Add test serialization

pytest.ini

@@ -1,6 +1,7 @@
 [pytest]
 markers =
     test_all_runtimes: Generate a test for each supported container runtime
+    serial: Tests that need to run serially
 testpaths = test
 addopts =
     -r a

test/conftest.py

@@ -102,3 +102,14 @@ def project_fixtures(tmp_path):
     yield dest
 
     shutil.rmtree(dest, ignore_errors=True)
+
+
+def pytest_collection_modifyitems(session, config, items):
+    # mark serial items as skipped if it looks like we're running with some obvious kinds of parallelism
+    numproc = getattr(config.known_args_namespace, 'numprocesses', None)
+
+    if isinstance(numproc, int) and numproc > 1:
+        for serial_item in (i for i in items if any(i.iter_markers(name='serial'))):
+            serial_item.add_marker(
+                pytest.mark.skip(reason='test requires serial execution (add --numprocesses 0 to allow)')
+            )

test/integration/test_transmit_worker_process.py

@@ -109,6 +109,10 @@ def test_remote_job_interface(self, tmp_path, project_fixtures, job_type):
         incoming_buffer.close()
         self.check_artifacts(str(process_dir), job_type)
 
+    # The keepalive tests will run serially since Python 3.13 somehow causes a longer delay
+    # in the tests, thus causing the keepalive timeout count to exceed the hardcoded value
+    # of 5.
+    @pytest.mark.serial
     @pytest.mark.parametrize("keepalive_setting", [
         0,  # keepalive explicitly disabled, default
         1,  # emit keepalives every 1s