_tracemalloc__get_object_traceback doesn't handle objects with pre-headers correctly

[colesbury]

In the main branch, the function _tracemalloc__get_object_traceback doesn't account for objects with preheaders:

cpython/Modules/_tracemalloc.c

Lines 1406 to 1414 in ea23271

	type = Py_TYPE(obj);
	if (PyType_IS_GC(type)) {
	ptr = (void *)((char *)obj - sizeof(PyGC_Head));
	}
	else {
	ptr = (void *)obj;
	}
	traceback = tracemalloc_get_traceback(DEFAULT_DOMAIN, (uintptr_t)ptr);

This means that when running with tracemalloc some allocation tracebacks are missing:

import warnings

class MyClass:
    def __del__(self):
        warnings.warn("Uh oh", ResourceWarning, source=self)

def func():
    m = MyClass()

func()

python3 -Wd -X tracemalloc=2 example.py

cc @markshannon

Linked PRs

• gh-101430: Update tracemalloc to handle presize properly. #101745

[colesbury]

Also _PyTraceMalloc_NewReference:

cpython/Modules/_tracemalloc.c

Lines 1726 to 1733 in 28db978

	uintptr_t ptr;
	PyTypeObject *type = Py_TYPE(op);
	if (PyType_IS_GC(type)) {
	ptr = (uintptr_t)((char *)op - sizeof(PyGC_Head));
	}
	else {
	ptr = (uintptr_t)op;
	}

[vstinner]

If someone is motivated, it would be nice to add an unit test to test_tracemalloc for this case.