bpo-44897: WIP: Integrate trashcan into _Py_Dealloc#27738
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This is a WIP/proof-of-concept of doing away with Py_TRASHCAN_BEGIN and Py_TRASHCAN_END and instead integrating the functionality into _Py_Dealloc. There are a few advantages: - all container objects have the risk of overflowing the C stack if a long reference chain of them is created and then deallocated. So, to be safe, the tp_dealloc methods for those objects should be protected from overflowing the stack. - the Py_TRASHCAN_BEGIN and Py_TRASHCAN_END macros are hard to understand and a bit hard to use correctly. Making the mechanism internal avoids confusion. The code can be slightly simplified as well. This proof-of-concept seems to pass tests but it will need some careful review. The exact rules related to calling GC Track/Untrack are subtle and this changes things a bit. I.e. tp_dealloc is called with GC objects already untracked. For 3rd party extensions, they are calling PyObject_GC_UnTrack() and so I believe they should still work. The fact that PyObject_CallFinalizerFromDealloc() wants GC objects to definitely be tracked is a bit of a mystery to me (there is an assert to check that). I changed the code to track objects if they are not already tracked but I'm not sure that's correct. There could be a performance hit, due to the _PyType_IS_GC() test that was added to the _Py_Dealloc() function. For non-GC objects, that's going to be a new branch and I'm worried it might hurt a bit. OTOH, maybe it's just in the noise. Profiling will need to be done.
nascheme requested review from 1st1, markshannon, methane and rhettinger as code owners
Since _Py_Dealloc will untrack, don't need to do it in tp_dealloc. If an object is resurrected, PyObject_CallFinalizerFromDealloc() will do the track call.
I'm not sure _PyType_IS_GC() is safe in case the object returns false from tp_is_gc().
This PR is stale because it has been open for 30 days with no activity. |
github-actionsbot removed the stale 
MemberAuthor
nascheme commented Apr 5, 2023
FYI, I believe this would have fixed GH-102356 and probably a lot of similar crashing bugs that have not yet been found. I would imagine most "container" (GC tracked) objects can be used to blow up the C stack on dealloc. |
MemberAuthor
nascheme commented Sep 27, 2024
Since this PR is so out of date, I've created a new one: GH-124716 |
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This is a WIP/proof-of-concept of doing away with Py_TRASHCAN_BEGIN and
Py_TRASHCAN_END and instead integrating the functionality into
_Py_Dealloc. There are a few advantages:
all container objects have the risk of overflowing the C stack if a
long reference chain of them is created and then deallocated. So, to
be safe, the tp_dealloc methods for those objects should be protected
from overflowing the stack.
the Py_TRASHCAN_BEGIN and Py_TRASHCAN_END macros are hard to
understand and a bit hard to use correctly. Making the mechanism
internal avoids confusion. The code can be slightly simplified as
well.
This proof-of-concept seems to pass tests but it will need some careful
review. The exact rules related to calling GC Track/Untrack are subtle
and this changes things a bit. I.e. tp_dealloc is called with GC
objects already untracked. For 3rd party extensions, they are calling
PyObject_GC_UnTrack() and so I believe they should still work.
The fact that PyObject_CallFinalizerFromDealloc() wants GC objects to
definitely be tracked is a bit of a mystery to me (there is an assert to
check that). I changed the code to track objects if they are not
already tracked but I'm not sure that's correct.
There could be a performance hit, due to the _PyType_IS_GC() test that
was added to the _Py_Dealloc() function. For non-GC objects, that's
going to be a new branch and I'm worried it might hurt a bit. OTOH,
maybe it's just in the noise. Profiling will need to be done.
https://bugs.python.org/issue44897