fix: race condition when initializing multiprocessing manager (#26)

Closes #18

### Summary of Changes

- use spawn instead of fork to not deadlock when running tests

---------

Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>

src/safeds_runner/server/pipeline_manager.py

@@ -35,12 +35,14 @@ class PipelineManager:
     """
 
     def __init__(self) -> None:
-        """Create a new PipelineManager object, which needs to be started by calling startup()."""
+        """Create a new PipelineManager object, which is lazily started, when needed."""
         self._placeholder_map: dict = {}
         self._websocket_target: simple_websocket.Server | None = None
 
     @cached_property
     def _multiprocessing_manager(self) -> SyncManager:
+        if multiprocessing.get_start_method() != "spawn":
+            multiprocessing.set_start_method("spawn", force=True)
         return multiprocessing.Manager()
 
     @cached_property
@@ -65,6 +67,7 @@ def _startup(self) -> None:
 
         This method should not be called during the bootstrap phase of the python interpreter, as it leads to a crash.
         """
+        _mq = self._messages_queue  # Initialize it here before starting a thread to avoid potential race condition
         if not self._messages_queue_thread.is_alive():
             self._messages_queue_thread.start()
 

tests/safeds_runner/server/test_runner_main.py

@@ -7,6 +7,7 @@
 
 
 def test_should_runner_start_successfully() -> None:
+    subprocess._USE_VFORK = False  # Do not fork the subprocess as it is unsafe to do
     process = subprocess.Popen(["poetry", "run", "safe-ds-runner", "start"], cwd=_project_root, stderr=subprocess.PIPE)
     while process.poll() is None:
         process_line = str(typing.cast(IO[bytes], process.stderr).readline(), "utf-8").strip()

tests/safeds_runner/server/test_websocket_mock.py

@@ -23,11 +23,11 @@ def __init__(self, messages: list[str]):
         self.received: list[str] = []
         self.close_reason: int | None = None
         self.close_message: str | None = None
-        self.condition_variable = threading.Condition()
+        self.condition_variable = threading.Condition(lock=threading.Lock())
 
     def send(self, msg: str) -> None:
-        self.received.append(msg)
         with self.condition_variable:
+            self.received.append(msg)
             self.condition_variable.notify_all()
 
     def receive(self) -> str | None:
@@ -44,7 +44,11 @@ def wait_for_messages(self, wait_for_messages: int = 1) -> None:
             with self.condition_variable:
                 if len(self.received) >= wait_for_messages:
                     return
-                self.condition_variable.wait()
+                self.condition_variable.wait(1.0)  # this should not be needed, but it seems the process can get stuck
+
+    def get_next_received_message(self) -> str:
+        with self.condition_variable:
+            return self.received.pop(0)
 
 
 @pytest.mark.parametrize(
@@ -208,7 +212,7 @@ def test_should_execute_pipeline_return_exception(
     mock_connection = MockWebsocketConnection(messages)
     ws_main(mock_connection, app_pipeline_manager)
     mock_connection.wait_for_messages(1)
-    exception_message = Message.from_dict(json.loads(mock_connection.received.pop(0)))
+    exception_message = Message.from_dict(json.loads(mock_connection.get_next_received_message()))
 
     assert exception_message.type == expected_response_runtime_error.type
     assert exception_message.id == expected_response_runtime_error.id
@@ -293,7 +297,7 @@ def test_should_execute_pipeline_return_valid_placeholder(
     # And compare with expected responses
     while len(expected_responses) > 0:
         mock_connection.wait_for_messages(1)
-        next_message = Message.from_dict(json.loads(mock_connection.received.pop(0)))
+        next_message = Message.from_dict(json.loads(mock_connection.get_next_received_message()))
         assert next_message == expected_responses.pop(0)
 
 
@@ -360,5 +364,5 @@ def test_should_successfully_execute_simple_flow(messages: list[str], expected_r
     mock_connection = MockWebsocketConnection(messages)
     ws_main(mock_connection, app_pipeline_manager)
     mock_connection.wait_for_messages(1)
-    query_result_invalid = Message.from_dict(json.loads(mock_connection.received.pop(0)))
+    query_result_invalid = Message.from_dict(json.loads(mock_connection.get_next_received_message()))
     assert query_result_invalid == expected_response