Scheduler: add a method to cancel all tasks

The dictionary which was iterated to cancel tasks is now "private".
Therefore, the scheduler should provide a public API for cancelling
tasks.

* Delete the task before cancelling it to prevent the done callback,
  however unlikely it may be, from deleting the task first

2 files changed, 9 insertions, 4 deletions

diff --git a/bot/cogs/help_channels.py b/bot/cogs/help_channels.py
index 4c83e0722..01bcc28f7 100644
--- a/bot/cogs/help_channels.py
+++ b/bot/cogs/help_channels.py
@@ -119,9 +119,7 @@ class HelpChannels(Scheduler, commands.Cog):
         for task in self.queue_tasks:
             task.cancel()
 
-        log.trace("Cog unload: cancelling the scheduled tasks")
-        for task in self.scheduled_tasks.values():
-            task.cancel()
+        self.cancel_all()
 
     def create_channel_queue(self) -> asyncio.Queue:
         """
diff --git a/bot/utils/scheduling.py b/bot/utils/scheduling.py
index 5760ec2d4..e9a9e6c2d 100644
--- a/bot/utils/scheduling.py
+++ b/bot/utils/scheduling.py
@@ -60,11 +60,18 @@ class Scheduler(metaclass=CogABCMeta):
             log.warning(f"{self.cog_name}: failed to unschedule {task_id} (no task found).")
             return
 
-        task.cancel()
         del self._scheduled_tasks[task_id]
+        task.cancel()
 
         log.debug(f"{self.cog_name}: unscheduled task #{task_id} {id(task)}.")
 
+    def cancel_all(self) -> None:
+        """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)
+
     def _task_done_callback(self, task_id: t.Hashable, done_task: asyncio.Task) -> None:
         """
         Delete the task and raise its exception if one exists.