From 85319e8a2265b86335a24e80711eb49fb3577c56 Mon Sep 17 00:00:00 2001
From: Pablo Galindo Salgado <pablogsal@gmail.com>
Date: Sun, 5 Apr 2026 22:11:44 +0100
Subject: [PATCH] gh-148144: Initialize visited on copied interpreter frames

_PyFrame_Copy() copied interpreter frames into generator and
frame-object storage without initializing the visited byte. Incremental
GC later reads frame->visited in mark_stacks() on non-start passes, so
copied frames could expose an uninitialized value once they became live
on a thread stack again.

Reset visited when copying a frame so copied frames start with defined
GC bookkeeping state. Preserve lltrace in Py_DEBUG builds.
---
 Include/internal/pycore_interpframe.h                        | 5 +++++
 .../2026-04-05-15-20-00.gh-issue-148144.f7qA0x.rst           | 3 +++
 2 files changed, 8 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-04-05-15-20-00.gh-issue-148144.f7qA0x.rst

diff --git a/Include/internal/pycore_interpframe.h b/Include/internal/pycore_interpframe.h
index 8db1aebdc11401..d744dd12cd0479 100644
--- a/Include/internal/pycore_interpframe.h
+++ b/Include/internal/pycore_interpframe.h
@@ -149,6 +149,11 @@ static inline void _PyFrame_Copy(_PyInterpreterFrame *src, _PyInterpreterFrame *
     int stacktop = (int)(src->stackpointer - src->localsplus);
     assert(stacktop >= 0);
     dest->stackpointer = dest->localsplus + stacktop;
+    // visited is GC bookkeeping for the current stack walk, not frame state.
+    dest->visited = 0;
+#ifdef Py_DEBUG
+    dest->lltrace = src->lltrace;
+#endif
     for (int i = 0; i < stacktop; i++) {
         dest->localsplus[i] = PyStackRef_MakeHeapSafe(src->localsplus[i]);
     }
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-04-05-15-20-00.gh-issue-148144.f7qA0x.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-04-05-15-20-00.gh-issue-148144.f7qA0x.rst
new file mode 100644
index 00000000000000..beda992a95bf94
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-04-05-15-20-00.gh-issue-148144.f7qA0x.rst
@@ -0,0 +1,3 @@
+Initialize ``_PyInterpreterFrame.visited`` when copying interpreter frames so
+incremental GC does not read an uninitialized byte from generator and
+frame-object copies.