Fix segmentation faults when handling unserialisable objects

Errors during `__repr__` itself as well as ones during the conversion to a bytes object were not handled, resulting in NULL pointer dereferencing.

Cf. #382

python/objToJSON.c

@@ -670,8 +670,15 @@ static void Object_beginTypeContext (JSOBJ _obj, JSONTypeContext *tc, JSONObject
   PyErr_Clear();
 
   objRepr = PyObject_Repr(obj);
-  PyObject* str = PyUnicode_AsEncodedString(objRepr, "utf-8", "~E~");
-  PyErr_Format (PyExc_TypeError, "%s is not JSON serializable", PyBytes_AsString(str));
+  if (!objRepr)
+  {
+    goto INVALID;
+  }
+  PyObject* str = PyUnicode_AsEncodedString(objRepr, "utf-8", "strict");
+  if (str)
+  {
+    PyErr_Format (PyExc_TypeError, "%s is not JSON serializable", PyBytes_AsString(str));
+  }
   Py_XDECREF(str);
   Py_DECREF(objRepr);
 

tests/test_ujson.py

@@ -636,22 +636,31 @@ def test_dumps(test_input, expected):
 
 
 class SomeObject:
+    def __init__(self, message, exception=None):
+        self._message = message
+        self._exception = exception
+
     def __repr__(self):
-        return "Some Object"
+        if self._exception:
+            raise self._exception
+        return self._message
 
 
 @pytest.mark.parametrize(
     "test_input, expected_exception, expected_message",
     [
         (set(), TypeError, "set() is not JSON serializable"),
         ({1, 2, 3}, TypeError, "{1, 2, 3} is not JSON serializable"),
-        (SomeObject(), TypeError, "Some Object is not JSON serializable"),
+        (SomeObject("Some Object"), TypeError, "Some Object is not JSON serializable"),
+        (SomeObject("\ud800"), UnicodeEncodeError, None),
+        (SomeObject(None, KeyboardInterrupt), KeyboardInterrupt, None),
     ],
 )
 def test_dumps_raises(test_input, expected_exception, expected_message):
     with pytest.raises(expected_exception) as e:
         ujson.dumps(test_input)
-    assert str(e.value) == expected_message
+    if expected_message:
+        assert str(e.value) == expected_message
 
 
 @pytest.mark.parametrize(