AntiSpam: prevent attempts to punish a user multiple times

A user may manage to send multiple message that violate filters before
the mute is applied. Because of a race condition, subsequent punish
attempts did not detect the mute role exists and therefore proceeded
to apply another mute.

To avoid the race condition, abort any subsequent punish attempts while
one is already ongoing for a given user. It could be possible to wait
instead of abort, but the first attempt failing very likely means
subsequent attempts would fail too.

Fixes #902

1 files changed, 3 insertions, 1 deletions

diff --git a/bot/exts/filters/antispam.py b/bot/exts/filters/antispam.py
index af8528a68..c9052b138 100644
--- a/bot/exts/filters/antispam.py
+++ b/bot/exts/filters/antispam.py
@@ -3,7 +3,7 @@ import logging
 from collections.abc import Mapping
 from dataclasses import dataclass, field
 from datetime import datetime, timedelta
-from operator import itemgetter
+from operator import attrgetter, itemgetter
 from typing import Dict, Iterable, List, Set
 
 from discord import Colour, Member, Message, NotFound, Object, TextChannel
@@ -18,6 +18,7 @@ from bot.constants import (
 )
 from bot.converters import Duration
 from bot.exts.moderation.modlog import ModLog
+from bot.utils import lock
 from bot.utils.messages import format_user, send_attachments
 
 
@@ -211,6 +212,7 @@ class AntiSpam(Cog):
                 await self.maybe_delete_messages(channel, relevant_messages)
                 break
 
+    @lock.lock_arg("antispam.punish", "member", attrgetter("id"))
     async def punish(self, msg: Message, member: Member, reason: str) -> None:
         """Punishes the given member for triggering an antispam rule."""
         if not any(role.id == self.muted_role.id for role in member.roles):