GH-94736: Fix _multiprocessing.SemLock subclassing (#94738)

* fix allocator and deallocator

* 📜🤖 Added by blurb_it.

* code review

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>

Lib/test/_test_multiprocessing.py

@@ -6020,3 +6020,14 @@ def install_tests_in_module_dict(remote_globs, start_method):
 
     remote_globs['setUpModule'] = setUpModule
     remote_globs['tearDownModule'] = tearDownModule
+
+
+@unittest.skipIf(not hasattr(_multiprocessing, 'SemLock'), 'SemLock not available')
+class SemLockTests(unittest.TestCase):
+
+    def test_semlock_subclass(self):
+        class SemLock(_multiprocessing.SemLock):
+            pass
+        name = f'test_semlock_subclass-{os.getpid()}'
+        s = SemLock(1, 0, 10, name, 0)
+        _multiprocessing.sem_unlink(name)

Misc/NEWS.d/next/Library/2022-07-11-10-41-48.gh-issue-94736.EbsgeK.rst

@@ -0,0 +1 @@
+Fix crash when deallocating an instance of a subclass of ``_multiprocessing.SemLock``. Patch by Kumar Aditya.

Modules/_multiprocessing/semaphore.c

@@ -454,9 +454,7 @@ static PyObject *
 newsemlockobject(PyTypeObject *type, SEM_HANDLE handle, int kind, int maxvalue,
                  char *name)
 {
-    SemLockObject *self;
+    SemLockObject *self = (SemLockObject *)type->tp_alloc(type, 0);
-
-    self = PyObject_New(SemLockObject, type);
     if (!self)
         return NULL;
     self->handle = handle;
@@ -573,7 +571,7 @@ semlock_dealloc(SemLockObject* self)
     if (self->handle != SEM_FAILED)
         SEM_CLOSE(self->handle);
     PyMem_Free(self->name);
-    PyObject_Free(self);
+    Py_TYPE(self)->tp_free((PyObject*)self);
 }
 
 /*[clinic input]