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bpo-36027: Fix a potential refcount bug in long_pow by mdickinson · Pull Request #26690 · python/cpython · GitHub

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bpo-36027: Fix a potential refcount bug in long_pow - #26690

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mdickinson merged 1 commit into
python:mainfrom
mdickinson:fix/potential-refcount-error-in-long-pow
Jun 13, 2021
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bpo-36027: Fix a potential refcount bug in long_pow#26690
mdickinson merged 1 commit into
python:mainfrom
mdickinson:fix/potential-refcount-error-in-long-pow

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mdickinson commented Jun 12, 2021 •
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This PR fixes a reference counting bug introduced in #13266 and identified by @tim-one in #26662: if we don't set temp to NULL, and if the value it has at this point is retained until the end of the function, then we end up doing an extra decref. I spent some time trying to identify a test-case that exercises this bug, and failed - I couldn't find a code path where temp isn't re-used at some point. Nevertheless, it's a potential point of fragility for maintenance of this code.

https://bugs.python.org/issue36027

mdickinson added type-bug An unexpected behavior, bug, or error and removed needs backport to 3.8 labels Jun 12, 2021

mdickinson commented Jun 12, 2021

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3.8 is in security-fix-only mode; adjusting the backport labels.

tim-one commented Jun 12, 2021

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Yes, this showed up for me because I introduced a new path that, for exponent 1 and no modulus, invoked REDUCE but not MULT. Without a modulus, REDUCE didn't overwrite temp, and without MULT nothing else did either. Nasty 😉.

tim-one reviewed Jun 12, 2021
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Ship it!

mdickinson merged commit 5924243 into python:main Jun 13, 2021

miss-islington commented Jun 13, 2021

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Thanks @mdickinson for the PR 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.9.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

miss-islington commented Jun 13, 2021

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Sorry @mdickinson, I had trouble checking out the 3.10 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 59242431991794064824cf2ab70886367613f29e 3.10

bedevere-bot commented Jun 13, 2021

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GH-26702 is a backport of this pull request to the 3.9 branch.

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Jun 13, 2021
Fix a potential reference-counting bug in long_pow (pythonGH-26690)
91a74e5 
(cherry picked from commit 5924243)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
mdickinson added a commit to mdickinson/cpython that referenced this pull request Jun 13, 2021
[3.10] Fix a potential reference-counting bug in long_pow (pythonGH-2…
def92d7 
…6690)

(cherry picked from commit 5924243)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

bedevere-bot commented Jun 13, 2021

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GH-26703 is a backport of this pull request to the 3.10 branch.

mdickinson added a commit that referenced this pull request Jun 13, 2021
Fix a potential reference-counting bug in long_pow (GH-26690) (#26702)
9ee8752 
(cherry picked from commit 5924243)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
mdickinson deleted the fix/potential-refcount-error-in-long-pow branch June 13, 2021 07:46
mdickinson added a commit that referenced this pull request Jun 13, 2021
[3.10] Fix a potential reference-counting bug in long_pow (GH-26690) (G…
929dd81 
…H-26703)

(cherry picked from commit 5924243)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
jdevries3133 pushed a commit to jdevries3133/cpython that referenced this pull request Jun 19, 2021
Fix a potential reference-counting bug in long_pow (pythonGH-26690)
6966890
pablogsal mentioned this pull request Jul 18, 2021
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