Fix exception when two dispatches have equal start time

The heapq will first look at the given time to compare,
but when it is identical it will look at the attached dispatch objects
for comparison.
But those just aren't ready for so much responsibility.
UNTIL NOW!
Cudos to Merlin for finding this bug!

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/dispatch/_dispatch.py

@@ -8,7 +8,7 @@
 from dataclasses import dataclass
 from datetime import datetime, timezone
 from enum import Enum
-from typing import Iterator, cast
+from typing import Any, Iterator, Self, cast
 
 from dateutil import rrule
 from frequenz.client.dispatch.types import Dispatch as BaseDispatch
@@ -264,3 +264,14 @@ def _until(self, now: datetime) -> datetime | None:
             return None
 
         return latest_past_start + self.duration
+
+    def __lt__(self, other: Self) -> Any:
+        """Compare two dispatches by their id.
+
+        Args:
+            other: The other dispatch.
+
+        Returns:
+            True if the current dispatch has a lower id than the other dispatch, False otherwise.
+        """
+        return self.id < other.id

tests/test_mananging_actor.py

@@ -4,6 +4,7 @@
 """Test the dispatch runner."""
 
 import asyncio
+import heapq
 from dataclasses import dataclass, replace
 from datetime import datetime, timedelta, timezone
 from typing import AsyncIterator, Iterator
@@ -128,6 +129,23 @@ async def test_simple_start_stop(
     assert test_env.actor.is_running is False
 
 
+def test_heapq_dispatch_compare(test_env: TestEnv) -> None:
+    """Test that the heapq compare function works."""
+    dispatch1 = test_env.generator.generate_dispatch()
+    dispatch2 = test_env.generator.generate_dispatch()
+
+    # Simulate two dispatches with the same 'until' time
+    now = datetime.now(timezone.utc)
+    until_time = now + timedelta(minutes=5)
+
+    # Create the heap
+    scheduled_events = []
+
+    # Push two events with the same 'until' time onto the heap
+    heapq.heappush(scheduled_events, (until_time, Dispatch(dispatch1)))
+    heapq.heappush(scheduled_events, (until_time, Dispatch(dispatch2)))
+
+
 async def test_dry_run(test_env: TestEnv, fake_time: time_machine.Coordinates) -> None:
     """Test the dry run mode."""
     dispatch = test_env.generator.generate_dispatch()