assertion failure at Objects/call.c:618: `PyObject *callmethod(PyThreadState *, PyObject *, const char *, struct __va_list_tag *): Assertion 'callable != NULL' failed.`

[YuanchengJiang]

Crash report

What happened?

import builtins
import sys

def test_import(name, *args, **kwargs):
    sys.modules[name] = object()
    return sys.modules[name]

builtins.__import__ = test_import

raise RuntimeError("Test")

python: ../Objects/call.c:618: PyObject *callmethod(PyThreadState *, PyObject *, const char *, struct __va_list_tag *): Assertion `callable != NULL' failed.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-142737: Handle lost io.open in _Py_FindSourceFile #142747
• [3.14] gh-142737: Handle lost io.open in _Py_FindSourceFile (GH-142747) #142773
• [3.13] gh-142737: Handle lost io.open in _Py_FindSourceFile (GH-142747) #142774

[johnslavik]

Full message (please include it when possible):

Exception ignored in the internal traceback machinery:
AttributeError: 'object' object has no attribute '_print_exception_bltin'
Traceback (most recent call last):
  File "/home/bswck/Python/cpython/t.py", line 12, in <module>
python: Objects/call.c:618: callmethod: Assertion `callable != NULL' failed.
Fatal Python error: Aborted
Aborted (core dumped)

The problem happens because _Py_FindSourceFile resorts to _PyObject_CallMethodFormat which assumes callable is healthy (the callable in this case is the result of getting open of io, which is NULL because io is an object() from test_import).
Maybe NULL-check the callable first? (PR in progress)

[ZeroIntensity]

Maybe NULL-check the callable first? (PR in progress)

I think the issue is here:

cpython/Python/traceback.c

Lines 417 to 420 in 14d3974

	open = PyObject_GetAttr(io, &_Py_ID(open));
	for (i = 0; i < npath; i++) {
	v = PyList_GetItem(syspath, i);
	if (v == NULL) {

We need to check for a NULL result from PyObject_GetAttr, not defer the check to _PyObject_CallMethodFormat, because:

1. That will slow things down for other callers.
2. It's generally unsafe to call arbitrary C API functions with an exception set.

[johnslavik]

Would slow down for other callers of _Py_FindSourceFile -- there are no other callers of _PyObject_CallMethodFormat than traceback.c itself.

I think we can skip the entire loop actually, if open is NULL?

[ZeroIntensity]

Would slow down for other callers of _Py_FindSourceFile -- there are no other callers of _PyObject_CallMethodFormat than traceback.c itself.

Then I assume that API was added with the intention that we'll use it somewhere else someday.

I think we can skip the entire loop actually, if open is NULL?

Yeah, that makes sense to me.