From f3157f306a7649d06c8c334bbe3cf714a1a32392 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adrian=20W=C3=A4lchli?= <aedu.waelchli@gmail.com>
Date: Mon, 19 Dec 2022 15:06:52 +0100
Subject: [PATCH] Set the default work start method to spawn on MacOS (#16089)

---
 src/lightning_app/CHANGELOG.md                |  4 +++
 src/lightning_app/core/work.py                |  2 +-
 tests/tests_app/runners/backends/__init__.py  |  0
 .../runners/backends/test_mp_process.py       | 27 +++++++++++++++++++
 tests/tests_app/runners/test_multiprocess.py  |  4 +--
 5 files changed, 34 insertions(+), 3 deletions(-)
 create mode 100644 tests/tests_app/runners/backends/__init__.py
 create mode 100644 tests/tests_app/runners/backends/test_mp_process.py

diff --git a/src/lightning_app/CHANGELOG.md b/src/lightning_app/CHANGELOG.md
index 4ee679fcd6..65d62b29da 100644
--- a/src/lightning_app/CHANGELOG.md
+++ b/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
 
 -
diff --git a/src/lightning_app/core/work.py b/src/lightning_app/core/work.py
index b34f42f2fe..dfc0c9e570 100644
--- a/src/lightning_app/core/work.py
+++ b/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,
diff --git a/tests/tests_app/runners/backends/__init__.py b/tests/tests_app/runners/backends/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/tests/tests_app/runners/backends/test_mp_process.py b/tests/tests_app/runners/backends/test_mp_process.py
new file mode 100644
index 0000000000..49a926b683
--- /dev/null
+++ b/tests/tests_app/runners/backends/test_mp_process.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()
diff --git a/tests/tests_app/runners/test_multiprocess.py b/tests/tests_app/runners/test_multiprocess.py
index 2e1a34ab38..48bbedf555 100644
--- a/tests/tests_app/runners/test_multiprocess.py
+++ b/tests/tests_app/runners/test_multiprocess.py
@@ -68,7 +68,7 @@ class ContextWork(LightningWork):
         assert _get_context().value == "work"
 
 
-class ContxtFlow(LightningFlow):
+class ContextFlow(LightningFlow):
     def __init__(self):
         super().__init__()
         self.work = ContextWork()
@@ -83,7 +83,7 @@ class ContxtFlow(LightningFlow):
 
 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(