gh-127582: Make object resurrection thread-safe for free threading. (GH-127612)

Objects may be temporarily "resurrected" in destructors when calling
finalizers or watcher callbacks. We previously undid the resurrection
by decrementing the reference count using `Py_SET_REFCNT`. This was not
thread-safe because other threads might be accessing the object
(modifying its reference count) if it was exposed by the finalizer,
watcher callback, or temporarily accessed by a racy dictionary or list
access.

This adds internal-only thread-safe functions for temporary object
resurrection during destructors.

1 files changed, 44 insertions, 0 deletions

diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index ce876b0..6b0b464 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -697,8 +697,52 @@ _PyObject_SetMaybeWeakref(PyObject *op)
     }
 }
 
+extern int _PyObject_ResurrectEndSlow(PyObject *op);
 #endif
 
+// Temporarily resurrects an object during deallocation. The refcount is set
+// to one.
+static inline void
+_PyObject_ResurrectStart(PyObject *op)
+{
+    assert(Py_REFCNT(op) == 0);
+#ifdef Py_REF_DEBUG
+    _Py_IncRefTotal(_PyThreadState_GET());
+#endif
+#ifdef Py_GIL_DISABLED
+    _Py_atomic_store_uintptr_relaxed(&op->ob_tid, _Py_ThreadId());
+    _Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 1);
+    _Py_atomic_store_ssize_relaxed(&op->ob_ref_shared, 0);
+#else
+    Py_SET_REFCNT(op, 1);
+#endif
+}
+
+// Undoes an object resurrection by decrementing the refcount without calling
+// _Py_Dealloc(). Returns 0 if the object is dead (the normal case), and
+// deallocation should continue. Returns 1 if the object is still alive.
+static inline int
+_PyObject_ResurrectEnd(PyObject *op)
+{
+#ifdef Py_REF_DEBUG
+    _Py_DecRefTotal(_PyThreadState_GET());
+#endif
+#ifndef Py_GIL_DISABLED
+    Py_SET_REFCNT(op, Py_REFCNT(op) - 1);
+    return Py_REFCNT(op) != 0;
+#else
+    uint32_t local = _Py_atomic_load_uint32_relaxed(&op->ob_ref_local);
+    Py_ssize_t shared = _Py_atomic_load_ssize_acquire(&op->ob_ref_shared);
+    if (_Py_IsOwnedByCurrentThread(op) && local == 1 && shared == 0) {
+        // Fast-path: object has a single refcount and is owned by this thread
+        _Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 0);
+        return 0;
+    }
+    // Slow-path: object has a shared refcount or is not owned by this thread
+    return _PyObject_ResurrectEndSlow(op);
+#endif
+}
+
 /* Tries to incref op and returns 1 if successful or 0 otherwise. */
 static inline int
 _Py_TryIncref(PyObject *op)