[3.14] gh-143547: Fix PyErr_FormatUnraisable() fallback (#143557) (#143603)

gh-143547: Fix PyErr_FormatUnraisable() fallback (#143557)

Hold a strong reference to 'hook' while calling the default
unraisable took to log hook failure.

(cherry picked from commit 39a2bcf)

Author: vstinner

Lib/test/test_sys.py

@@ -1480,6 +1480,7 @@ def hook_func(args):
     def test_custom_unraisablehook_fail(self):
         _testcapi = import_helper.import_module('_testcapi')
         from _testcapi import err_writeunraisable
+
         def hook_func(*args):
             raise Exception("hook_func failed")
 

Misc/NEWS.d/next/Library/2026-01-08-14-53-46.gh-issue-143547.wHBVlr.rst

@@ -0,0 +1,3 @@
+Fix :func:`sys.unraisablehook` when the hook raises an exception and changes
+:func:`sys.unraisablehook`: hold a strong reference to the old hook. Patch
+by Victor Stinner.

Python/errors.c

@@ -1632,6 +1632,7 @@ format_unraisable_v(const char *format, va_list va, PyObject *obj)
     _Py_EnsureTstateNotNULL(tstate);
 
     PyObject *err_msg = NULL;
+    PyObject *hook = NULL;
     PyObject *exc_type, *exc_value, *exc_tb;
     _PyErr_Fetch(tstate, &exc_type, &exc_value, &exc_tb);
 
@@ -1676,7 +1677,6 @@ format_unraisable_v(const char *format, va_list va, PyObject *obj)
         goto error;
     }
 
-    PyObject *hook;
     if (_PySys_GetOptionalAttr(&_Py_ID(unraisablehook), &hook) < 0) {
         Py_DECREF(hook_args);
         err_msg_str = NULL;
@@ -1689,21 +1689,18 @@ format_unraisable_v(const char *format, va_list va, PyObject *obj)
     }
 
     if (_PySys_Audit(tstate, "sys.unraisablehook", "OO", hook, hook_args) < 0) {
-        Py_DECREF(hook);
         Py_DECREF(hook_args);
         err_msg_str = "Exception ignored in audit hook";
         obj = NULL;
         goto error;
     }
 
     if (hook == Py_None) {
-        Py_DECREF(hook);
         Py_DECREF(hook_args);
         goto default_hook;
     }
 
     PyObject *res = PyObject_CallOneArg(hook, hook_args);
-    Py_DECREF(hook);
     Py_DECREF(hook_args);
     if (res != NULL) {
         Py_DECREF(res);
@@ -1733,6 +1730,7 @@ format_unraisable_v(const char *format, va_list va, PyObject *obj)
     Py_XDECREF(exc_value);
     Py_XDECREF(exc_tb);
     Py_XDECREF(err_msg);
+    Py_XDECREF(hook);
     _PyErr_Clear(tstate); /* Just in case */
 }