[3.9] bpo-41690: Use a loop to collect args in the parser instead of …

…recursion (GH-22053) (GH-22067)

This program can segfault the parser by stack overflow:

```
import ast

code = "f(" + ",".join(['a' for _ in range(100000)]) + ")"
print("Ready!")
ast.parse(code)
```

the reason is that the rule for arguments has a simple recursion when collecting args:

args[expr_ty]:
    [...]
    | a=named_expression b=[',' c=args { c }] {
        [...] }.
(cherry picked from commit 4a97b15)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Grammar/python.gram

@@ -537,22 +537,11 @@ arguments[expr_ty] (memo):
     | a=args [','] &')' { a }
     | incorrect_arguments
 args[expr_ty]:
-    | a=starred_expression b=[',' c=args { c }] {
-        _Py_Call(_PyPegen_dummy_name(p),
-                 (b) ? CHECK(_PyPegen_seq_insert_in_front(p, a, ((expr_ty) b)->v.Call.args))
-                     : CHECK(_PyPegen_singleton_seq(p, a)),
-                 (b) ? ((expr_ty) b)->v.Call.keywords : NULL,
-                 EXTRA) }
+    | a=','.(starred_expression | named_expression !'=')+ b=[',' k=kwargs {k}] { _PyPegen_collect_call_seqs(p, a, b) }
     | a=kwargs { _Py_Call(_PyPegen_dummy_name(p),
                           CHECK_NULL_ALLOWED(_PyPegen_seq_extract_starred_exprs(p, a)),
                           CHECK_NULL_ALLOWED(_PyPegen_seq_delete_starred_exprs(p, a)),
                           EXTRA) }
-    | a=named_expression b=[',' c=args { c }] {
-        _Py_Call(_PyPegen_dummy_name(p),
-                 (b) ? CHECK(_PyPegen_seq_insert_in_front(p, a, ((expr_ty) b)->v.Call.args))
-                     : CHECK(_PyPegen_singleton_seq(p, a)),
-                 (b) ? ((expr_ty) b)->v.Call.keywords : NULL,
-                 EXTRA) }
 kwargs[asdl_seq*]:
     | a=','.kwarg_or_starred+ ',' b=','.kwarg_or_double_starred+ { _PyPegen_join_sequences(p, a, b) }
     | ','.kwarg_or_starred+

Misc/NEWS.d/next/Core and Builtins/2020-09-02-12-00-57.bpo-41690.Ny-Sfy.rst

@@ -0,0 +1,2 @@
+Fix a possible stack overflow in the parser when parsing functions and
+classes with a huge ammount of arguments. Patch by Pablo Galindo.