Set the default work start method to spawn on MacOS (#16089)

Author: awaelchli

src/lightning_app/CHANGELOG.md

@@ -28,9 +28,13 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 ### Changed
 
 
+- The default `start_method` for creating Work processes locally on MacOS is now 'spawn' (previously 'fork') ([#16089](https://github.com/Lightning-AI/lightning/pull/16089))
+
+
 - The utility `lightning.app.utilities.cloud.is_running_in_cloud` now returns `True` during loading of the app locally when running with `--cloud` ([#16045](https://github.com/Lightning-AI/lightning/pull/16045))
 
 
+
 ### Deprecated
 
 -

src/lightning_app/core/work.py

@@ -51,7 +51,7 @@ class LightningWork:
 
     _run_executor_cls: Type[WorkRunExecutor] = WorkRunExecutor
     # TODO: Move to spawn for all Operating System.
-    _start_method = "spawn" if sys.platform == "win32" else "fork"
+    _start_method = "spawn" if sys.platform in ("darwin", "win32") else "fork"
 
     def __init__(
         self,

tests/tests_app/runners/backends/__init__.py

@@ -0,0 +1,27 @@
+from unittest import mock
+from unittest.mock import MagicMock, Mock
+
+from lightning_app import LightningApp, LightningWork
+from lightning_app.runners.backends import MultiProcessingBackend
+
+
+@mock.patch("lightning_app.runners.backends.mp_process.multiprocessing")
+def test_backend_create_work_with_set_start_method(multiprocessing_mock):
+    backend = MultiProcessingBackend(entrypoint_file="fake.py")
+    work = Mock(spec=LightningWork)
+    work._start_method = "test_start_method"
+
+    app = LightningApp(work)
+    app.caller_queues = MagicMock()
+    app.delta_queue = MagicMock()
+    app.readiness_queue = MagicMock()
+    app.error_queue = MagicMock()
+    app.request_queues = MagicMock()
+    app.response_queues = MagicMock()
+    app.copy_request_queues = MagicMock()
+    app.copy_response_queues = MagicMock()
+    app.flow_to_work_delta_queues = MagicMock()
+
+    backend.create_work(app=app, work=work)
+    multiprocessing_mock.get_context.assert_called_with("test_start_method")
+    multiprocessing_mock.get_context().Process().start.assert_called_once()

tests/tests_app/runners/backends/test_mp_process.py

@@ -68,7 +68,7 @@ def run(self):
         assert _get_context().value == "work"
 
 
-class ContxtFlow(LightningFlow):
+class ContextFlow(LightningFlow):
     def __init__(self):
         super().__init__()
         self.work = ContextWork()
@@ -83,7 +83,7 @@ def run(self):
 
 def test_multiprocess_runtime_sets_context():
     """Test that the runtime sets the global variable COMPONENT_CONTEXT in Flow and Work."""
-    MultiProcessRuntime(LightningApp(ContxtFlow())).dispatch()
+    MultiProcessRuntime(LightningApp(ContextFlow())).dispatch()
 
 
 @pytest.mark.parametrize(