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GeezerVison is showing github.com — gh-135106: [crash repro] Randomly deposit objects in `_Py_Dealloc` by colesbury · Pull Request #135589 · python/cpython

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@colesbury

@colesbury colesbury commented Jun 16, 2025 •
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Returns 1 a little less than 1% of the time in
_Py_RecursionLimit_GetMargin() to force the use of the trashcan mechanism.

This isn't intended to be merged. It's just to:

  1. Demonstrate the crash
  2. Test on a variety of platforms and configs from the CI
  • Issue: test_weakref.test_threaded_weak_key_dict_deepcopy crash: merged objects should have ob_tid == 0 #135106

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@colesbury
pythongh-135106: Randomly return 1 in _Py_RecursionLimit_GetMargin
030abcd 
Returns `1` a little less than 1% of the time in
_Py_RecursionLimit_GetMargin() to force the use of the trashcan
mechanism.
@bedevere-app bedevere-app Bot mentioned this pull request Jun 16, 2025
test_weakref.test_threaded_weak_key_dict_deepcopy crash: merged objects should have ob_tid == 0 #135106
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@colesbury colesbury added the skip news label Jun 16, 2025
AA-Turner
AA-Turner reviewed Jun 16, 2025
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Comment thread Include/internal/pycore_pystate.h Resolved

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@colesbury colesbury added the DO-NOT-MERGE label Jun 16, 2025
@colesbury colesbury changed the title gh-135106: Randomly return 1 in _Py_RecursionLimit_GetMargin gh-135106: [crash repro] Randomly return 1 in _Py_RecursionLimit_GetMargin Jun 16, 2025
@colesbury @AA-Turner
Update Include/internal/pycore_pystate.h
92e82ac 
Co-authored-by: Adam Turner <[email protected]>
@markshannon

markshannon commented Jun 17, 2025

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I don't think this shows anything more than that randomly changing the stack limit causes crashes.

The trashcan mechanism relies on the margin being larger for frames higher up the call stack. This breaks that invariant.

You could try changing margin > 2 and margin >= 4 to much larger numbers to see if that provokes a failure.
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@hugovk
hugovk force-pushed the gh-135106-crash-demo branch from f0ef510 to 92e82ac Compare June 17, 2025 07:32
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@colesbury colesbury changed the title gh-135106: [crash repro] Randomly return 1 in _Py_RecursionLimit_GetMargin gh-135106: [crash repro] Randomly deposit objects in _Py_Dealloc Jun 17, 2025
@colesbury
Update test so that RNG is in _Py_Dealloc
7c86a25
@colesbury
colesbury force-pushed the gh-135106-crash-demo branch from da82cbb to 7c86a25 Compare June 17, 2025 17:08
@colesbury

colesbury commented Jun 17, 2025

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I don't think this shows anything more than that randomly changing the stack limit causes crashes.

The only thing that matters here is the condition to deposit objects -- there's no invariant to be broken. I've updated the test to make that more clear. It should always be safe to deposit an object conservatively (i.e., to treat the available stack as smaller).

Here's the equivalent test with the old trashcan behavior, which works without crashing:
https://github.com/colesbury/cpython/tree/gh-135106-test-old-behavior
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@colesbury colesbury closed this Jun 25, 2025
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@AA-Turner AA-Turner AA-Turner left review comments
@ericsnowcurrently ericsnowcurrently Awaiting requested review from ericsnowcurrently ericsnowcurrently will be requested when the pull request is marked ready for review ericsnowcurrently is a code owner
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@colesbury @markshannon @hugovk @AA-Turner
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