gh-99578: Fix refleak in _imp.create_builtin() (GH-99642)

Fix a reference bug in _imp.create_builtin() after the creation of
the first sub-interpreter for modules "builtins" and "sys".
(cherry picked from commit cb2ef8b)

Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/test/test_imp.py

@@ -1,3 +1,4 @@
+import gc
 import importlib
 import importlib.util
 import os
@@ -379,6 +380,35 @@ def test_find_and_load_checked_pyc(self):
  self.assertEqual(mod.x, 42)
 
 
+ @support.cpython_only
+ def test_create_builtin_subinterp(self):
+ # gh-99578: create_builtin() behavior changes after the creation of the
+ # first sub-interpreter. Test both code paths, before and after the
+ # creation of a sub-interpreter. Previously, create_builtin() had
+ # a reference leak after the creation of the first sub-interpreter.
+
+ import builtins
+ create_builtin = support.get_attribute(_imp, "create_builtin")
+ class Spec:
+ name = "builtins"
+ spec = Spec()
+
+ def check_get_builtins():
+ refcnt = sys.getrefcount(builtins)
+ mod = _imp.create_builtin(spec)
+ self.assertIs(mod, builtins)
+ self.assertEqual(sys.getrefcount(builtins), refcnt + 1)
+ # Check that a GC collection doesn't crash
+ gc.collect()
+
+ check_get_builtins()
+
+ ret = support.run_in_subinterp("import builtins")
+ self.assertEqual(ret, 0)
+
+ check_get_builtins()
+
+
 class ReloadTests(unittest.TestCase):
 
  """Very basic tests to make sure that imp.reload() operates just like

Misc/NEWS.d/next/Core and Builtins/2022-11-21-11-27-14.gh-issue-99578.DcKoBJ.rst

@@ -0,0 +1,3 @@
+Fix a reference bug in :func:`_imp.create_builtin()` after the creation of the
+first sub-interpreter for modules ``builtins`` and ``sys``. Patch by Victor
+Stinner.

Python/import.c

@@ -1006,7 +1006,8 @@ create_builtin(PyThreadState *tstate, PyObject *name, PyObject *spec)
  if (_PyUnicode_EqualToASCIIString(name, p->name)) {
  if (p->initfunc == NULL) {
  /* Cannot re-init internal module ("sys" or "builtins") */
- return PyImport_AddModuleObject(name);
+ mod = PyImport_AddModuleObject(name);
+ return Py_XNewRef(mod);
  }
 
  mod = (*p->initfunc)();