gh-112529: Remove PyGC_Head from object pre-header in free-threaded build (#114564)

* gh-112529: Remove PyGC_Head from object pre-header in free-threaded build

This avoids allocating space for PyGC_Head in the free-threaded build.
The GC implementation for free-threaded CPython does not use the
PyGC_Head structure.

 * The trashcan mechanism uses the `ob_tid` field instead of `_gc_prev`
   in the free-threaded build.
 * The GDB libpython.py file now determines the offset of the managed
   dict field based on whether the running process is a free-threaded
   build. Those are identified by the `ob_ref_local` field in PyObject.
 * Fixes `_PySys_GetSizeOf()` which incorrectly incorrectly included the
   size of `PyGC_Head` in the size of static `PyTypeObject`.

1 files changed, 9 insertions, 1 deletions

diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index f558a00..437d7f8 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1878,7 +1878,15 @@ _PySys_GetSizeOf(PyObject *o)
         return (size_t)-1;
     }
 
-    return (size_t)size + _PyType_PreHeaderSize(Py_TYPE(o));
+    size_t presize = 0;
+    if (!Py_IS_TYPE(o, &PyType_Type) ||
+         PyType_HasFeature((PyTypeObject *)o, Py_TPFLAGS_HEAPTYPE))
+    {
+        /* Add the size of the pre-header if "o" is not a static type */
+        presize = _PyType_PreHeaderSize(Py_TYPE(o));
+    }
+
+    return (size_t)size + presize;
 }
 
 static PyObject *