https://github.com/python/cpython/commit/f554237b8ef6c60df651ac17eb0ef0c095cef185
commit: f554237b8ef6c60df651ac17eb0ef0c095cef185
branch: main
author: Mark Shannon <m...@hotpy.org>
committer: markshannon <m...@hotpy.org>
date: 2025-05-05T13:44:50+01:00
summary:
GH-133261: Make sure that the GC doesn't untrack objects in trashcan (GH-133431)
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2025-05-05-12-03-46.gh-issue-133261.bL1gqz.rst
M Objects/object.c
M Python/gc.c
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-05-12-03-46.gh-issue-133261.bL1gqz.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-05-12-03-46.gh-issue-133261.bL1gqz.rst
new file mode 100644
index 00000000000000..1f40ea7eb8843b
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-05-12-03-46.gh-issue-133261.bL1gqz.rst
@@ -0,0 +1,3 @@
+Fix bug where the cycle GC could untrack objects in the trashcan because
+they looked like they were immortal. When objects are added to the trashcan,
+we take care to ensure they keep a mortal reference count.
diff --git a/Objects/object.c b/Objects/object.c
index 0974a231ec101a..f5fe25eb5aaf8f 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -2930,6 +2930,33 @@ Py_ReprLeave(PyObject *obj)
/* Trashcan support. */
+#ifndef Py_GIL_DISABLED
+/* We need to store a pointer in the refcount field of
+ * an object. It is important that we never store 0 (NULL).
+* It is also important to not make the object appear immortal,
+* or it might be untracked by the cycle GC. */
+static uintptr_t
+pointer_to_safe_refcount(void *ptr)
+{
+ uintptr_t full = (uintptr_t)ptr;
+ assert((full & 3) == 0);
+ uint32_t refcnt = (uint32_t)full;
+ if (refcnt >= (uint32_t)_Py_IMMORTAL_MINIMUM_REFCNT) {
+ full = full - ((uintptr_t)_Py_IMMORTAL_MINIMUM_REFCNT) + 1;
+ }
+ return full + 2;
+}
+
+static void *
+safe_refcount_to_pointer(uintptr_t refcnt)
+{
+ if (refcnt & 1) {
+ refcnt += _Py_IMMORTAL_MINIMUM_REFCNT - 1;
+ }
+ return (void *)(refcnt - 2);
+}
+#endif
+
/* Add op to the gcstate->trash_delete_later list. Called when the current
* call-stack depth gets large. op must be a currently untracked gc'ed
* object, with refcount 0. Py_DECREF must already have been called on it.
@@ -2941,11 +2968,10 @@ _PyTrash_thread_deposit_object(PyThreadState *tstate,
PyObject *op)
#ifdef Py_GIL_DISABLED
op->ob_tid = (uintptr_t)tstate->delete_later;
#else
- /* Store the delete_later pointer in the refcnt field.
- * As this object may still be tracked by the GC,
- * it is important that we never store 0 (NULL). */
- uintptr_t refcnt = (uintptr_t)tstate->delete_later;
- *((uintptr_t*)op) = refcnt+1;
+ /* Store the delete_later pointer in the refcnt field. */
+ uintptr_t refcnt = pointer_to_safe_refcount(tstate->delete_later);
+ *((uintptr_t*)op) = refcnt;
+ assert(!_Py_IsImmortal(op));
#endif
tstate->delete_later = op;
}
@@ -2967,7 +2993,7 @@ _PyTrash_thread_destroy_chain(PyThreadState *tstate)
/* Get the delete_later pointer from the refcnt field.
* See _PyTrash_thread_deposit_object(). */
uintptr_t refcnt = *((uintptr_t*)op);
- tstate->delete_later = (PyObject *)(refcnt - 1);
+ tstate->delete_later = safe_refcount_to_pointer(refcnt);
op->ob_refcnt = 0;
#endif
diff --git a/Python/gc.c b/Python/gc.c
index b7b48c8af39c4a..7b0e6d6e803504 100644
--- a/Python/gc.c
+++ b/Python/gc.c
@@ -493,6 +493,7 @@ update_refs(PyGC_Head *containers)
next = GC_NEXT(gc);
PyObject *op = FROM_GC(gc);
if (_Py_IsImmortal(op)) {
+ assert(!_Py_IsStaticImmortal(op));
_PyObject_GC_UNTRACK(op);
gc = next;
continue;
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