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, 11 insertions, 1 deletions

diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index c4a648a..0bb739b 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -1752,8 +1752,18 @@ module_exec(PyObject *module)
         return 1;
     }
 
+    Py_ssize_t sizeof_gc_head = 0;
+#ifndef Py_GIL_DISABLED
+    sizeof_gc_head = sizeof(PyGC_Head);
+#endif
+
     if (PyModule_Add(module, "SIZEOF_PYGC_HEAD",
-                        PyLong_FromSsize_t(sizeof(PyGC_Head))) < 0) {
+                        PyLong_FromSsize_t(sizeof_gc_head)) < 0) {
+        return 1;
+    }
+
+    if (PyModule_Add(module, "SIZEOF_MANAGED_PRE_HEADER",
+                        PyLong_FromSsize_t(2 * sizeof(PyObject*))) < 0) {
         return 1;
     }