Log exception in scan and command methods

src/fastcs/backend.py

@@ -56,6 +56,18 @@ async def _start_scan_tasks(
     ):
         self._scan_tasks = {self._loop.create_task(coro()) for coro in coros}
 
+        for task in self._scan_tasks:
+            task.add_done_callback(self._scan_done)
+
+    def _scan_done(self, task: asyncio.Task):
+        try:
+            task.result()
+        except Exception as e:
+            raise FastCSException(
+                "Exception raised in scan method of "
+                f"{self._controller.__class__.__name__}"
+            ) from e
+
     def _stop_scan_tasks(self):
         for task in self._scan_tasks:
             if not task.done():

tests/test_backend.py

@@ -6,6 +6,7 @@
 from fastcs.controller import Controller
 from fastcs.cs_methods import Command
 from fastcs.datatypes import Int
+from fastcs.exceptions import FastCSException
 from fastcs.wrappers import command, scan
 
 
@@ -127,3 +128,40 @@ async def test_wrapper():
 
     assert len(backend._scan_tasks) == 1
     assert len(backend._initial_coros) == 2
+
+
+def test_scan_raises_exception_via_callback():
+    class MyTestController(Controller):
+        def __init__(self):
+            super().__init__()
+
+        @scan(0.1)
+        async def raise_exception(self):
+            raise ValueError("Scan Exception")
+
+    controller = MyTestController()
+    loop = asyncio.get_event_loop()
+    backend = Backend(controller, loop)
+
+    exception_info = {}
+    # This will intercept the exception raised in _scan_done
+    loop.set_exception_handler(
+        lambda _loop, context: exception_info.update(
+            {"exception": context.get("exception")}
+        )
+    )
+
+    async def test_scan_wrapper():
+        await backend.serve()
+        # This allows scan time to run
+        await asyncio.sleep(0.2)
+        # _scan_done should raise an exception
+        assert isinstance(exception_info["exception"], FastCSException)
+        for task in backend._scan_tasks:
+            internal_exception = task.exception()
+            assert internal_exception
+            # The task exception comes from scan method raise_exception
+            assert isinstance(internal_exception, ValueError)
+            assert "Scan Exception" == str(internal_exception)
+
+    loop.run_until_complete(test_scan_wrapper())