Scheduler: fix dict size changing while cancelling all tasks

* Make a copy of the dict
* Add a `ignore_missing` param to `cancel_task` to suppress the warning
  for a task not being found

1 files changed, 10 insertions, 5 deletions

diff --git a/bot/utils/scheduling.py b/bot/utils/scheduling.py
index f8b9d2d48..8b778a093 100644
--- a/bot/utils/scheduling.py
+++ b/bot/utils/scheduling.py
@@ -51,13 +51,18 @@ class Scheduler(metaclass=CogABCMeta):
         self._scheduled_tasks[task_id] = task
         log.debug(f"{self.cog_name}: scheduled task #{task_id} {id(task)}.")
 
-    def cancel_task(self, task_id: t.Hashable) -> None:
-        """Unschedule the task identified by `task_id`."""
+    def cancel_task(self, task_id: t.Hashable, ignore_missing: bool = False) -> None:
+        """
+        Unschedule the task identified by `task_id`.
+
+        If `ignore_missing` is True, a warning will not be sent if a task isn't found.
+        """
         log.trace(f"{self.cog_name}: cancelling task #{task_id}...")
         task = self._scheduled_tasks.get(task_id)
 
         if not task:
-            log.warning(f"{self.cog_name}: failed to unschedule {task_id} (no task found).")
+            if not ignore_missing:
+                log.warning(f"{self.cog_name}: failed to unschedule {task_id} (no task found).")
             return
 
         del self._scheduled_tasks[task_id]
@@ -69,8 +74,8 @@ class Scheduler(metaclass=CogABCMeta):
         """Unschedule all known tasks."""
         log.debug(f"{self.cog_name}: unscheduling all tasks")
 
-        for task_id in self._scheduled_tasks:
-            self.cancel_task(task_id)
+        for task_id in self._scheduled_tasks.copy():
+            self.cancel_task(task_id, ignore_missing=True)
 
     def _task_done_callback(self, task_id: t.Hashable, done_task: asyncio.Task) -> None:
         """