Leave space for the empty GC head.

Author: ericsnowcurrently

Include/internal/pycore_global_objects.h

@@ -8,6 +8,7 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#include "pycore_gc.h"              // PyGC_Head
 #include "pycore_global_strings.h"  // struct _Py_global_strings
 
 
@@ -41,6 +42,7 @@ struct _Py_global_objects {
 
         struct _Py_global_strings strings;
 
+        PyGC_Head _tuple_empty_gc_not_used;
         PyTupleObject tuple_empty;
     } singletons;
 };

Include/internal/pycore_runtime_init.h

@@ -934,7 +934,7 @@ extern "C" {
         }, \
         \
         .tuple_empty = { \
-            _PyVarObject_IMMORTAL_INIT(&PyTuple_Type, 0) \
+            .ob_base = _PyVarObject_IMMORTAL_INIT(&PyTuple_Type, 0) \
         }, \
     }, \
 }

Tools/scripts/generate_global_objects.py

@@ -250,7 +250,7 @@ def generate_runtime_init(identifiers, strings):
                             printer.write(f'INIT_ID({name}),')
                 printer.write('')
                 with printer.block('.tuple_empty =', ','):
-                    printer.write('_PyVarObject_IMMORTAL_INIT(&PyTuple_Type, 0)')
+                    printer.write('.ob_base = _PyVarObject_IMMORTAL_INIT(&PyTuple_Type, 0)')
         printer.write(END)
         printer.write(after)