Use asyncio.Event to deterministically ensure simulator start (#2734)

Author: alexrudd2

pymodbus/server/simulator/http_server.py

@@ -79,7 +79,7 @@ class CallTypeResponse:
     clear_after: int = 1
 
 
-class ModbusSimulatorServer:
+class ModbusSimulatorServer:  # pylint: disable=too-many-instance-attributes
     """**ModbusSimulatorServer**.
 
     :param modbus_server: Server name in json file (default: "server")
@@ -219,6 +219,7 @@ def __init__(
         self.call_response = CallTypeResponse()
         app_key = getattr(web, 'AppKey', str)  # fall back to str for aiohttp < 3.9.0
         self.api_key = app_key("modbus_server")
+        self.ready_event = asyncio.Event()
 
     async def start_modbus_server(self, app):
         """Start Modbus server as asyncio task."""
@@ -253,6 +254,7 @@ async def run_forever(self, only_start=False):
             await self.runner.setup()
             self.site = web.TCPSite(self.runner, self.http_host, self.http_port)
             await self.site.start()
+            self.ready_event.set()
         except Exception as exc:
             Log.error("Error starting http server, reason: {}", exc)
             raise exc

test/server/test_simulator_api.py

@@ -1,5 +1,4 @@
 """Test simulator API."""
-import asyncio
 import json
 
 import pytest
@@ -127,20 +126,16 @@ async def simulator(self, tmp_path):
             json_file = config_path
         )
 
-        # Run the simulator in the current event loop. Store the task so they live
-        # until the test is done.
-        loop = asyncio.get_running_loop()
-        task = loop.create_task(simulator.run_forever(only_start=True))
+        # this will finish almost immediately; no need to keep a task
+        await simulator.run_forever(only_start=True)
 
-        # TODO: Make a better way to wait for the simulator to start
-        await asyncio.sleep(1)
+        # wait for the simulator to start
+        await simulator.ready_event.wait()
 
-        yield simulator
-
-        # Stop the simulator after the test is done
-        task.cancel()
-        await task
-        await simulator.stop()
+        try:
+            yield simulator
+        finally:
+            await simulator.stop()
 
     @pytest.mark.asyncio
     async def test_registers_json_valid(self, client, simulator):