[3.14] gh-146480: Override the exception in _PyErr_SetKeyError() (GH-146486) (#146511)

gh-146480: Override the exception in _PyErr_SetKeyError() (GH-146486)

If _PyErr_SetKeyError() is called with an exception set, it now
replaces the current exception with KeyError (as expected), instead
of setting a SystemError or failing with a fatal error (in debug
mode).
(cherry picked from commit d4153a9f76)

Co-authored-by: Victor Stinner <vstinner@python.org>

Python/errors.c

@@ -247,13 +247,23 @@ PyErr_SetObject(PyObject *exception, PyObject *value)
     _PyErr_SetObject(tstate, exception, value);
 }
 
-/* Set a key error with the specified argument, wrapping it in a
- * tuple automatically so that tuple keys are not unpacked as the
- * exception arguments. */
+/* Set a key error with the specified argument. This function should be used to
+ * raise a KeyError with an argument instead of PyErr_SetObject(PyExc_KeyError,
+ * arg) which has a special behavior. PyErr_SetObject() unpacks arg if it's a
+ * tuple, and it uses arg instead of creating a new exception if arg is an
+ * exception.
+ *
+ * If an exception is already set, override the exception. */
 void
 _PyErr_SetKeyError(PyObject *arg)
 {
     PyThreadState *tstate = _PyThreadState_GET();
+
+    // PyObject_CallOneArg() must not be called with an exception set,
+    // otherwise _Py_CheckFunctionResult() can fail if the function returned
+    // a result with an excception set.
+    _PyErr_Clear(tstate);
+
     PyObject *exc = PyObject_CallOneArg(PyExc_KeyError, arg);
     if (!exc) {
         /* caller will expect error to be set anyway */