[App] Enable running with spawn context

src/lightning_app/CHANGELOG.md

@@ -19,6 +19,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Added the property `ready` of the LightningFlow to inform when the `Open App` should be visible ([#15921](https://github.com/Lightning-AI/lightning/pull/15921))
 
+- Added private work attributed `_start_method` to customize how to start the works ([#15923](https://github.com/Lightning-AI/lightning/pull/15923))
+
 
 ### Changed
 

src/lightning_app/core/queues.py

@@ -198,7 +198,8 @@ class MultiProcessQueue(BaseQueue):
     def __init__(self, name: str, default_timeout: float):
         self.name = name
         self.default_timeout = default_timeout
-        self.queue = multiprocessing.Queue()
+        context = multiprocessing.get_context("spawn")
+        self.queue = context.Queue()
 
     def put(self, item):
         self.queue.put(item)

src/lightning_app/core/work.py

@@ -46,6 +46,7 @@ class LightningWork:
     )
 
     _run_executor_cls: Type[WorkRunExecutor] = WorkRunExecutor
+    _start_method = "fork"
 
     def __init__(
         self,

src/lightning_app/runners/backends/mp_process.py

@@ -31,7 +31,10 @@ def start(self):
             flow_to_work_delta_queue=self.app.flow_to_work_delta_queues[self.work.name],
             run_executor_cls=self.work._run_executor_cls,
         )
-        self._process = multiprocessing.Process(target=self._work_runner)
+
+        start_method = getattr(self.work, "_start_method", "fork")
+        context = multiprocessing.get_context(start_method)
+        self._process = context.Process(target=self._work_runner)
         self._process.start()
 
     def kill(self):