bpo-29587: _PyErr_ChainExceptions() checks exception (GH-19902)

_PyErr_ChainExceptions() now ensures that the first parameter is an
exception type, as done by _PyErr_SetObject().

* The following function now check PyExceptionInstance_Check() in an
  assertion using a new _PyBaseExceptionObject_cast() helper
  function:

  * PyException_GetTraceback(), PyException_SetTraceback()
  * PyException_GetCause(), PyException_SetCause()
  * PyException_GetContext(), PyException_SetContext()

* PyExceptionClass_Name() now checks PyExceptionClass_Check() with an
  assertion.

* Remove XXX comment and add gi_exc_state variable to _gen_throw().

* Remove comment from test_generators

1 files changed, 11 insertions, 1 deletions

diff --git a/Python/errors.c b/Python/errors.c
index 9e53d05..f856a79 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -107,7 +107,8 @@ _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value)
     if (exception != NULL &&
         !PyExceptionClass_Check(exception)) {
         _PyErr_Format(tstate, PyExc_SystemError,
-                      "exception %R not a BaseException subclass",
+                      "_PyErr_SetObject: "
+                      "exception %R is not a BaseException subclass",
                       exception);
         return;
     }
@@ -484,6 +485,15 @@ _PyErr_ChainExceptions(PyObject *exc, PyObject *val, PyObject *tb)
         return;
 
     PyThreadState *tstate = _PyThreadState_GET();
+
+    if (!PyExceptionClass_Check(exc)) {
+        _PyErr_Format(tstate, PyExc_SystemError,
+                      "_PyErr_ChainExceptions: "
+                      "exception %R is not a BaseException subclass",
+                      exc);
+        return;
+    }
+
     if (_PyErr_Occurred(tstate)) {
         PyObject *exc2, *val2, *tb2;
         _PyErr_Fetch(tstate, &exc2, &val2, &tb2);