LIU-284: Add exception logging in EventFirer

Event sources will generally not be aware of the exceptions that a
listener could throw and should not be handling them.

To preserve existing behaviour, exceptions are logged and immediately
rethrown. However arguably they should be swallowed at this point to
prevent other listeners from being affected.

daliuge-engine/dlg/event.py

@@ -123,4 +123,8 @@ def _fireEvent(self, eventType: str, **attrs):
             setattr(e, k, v)
 
         for l in listeners:
-            l.handleEvent(e)
+            try:
+                l.handleEvent(e)
+            except:
+                logger.exception("Exception in listener %s while handling event %s", l, e)
+                raise

daliuge-engine/test/test_event.py

@@ -0,0 +1,54 @@
+from typing import Optional
+from dlg.event import EventFirer, EventHandler, Event
+import pytest
+
+
+class MockEventSource(EventFirer):
+    def fireEvent(self, eventType, **kwargs):
+        self._fireEvent(eventType, **kwargs)
+
+
+class MockThrowingEventHandler(EventHandler):
+    def __init__(self) -> None:
+        self.wasCalled = False
+
+    def handleEvent(self, e: Event) -> None:
+        self.wasCalled = True
+        raise RuntimeError("MockThrow", e)
+
+
+class MockEventHandler(EventHandler):
+    def __init__(self) -> None:
+        self.lastEvent: Optional[Event] = None
+
+    def handleEvent(self, e: Event) -> None:
+        self.lastEvent = e
+
+
+def test_listener_exception_interrupts_later_handlers():
+    eventSource = MockEventSource()
+    handler1 = MockEventHandler()
+    handler2 = MockEventHandler()
+    throwingHandler = MockThrowingEventHandler()
+    eventSource.subscribe(handler1, "raise")
+    eventSource.subscribe(throwingHandler, "raise")
+    eventSource.subscribe(handler2, "raise")
+
+    with pytest.raises(RuntimeError):
+        eventSource.fireEvent("raise", prop="value")
+
+    assert throwingHandler.wasCalled
+    assert handler1.lastEvent is not None
+    assert getattr(handler1.lastEvent, "prop") == "value"
+    assert handler2.lastEvent is None
+
+
+def test_listener_exception_is_logged(caplog: pytest.LogCaptureFixture):
+    eventSource = MockEventSource()
+    eventSource.subscribe(MockThrowingEventHandler(), "raise")
+
+    with pytest.raises(RuntimeError) as excinfo:
+        eventSource.fireEvent("raise")
+
+    assert "MockThrow" in str(excinfo.value)
+    assert "MockThrow" in caplog.text, "Expected exception to have been logged"