test(companion): Assert HTTP worker path is unchanged

Add two arbiter regression tests. A worker exit is still reaped normally (tmp
closed, child_exit called) while a companion manager pid is registered, so the
companion reap branch does not swallow worker exits. And an HTTP worker is
still spawned and recorded as before when companions are configured, so the
companion config never touches the worker path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/design/companion-process-manager.md

@@ -699,7 +699,7 @@ No per-companion logic in Arbiter.
 - [x] Add tests for state transitions.
 - [x] Add tests for control commands.
 - [x] Add tests for transactional reread.
-- [ ] Add tests that HTTP worker behavior is unchanged.
+- [x] Add tests that HTTP worker behavior is unchanged.
 
 ## 21. Test Plan
 

tests/test_arbiter.py

@@ -205,6 +205,37 @@ def test_stop_companion_manager_clears_pid_when_already_gone():
     assert arbiter.companion_manager_pid == 0
 
 
+@mock.patch('os.waitpid')
+def test_worker_reap_unaffected_by_companion_manager(mock_os_waitpid):
+    # A worker exit is still reaped normally while a companion manager runs;
+    # the companion reap branch must not swallow worker exits.
+    mock_os_waitpid.side_effect = [(42, 0), (0, 0)]
+    arbiter = gunicorn.arbiter.Arbiter(DummyApplication())
+    arbiter.cfg.settings['child_exit'] = mock.Mock()
+    arbiter.companion_manager_pid = 9999
+    mock_worker = mock.Mock()
+    arbiter.WORKERS = {42: mock_worker}
+    arbiter.reap_workers()
+    mock_worker.tmp.close.assert_called_with()
+    arbiter.cfg.child_exit.assert_called_with(arbiter, mock_worker)
+    assert arbiter.companion_manager_pid == 9999
+
+
+@mock.patch('os.fork', return_value=77)
+def test_spawn_worker_unaffected_by_companions(mock_os_fork):
+    # With companions configured, an HTTP worker is still spawned and recorded
+    # exactly as before; companion config does not touch the worker path.
+    arbiter = gunicorn.arbiter.Arbiter(DummyApplication())
+    arbiter.cfg.set("companion_workers", [{"name": "rq", "target": "pkg:run"}])
+    arbiter.pid = 1234
+    arbiter.WORKERS = {}  # instance dict, do not mutate the shared class attr
+    mock_worker = mock.Mock()
+    arbiter.worker_class = mock.Mock(return_value=mock_worker)
+    pid = arbiter.spawn_worker()
+    assert pid == 77
+    assert arbiter.WORKERS[77] is mock_worker
+
+
 def test_close_gunicorn_fds_in_manager_child():
     arbiter = gunicorn.arbiter.Arbiter(DummyApplication())
     listener = mock.Mock()