From 7d2b37ddd64ba4d3d4b8b5efdbca146a4a13807d Mon Sep 17 00:00:00 2001
From: Changjoon <sisyphus9402@gmail.com>
Date: Sun, 19 Apr 2026 22:35:09 +0900
Subject: [PATCH] Fix segfault on cyclic or deeply-nested AST in compile()
 (#7630)

---
 crates/vm/src/stdlib/_ast/node.rs  | 17 ++++++++--
 extra_tests/snippets/stdlib_ast.py | 51 ++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/crates/vm/src/stdlib/_ast/node.rs b/crates/vm/src/stdlib/_ast/node.rs
index 6f74e7151..4ee3893b6 100644
--- a/crates/vm/src/stdlib/_ast/node.rs
+++ b/crates/vm/src/stdlib/_ast/node.rs
@@ -31,7 +31,14 @@ impl<T: Node> Node for Vec<T> {
         source_file: &SourceFile,
         object: PyObjectRef,
     ) -> PyResult<Self> {
-        vm.extract_elements_with(&object, |obj| Node::ast_from_object(vm, source_file, obj))
+        // Recursion guard for each element: prevents stack overflow when a
+        // sequence element transitively references the sequence itself
+        // (e.g. `l = ast.List(...); l.elts = [l]`). See issue #4862.
+        vm.extract_elements_with(&object, |obj| {
+            vm.with_recursion("while traversing AST node", || {
+                Node::ast_from_object(vm, source_file, obj)
+            })
+        })
     }
 }
 
@@ -45,7 +52,13 @@ impl<T: Node> Node for Box<T> {
         source_file: &SourceFile,
         object: PyObjectRef,
     ) -> PyResult<Self> {
-        T::ast_from_object(vm, source_file, object).map(Self::new)
+        // Recursion guard: every descent through a Box<AstNode> increments the
+        // VM's recursion depth so cyclic or pathologically deep ASTs raise
+        // RecursionError instead of overflowing the native stack.
+        // See issue #4862.
+        vm.with_recursion("while traversing AST node", || {
+            T::ast_from_object(vm, source_file, object).map(Self::new)
+        })
     }
 
     fn is_none(&self) -> bool {
diff --git a/extra_tests/snippets/stdlib_ast.py b/extra_tests/snippets/stdlib_ast.py
index dc626506f..7b5c69df4 100644
--- a/extra_tests/snippets/stdlib_ast.py
+++ b/extra_tests/snippets/stdlib_ast.py
@@ -37,3 +37,54 @@ assert i.level == 3
 assert i.module is None
 assert i.names[0].name == "a"
 assert i.names[0].asname is None
+
+
+# Regression test for issue #4862:
+# A cyclic AST fed to compile() used to overflow the Rust stack and SIGSEGV.
+# After the fix, the recursion guard in ast_from_object raises RecursionError,
+# matching CPython's behavior. Covers both Box<T> descents (UnaryOp, BinOp,
+# Call, Attribute) and Vec<T> descents (List, Tuple).
+import warnings
+
+
+def _cyclic_cases():
+    # Box<Expr> descents
+    u = ast.UnaryOp(op=ast.Not(), lineno=0, col_offset=0)
+    u.operand = u
+    yield "UnaryOp", u
+
+    b = ast.BinOp(
+        op=ast.Add(),
+        right=ast.Constant(value=0, lineno=0, col_offset=0),
+        lineno=0,
+        col_offset=0,
+    )
+    b.left = b
+    yield "BinOp", b
+
+    c = ast.Call(args=[], keywords=[], lineno=0, col_offset=0)
+    c.func = c
+    yield "Call", c
+
+    a = ast.Attribute(attr="x", ctx=ast.Load(), lineno=0, col_offset=0)
+    a.value = a
+    yield "Attribute", a
+
+    # Vec<Expr> descents
+    lst = ast.List(ctx=ast.Load(), lineno=0, col_offset=0)
+    lst.elts = [lst]
+    yield "List", lst
+
+    tup = ast.Tuple(ctx=ast.Load(), lineno=0, col_offset=0)
+    tup.elts = [tup]
+    yield "Tuple", tup
+
+
+with warnings.catch_warnings():
+    warnings.simplefilter("ignore")
+    for name, node in _cyclic_cases():
+        try:
+            compile(ast.Expression(node), "<cyclic>", "eval")
+            raise AssertionError(f"cyclic {name} should raise RecursionError")
+        except RecursionError:
+            pass  # expected; matches CPython