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gh-141805: avoid increase the set->used twice when the __eq__ trigger another add #142007

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@kemingy kemingy commented Nov 27, 2025

gh-141805: goes directly to the found_unused_or_dummy when finding a dummy slot in set.add

This avoids triggering the __eq__ in the next iteration by the PyObject_RichCompareBool, which might introduce another set.add in the user code. Like:

tasks = set()
class Corrupt:
 def __eq__(self, value):
 tasks.add(self)
 return False

As described in #141805 (comment)

yihong0618 reacted with thumbs up emoji 
...er add
Signed-off-by: Keming <kemingy94@gmail.com>
@kemingy kemingy changed the title (削除) fix: avoid increase the set->used twice when the __eq__ trigger another add (削除ここまで) (追記) gh-141805: avoid increase the set->used twice when the __eq__ trigger another add (追記ここまで) Nov 27, 2025 
Signed-off-by: Keming <kemingy94@gmail.com>
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@efimov-mikhail efimov-mikhail left a comment

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This PR looks correct for me, just little nitpicks.

@efimov-mikhail efimov-mikhail added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Dec 20, 2025
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LGTM!

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kemingy commented Jan 8, 2026

Kindly ping @rhettinger, please take a look when you are free. Thanks!

else if (entry->hash == -1) {
assert (entry->key == dummy);
freeslot = entry;
goto found_unused_or_dummy;
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@serhiy-storchaka serhiy-storchaka Jan 13, 2026

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No, this is incorrect.

Imagine you added two elements with the same hash, A and B. Then A hes been removed, and replaced with a dummy (this is why dummy is needed). Then we try to add B again. With your code, it will find a dummy in place of A, and replace it with B. Now you have two Bs in a set.

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@serhiy-storchaka serhiy-storchaka Jan 13, 2026

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#143781 adds a test for this.

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@serhiy-storchaka serhiy-storchaka serhiy-storchaka left review comments

@efimov-mikhail efimov-mikhail efimov-mikhail approved these changes

@rhettinger rhettinger Awaiting requested review from rhettinger rhettinger is a code owner

+1 more reviewer

@dr-carlos dr-carlos dr-carlos requested changes

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