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Use _PyObject_IsUniquelyReferenced instead of Py_REFCNT(x) == 1 for free-threaded compatibility #140061
Description
Activity
- changed the title [-]Widely use _PyObject_IsUniquelyReferenced to check if object is uniquely referenced[/-] [+]Use _PyObject_IsUniquelyReferenced instead of `Py_REFCNT(x) == 1` for free-threaded compatibility[/+] on Oct 13, 2025
- added a commit that references this issue on Oct 15, 2025
- added a commit that references this issue on Oct 15, 2025
As a heads up this migration led to a subtle free-threading only bug in _PyBytes_Resize (GH-157373). _IsUniquelyReferenced returns false when a reference crosses a thread boundary while Py_REFCNT(x) == 1 returns true. That resulted in an immortal constant object being returned because resize thought the bytes was shared (and thus immutable) in a case where code could still modify the bytes.
I used AI to spot-check the rest of the migrations here (Claude Fable 5.1) and it found one other behavior change worth noting: PySet_Add on a single reference frozenset which crosses thread boundaries now raises SystemError rather than passing.
I used AI to spot-check the rest of the migrations here (Claude Fable 5.1) and it found one other behavior change worth noting: PySet_Add on a single reference frozenset which crosses thread boundaries now raises SystemError rather than passing.
IMO PySet_Add(frozenset, key) works as expected here: you must not modify a frozenset from a thread different than the one which created it. If you think that the documentation isn't explicit about this point, maybe you can suggest a change for PySet_Add() documentation.
Feature or enhancement
Proposal:
There are a lot of places where Py_REFCNT(..)==1 used. IIUC it is not correct for no-gil version.
Has this already been discussed elsewhere?
This is a minor feature, which does not need previous discussion elsewhere
Links to previous discussion of this feature:
No response
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