Python: Improve exception handling for the coroutine passed to slint.run_event_loop()

If an exception is thrown, propagate it. Otherwise it becomes difficult to debug...

cc #4137

Author: tronical

api/python/slint/README.md

@@ -339,8 +339,6 @@ For the common use case of interacting with REST APIs, we recommend the [`aiohtt
 ### Known Limitations
 
 - Pipes and sub-processes are only supported on Unix-like platforms.
-- Exceptions thrown in the coroutine passed to `slint.run_event_loop()` don't cause the loop to terminate. This behaviour may
-  change in a future release.
 
 ## Third-Party Licenses
 

api/python/slint/slint/__init__.py

@@ -464,10 +464,20 @@ async def main_receiver(image_model: slint.ListModel) -> None:
     async def run_inner() -> None:
         global quit_event
         loop = typing.cast(SlintEventLoop, asyncio.get_event_loop())
+
+        tasks: typing.List[asyncio.Task[typing.Any]] = [
+            asyncio.ensure_future(quit_event.wait(), loop=loop)
+        ]
+
+        main_task = None
         if main_coro:
-            loop.create_task(main_coro)
+            main_task = loop.create_task(main_coro)
+            tasks.append(main_task)
+
+        done, pending = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)
 
-        await quit_event.wait()
+        if main_task is not None and main_task in done:
+            main_task.result()  # propagate exception if thrown
 
     global quit_event
     quit_event = asyncio.Event()

api/python/slint/tests/test_async.py

@@ -207,3 +207,11 @@ async def launch_process(exception_check: typing.List[Exception]) -> None:
     slint.run_event_loop(launch_process(exception_check))
     if len(exception_check) > 0:
         raise exception_check[0]
+
+
+def test_exception_thrown() -> None:
+    async def throws() -> None:
+        raise RuntimeError("Boo")
+
+    with pytest.raises(RuntimeError, match="Boo"):
+        slint.run_event_loop(throws())