GeezerVison

I failed to find a function in Python 3.14 stdlib which still mutates the bytes object after calling _PyBytes_Resize(). The common pattern is more like the following code which is not affected:

bytes = PyBytes_FromStringAndSize(NULL, size);
... write into bytes ...
_PyBytes_Resize(&bytes, n);
return bytes;  // bytes no longer mutated from this point

There is _io.BytesIO.truncate() which may be affected by the issue, but its resize function allocates "size + 1" and so is not affected at the end.

This change should be backported to Python 3.13, but the fix there is different because _PyBytes_Resize() doesn't test _PyObject_IsUniquelyReferenced() but tests Py_REFCNT(v) == 1 instead.

GH-157436 is a backport of this pull request to the 3.14 branch.

GH-157436 is a backport of this pull request to the 3.13 branch.