Propagate exceptions from action threads

Author: rwb27

src/labthings/actions/thread.py

@@ -85,6 +85,7 @@ def __init__(
         self._request_time: datetime.datetime = datetime.datetime.now()
         self._start_time: Optional[datetime.datetime] = None  # Task start time
         self._end_time: Optional[datetime.datetime] = None  # Task end time
+        self._exception: Optional[Exception] = None  # Propagate exceptions helpfully
 
         # Public state properties
         self.progress: Optional[int] = None  # Percent progress of the task
@@ -218,6 +219,7 @@ def wrapped(*args, **kwargs):
                 logging.error(traceback.format_exc())
                 self._return_value = str(e)
                 self._status = "error"
+                self._exception = e
                 raise e
             finally:
                 self._end_time = datetime.datetime.now()

src/labthings/views/__init__.py

@@ -234,6 +234,13 @@ def dispatch_request(self, *args, **kwargs):
         if task.output and isinstance(task.output, ResponseBase):
             return self.represent_response((task.output, 200))
 
+        # If the action fails quickly, propagate the exception.
+        # This allows us to handle validation errors nicely.
+        # TODO: do we want to do this for all exceptions, or just
+        # werkzeug.exceptions.HTTPException instances?
+        if task._exception is not None:
+            raise e
+
         return self.represent_response((ActionSchema().dump(task), 201))