gh-143547: Fix PyErr_FormatUnraisable() fallback

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

Fix test_sys.UnraisableHookTest: use the right decorator function to
disable colors. Previously, tests were always skipped.

Author: vstinner

Lib/test/test_sys.py

@@ -1350,7 +1350,7 @@ def test_disable_gil_abi(self):
 
 
 @test.support.cpython_only
-@force_not_colorized
+@test.support.force_not_colorized_test_class
 class UnraisableHookTest(unittest.TestCase):
     def test_original_unraisablehook(self):
         _testcapi = import_helper.import_module('_testcapi')
@@ -1492,6 +1492,7 @@ 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")
 

Python/errors.c

@@ -1692,6 +1692,7 @@ format_unraisable_v(const char *format, va_list va, PyObject *obj)
         }
     }
 
+    PyObject *hook = NULL;
     PyObject *hook_args = make_unraisable_hook_args(
         tstate, exc_type, exc_value, exc_tb, err_msg, obj);
     if (hook_args == NULL) {
@@ -1700,7 +1701,6 @@ 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;
@@ -1727,7 +1727,6 @@ format_unraisable_v(const char *format, va_list va, PyObject *obj)
     }
 
     PyObject *res = PyObject_CallOneArg(hook, hook_args);
-    Py_DECREF(hook);
     Py_DECREF(hook_args);
     if (res != NULL) {
         Py_DECREF(res);
@@ -1757,6 +1756,7 @@ 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 */
 }