bpo-42540: reallocation of id_mutex should not force default allocator (GH-29564)

Unlike the other locks reinitialized by _PyRuntimeState_ReInitThreads, the "interpreters.main->id_mutex" is not freed by _PyRuntimeState_Fini and should not force the default raw allocator.

Author: colesbury

Lib/test/test_os.py

@@ -4500,6 +4500,22 @@ def test_times(self):
  self.assertEqual(times.elapsed, 0)
 
 
+@requires_os_func('fork')
+class ForkTests(unittest.TestCase):
+ def test_fork(self):
+ # bpo-42540: ensure os.fork() with non-default memory allocator does
+ # not crash on exit.
+ code = """if 1:
+ import os
+ from test import support
+ pid = os.fork()
+ if pid != 0:
+ support.wait_process(pid, exitcode=0)
+ """
+ assert_python_ok("-c", code)
+ assert_python_ok("-c", code, PYTHONMALLOC="malloc_debug")
+
+
 # Only test if the C version is provided, otherwise TestPEP519 already tested
 # the pure Python implementation.
 if hasattr(os, "_fspath"):

Misc/NEWS.d/next/Core and Builtins/2021-11-15-12-08-27.bpo-42540.V2w107.rst

@@ -0,0 +1,2 @@
+Fix crash when :func:`os.fork` is called with an active non-default
+memory allocator.

Python/pystate.c

@@ -148,12 +148,15 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
  _PyMem_SetDefaultAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
  int reinit_interp = _PyThread_at_fork_reinit(&runtime->interpreters.mutex);
- int reinit_main_id = _PyThread_at_fork_reinit(&runtime->interpreters.main->id_mutex);
  int reinit_xidregistry = _PyThread_at_fork_reinit(&runtime->xidregistry.mutex);
  int reinit_unicode_ids = _PyThread_at_fork_reinit(&runtime->unicode_ids.lock);
 
  PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
+ /* bpo-42540: id_mutex is freed by _PyInterpreterState_Delete, which does
+ * not force the default allocator. */
+ int reinit_main_id = _PyThread_at_fork_reinit(&runtime->interpreters.main->id_mutex);
+
  if (reinit_interp < 0
  || reinit_main_id < 0
  || reinit_xidregistry < 0