PyRefTracer_DESTROY is not called for objects deferred by the trashcan mechanism

[corona10]

Bug report

This works correctly until Python 3.13, but #132280 introduces a regression.
A simple PR will fix this issue, and I have already prepared a fix for this issue.

import os
import subprocess
import sys
import sysconfig
import tempfile

CSRC = r"""
#include <Python.h>

static Py_ssize_t list_destroys = 0;

static int
tracer(PyObject *obj, PyRefTracerEvent event, void *data)
{
    if (event == PyRefTracer_DESTROY && PyList_CheckExact(obj)) {
        list_destroys++;
    }
    return 0;
}

static PyObject *
start(PyObject *self, PyObject *args)
{
    list_destroys = 0;
    if (PyRefTracer_SetTracer(tracer, NULL) < 0) {
        return NULL;
    }
    Py_RETURN_NONE;
}

static PyObject *
stop(PyObject *self, PyObject *args)
{
    if (PyRefTracer_SetTracer(NULL, NULL) < 0) {
        return NULL;
    }
    return PyLong_FromSsize_t(list_destroys);
}

static PyMethodDef methods[] = {
    {"start", start, METH_NOARGS, NULL},
    {"stop", stop, METH_NOARGS, NULL},
    {NULL, NULL, 0, NULL},
};

static PyModuleDef_Slot slots[] = {
#ifdef Py_GIL_DISABLED
    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
#endif
    {0, NULL},
};

static struct PyModuleDef module = {
    PyModuleDef_HEAD_INIT,
    .m_name = "reftrace_repro",
    .m_size = 0,
    .m_methods = methods,
    .m_slots = slots,
};

PyMODINIT_FUNC
PyInit_reftrace_repro(void)
{
    return PyModuleDef_Init(&module);
}
"""


def build_module():
    tmpdir = tempfile.mkdtemp(prefix="reftrace_repro_")
    src = os.path.join(tmpdir, "reftrace_repro.c")
    with open(src, "w") as f:
        f.write(CSRC)
    ext = os.path.join(tmpdir, "reftrace_repro.so")
    cmd = ["cc", "-O2", "-shared", "-fPIC",
           "-I", sysconfig.get_path("include"),
           "-I", sysconfig.get_path("platinclude"),
           "-o", ext, src]
    if sys.platform == "darwin":
        cmd.insert(3, "-undefined")
        cmd.insert(4, "dynamic_lookup")
    subprocess.run(cmd, check=True)
    sys.path.insert(0, tmpdir)


def run(depth):
    import reftrace_repro

    chain = None
    for _ in range(depth):
        chain = [chain]
    reftrace_repro.start()
    del chain
    destroys = reftrace_repro.stop()
    missing = depth - destroys
    print(f"depth={depth:>7}  DESTROY events={destroys:>7}  "
          f"missing={missing:>5}  [{'BUG' if missing else 'OK'}]")


if __name__ == "__main__":
    print(sys.version)
    build_module()
    run(100)        
    run(200_000)

Status

$ python3.13 repro.py
3.13.11 (main, Dec 17 2025, 20:55:16) [Clang 21.1.4 ]
depth=    100  DESTROY events=    100  missing=    0  [OK]
depth= 200000  DESTROY events= 200000  missing=    0  [OK]

$ python3.14 repro.py
3.14.3 (main, Feb  3 2026, 15:32:20) [Clang 17.0.0 (clang-1700.6.3.2)]
depth=    100  DESTROY events=    100  missing=    0  [OK]
depth= 200000  DESTROY events= 199923  missing=   77  [BUG]

$ python3.15 repro.py
3.15.0a3 (main, Dec 17 2025, 21:18:28) [Clang 21.1.4 ]
depth=    100  DESTROY events=    100  missing=    0  [OK]
depth= 200000  DESTROY events= 199570  missing=  430  [BUG]

$ ./python.exe repro.py   # main (unpatched)
3.16.0a0 (heads/main:e4b22adf249, Aug 26 2026, 01:02:19) [Clang 21.0.0]
depth=    100  DESTROY events=    100  missing=    0  [OK]
depth= 200000  DESTROY events= 199923  missing=   77  [BUG]

Linked PRs

• gh-156371: Fix missing PyRefTracer_DESTROY events for trashcan objects #156372

[markshannon]

OOI, how did you find this issue?

[corona10]

OOI, how did you find this issue?

While investigating other issues with Claude Code, I came across this issue. :)
To clarify, I independently validated the regression and the analysis, and I also directed the proposed solution myself, so this is not AI-generated slop.