Incidents: implement `resolve_message`

See docstring. The exception log is DEBUG level as failure does not
necessarily indicate that we have done something wrong. We rely on
the API to tell us that the message no longer exists in situations
where we have 2 coroutines racing to archive the same message.

1 files changed, 31 insertions, 0 deletions

diff --git a/bot/cogs/moderation/incidents.py b/bot/cogs/moderation/incidents.py
index d2b4581e7..d994054c8 100644
--- a/bot/cogs/moderation/incidents.py
+++ b/bot/cogs/moderation/incidents.py
@@ -107,6 +107,37 @@ class Incidents(Cog):
 
         log.debug("Crawl task finished!")
 
+    async def resolve_message(self, message_id: int) -> t.Optional[discord.Message]:
+        """
+        Get `discord.Message` for `message_id` from cache, or API.
+
+        We first look into the local cache to see if the message is present.
+
+        If not, we try to fetch the message from the API. This is necessary for messages
+        which were sent before the bot's current session.
+
+        However, in an edge-case, it is also possible that the message was already deleted,
+        and the API will return a 404. In such a case, None will be returned. This signals
+        that the event for `message_id` should be ignored.
+        """
+        await self.bot.wait_until_guild_available()  # First make sure that the cache is ready
+        log.debug(f"Resolving message for: {message_id=}")
+        message: discord.Message = self.bot._connection._get_message(message_id)  # noqa: Private attribute
+
+        if message is not None:
+            log.debug("Message was found in cache")
+            return message
+
+        log.debug("Message not found, attempting to fetch")
+        try:
+            message = await self.bot.get_channel(Channels.incidents).fetch_message(message_id)
+        except Exception as exc:
+            log.debug(f"Failed to fetch message: {exc}")
+            return None
+        else:
+            log.debug("Message fetched successfully!")
+            return message
+
     @Cog.listener()
     async def on_message(self, message: discord.Message) -> None:
         """Pass `message` to `add_signals` if and only if it satisfies `is_incident`."""