prevert ast errors from being normalized before ast_error_finish is called (closes #15846)

Lib/test/test_ast.py

@@ -386,6 +386,12 @@ class ASTHelpers_Test(unittest.TestCase):
         b = compile('foo(1 + 1)', '<unknown>', 'exec', ast.PyCF_ONLY_AST)
         self.assertEqual(ast.dump(a), ast.dump(b))
 
+    def test_parse_in_error(self):
+        try:
+            1/0
+        except Exception:
+            self.assertRaises(SyntaxError, ast.parse, r"'\U'")
+
     def test_dump(self):
         node = ast.parse('spam(eggs, "and cheese")')
         self.assertEqual(ast.dump(node),

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.2.4
 Core and Builtins
 -----------------
 
+- Issue #15846: Fix SystemError which happened when using ast.parse in an
+  exception handler on code with syntax errors.
+
 - Issue #15761: Fix crash when PYTHONEXECUTABLE is set on Mac OS X.
 
 - Issue #15801: Make sure mappings passed to '%' formatting are actually

Python/ast.c

@@ -92,7 +92,15 @@ ast_error(const node *n, const char *errstr)
     PyObject *u = Py_BuildValue("zii", errstr, LINENO(n), n->n_col_offset);
     if (!u)
         return 0;
+    /*
+     * Prevent the error from being chained. PyErr_SetObject will normalize the
+     * exception in order to chain it. ast_error_finish, however, requires the
+     * error not to be normalized.
+     */
+    PyObject *save = PyThreadState_GET()->exc_value;
+    PyThreadState_GET()->exc_value = NULL;
     PyErr_SetObject(PyExc_SyntaxError, u);
+    PyThreadState_GET()->exc_value = save;
     Py_DECREF(u);
     return 0;
 }