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.
(cherry picked from commit 736684b)

Co-authored-by: Sam Gross <[email protected]>

Author: miss-islington

Lib/test/test_os.py

@@ -4501,6 +4501,22 @@ 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

@@ -147,12 +147,15 @@ _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