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

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kemingy wants to merge 6 commits into python:main from kemingy:gh-141805
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29 changes: 29 additions & 0 deletions Lib/test/test_set.py
View file Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -675,6 +675,35 @@ def __hash__(self):
with self.assertRaises(KeyError):
myset.discard(elem2)

def test_set_add_to_dummy_slot(self):
# gh-141805
tasks = set()

class Dummy:
def __hash__(self):
return 0

class CorruptTrigger:
triggered = False

def __hash__(self):
return 0

def __eq__(self, value):
if not self.triggered:
self.triggered = True
tasks.add(self)
return False

tasks.add(Dummy())
tasks.add(Dummy())
tasks.pop()
self.assertEqual(len(tasks), 1)
tasks.add(CorruptTrigger())
self.assertEqual(len(tasks), 2)
# cover the case in gh-141805 that triggers segfault
repr(tasks)


class SetSubclass(set):
pass
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix set corruption due to invalid length calculation when set additions occur
in the element comparison (:meth:`object.__eq__`) methods.
1 change: 1 addition & 0 deletions Objects/setobject.c
View file Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -298,6 +298,7 @@ set_add_entry_takeref(PySetObject *so, PyObject *key, Py_hash_t hash)
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.

}
entry++;
} while (probes--);
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