add test that GIL stays disabled

Author: mscheltienne

tests/test_gil.py

@@ -0,0 +1,26 @@
+"""Test that the GIL is disabled on free-threaded Python builds."""
+
+from __future__ import annotations
+
+import sys
+import sysconfig
+
+import pytest
+
+_is_free_threaded = bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
+
+
+@pytest.mark.skipif(not _is_free_threaded, reason="not a free-threaded python build")
+def test_gil_disabled() -> None:
+    """Assert that the GIL is disabled on free-threaded Python.
+
+    On free-threaded builds (3.13t, 3.14t, ...), the GIL should be disabled by default.
+    If a C extension that hasn't declared ``Py_MOD_GIL_NOT_USED`` gets imported, the GIL
+    is silently re-enabled and a ``RuntimeWarning`` is emitted. Combined with
+    ``filterwarnings = ["error"]`` in our pytest config, this test ensures that no such
+    extension has been loaded during the test session.
+    """
+    assert not sys._is_gil_enabled(), (
+        "The GIL has been re-enabled. A C extension that does not declare "
+        "Py_MOD_GIL_NOT_USED was loaded during the test session."
+    )