Use a new loop in run_app()

CHANGES/5572.bugfix

@@ -0,0 +1 @@
+Always create a new event loop in run_app() to avoid an error caused by using `asyncio.run()` beforehand.

aiohttp/web.py

@@ -479,7 +479,8 @@ def run_app(
     reuse_port: Optional[bool] = None,
 ) -> None:
     """Run an app locally"""
-    loop = asyncio.get_event_loop()
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
     loop.set_debug(debug)
 
     # Configure if and only if in debugging mode and using the default logger

tests/test_web_runner.py

@@ -211,3 +211,22 @@ async def mock_create_server(*args, **kwargs):
     assert server is runner.server
     assert host is None
     assert port == 8080
+
+
+def test_run_after_asyncio_run() -> None:
+    async def nothing():
+        pass
+
+    async def shutdown():
+        raise web.GracefulExit()
+
+    # asyncio.run() creates a new loop and closes it.
+    asyncio.run(nothing())
+
+    app = web.Application()
+    # create_task() will delay the function until app is run.
+    app.on_startup.append(lambda a: asyncio.create_task(shutdown()))
+    try:
+        web.run_app(app)
+    except RuntimeError:
+        pytest.fail("run_app() should work after asyncio.run().")