type attributes to be set with PyObject_SetAttr, thereby updating the cache.
msg61447 - (view)	Author: Paul Pogonyshev (_doublep)	Date: 2008-01-21 20:53
I personally think that this bug is a showstopper for the caching patch in 2.6. Well, the problem can be deemed insignificant, but it is sure a break of backward compatibility and, worse yet, it results in _very_ obscure fails. Even if type dictionary changes are not all that common, I'm sure there are extensions out there that do it. For Py3k things can be different. I'm not sure what would be the best way, but at least Py3k is not required to be compatible with 2.x in all aspects.
msg70483 - (view)	Author: Benjamin Peterson (benjamin.peterson) *	Date: 2008-07-31 02:18
Ping
msg70490 - (view)	Author: Raymond Hettinger (rhettinger) *	Date: 2008-07-31 07:57
Guido, what say you, live with it, revert it, or apply Py_TPFLAGS_HAVE_VERSION_TAG to all core types?
msg70523 - (view)	Author: Armin Rigo (arigo) *	Date: 2008-07-31 16:07
Maybe there is a better solution along the following line: conditionally define Py_TPFLAGS_DEFAULT so that when compiling the Python core it includes the Py_TPFLAGS_HAVE_VERSION_TAG, but when compiling extension modules it does not. This should ensure compatibility with many existing extension modules. (It would still break if they play tricks like modifying the tp_dict of types other than their own.) In addition, to support the method cache in newer C extension modules: * C types defined by C extension modules can include the Py_TPFLAGS_HAVE_VERSION_TAG explicitly to enable the cache; * new C API functions should be introduced to give an official way to access the tp_dict of a type, e.g. PyType_{Get,Set,Del}DictItemString().
msg71466 - (view)	Author: Guido van Rossum (gvanrossum) *	Date: 2008-08-19 19:16
I like Armin's latest proposal: have Py_TPFLAGS_DEFAULT include Py_TPFLAGS_HAVE_VERSION_TAG when compiling the core only. ISTR there's a way to do this, but I can't find it right now.
msg71468 - (view)	Author: Guido van Rossum (gvanrossum) *	Date: 2008-08-19 19:25
Please review the patch here: http://codereview.appspot.com/3005
msg71476 - (view)	Author: Guido van Rossum (gvanrossum) *	Date: 2008-08-19 20:14
Submitted as r65874. I will block this for 3.0; 3.0 extensions that want to mess with tp_dict must explicitly disable this flag.
msg71479 - (view)	Author: Benjamin Peterson (benjamin.peterson) *	Date: 2008-08-19 20:44
Do we want a test?
msg71484 - (view)	Author: Guido van Rossum (gvanrossum) *	Date: 2008-08-19 21:06
Sure, go right ahead.