bpo-11105: Do not crash when compiling recursive ASTs (GH-20594)

When compiling an AST object with a direct / indirect reference
cycles, on the conversion phase because of exceeding amount of
calls, a segfault was raised. This patch adds recursion guards to
places for preventing user inputs to not to crash AST but instead
raise a RecursionError.

1 files changed, 14 insertions, 0 deletions

diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index 249e4bf..6a6f06c 100644
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -1098,6 +1098,20 @@ Module(
         exec(code, ns)
         self.assertIn('sleep', ns)
 
+    def test_recursion_direct(self):
+        e = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
+        e.operand = e
+        with self.assertRaises(RecursionError):
+            compile(ast.Expression(e), "<test>", "eval")
+
+    def test_recursion_indirect(self):
+        e = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
+        f = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
+        e.operand = f
+        f.operand = e
+        with self.assertRaises(RecursionError):
+            compile(ast.Expression(e), "<test>", "eval")
+
 
 class ASTValidatorTests(unittest.TestCase):