remove nonexistent channels' settings on posting error

librarian/discord/bot.py

@@ -2,8 +2,10 @@
 import logging
 
 import discord
+import discord.errors
 from discord.ext import commands
 
+from librarian.discord import errors
 from librarian.discord.cogs import (
     pulls,
     server,
@@ -53,9 +55,12 @@ async def on_ready(self):
         await self.start_routines()
 
     async def post_or_update(self, channel_id, message_id=None, content=None, embed=None):
-        channel = self.get_channel(channel_id)
-        if channel is None:
-            channel = await self.fetch_channel(channel_id)
+        try:
+            channel = self.get_channel(channel_id)
+            if channel is None:
+                channel = await self.fetch_channel(channel_id)
+        except discord.errors.NotFound:
+            raise errors.NoDiscordChannel(channel_id)
 
         if message_id is None:
             message = await channel.send(content=content, embed=embed)  # type: discord.Message

librarian/discord/cogs/background/github.py

@@ -11,7 +11,7 @@
 from sqlalchemy import exc as sql_exc
 
 from librarian import storage
-from librarian.discord import formatters
+from librarian.discord import formatters, errors
 from librarian.discord.settings import custom
 from librarian.discord.cogs.background import base
 
@@ -265,11 +265,16 @@ async def sort_for_updates(self, pulls: typing.List[storage.models.pull.Pull]) -
                     "%s: failed to post an update for pull #%d in channel #%d: %s",
                     self.name, item[0], item[1], result
                 )
+                await self.handle_update_exception(result, item)
             elif result is not None:
                 new_messages.append(result)
 
         self.storage.discord.save_messages(*new_messages)
 
+    async def handle_update_exception(self, exc: errors.LibrarianException, item: typing.Tuple[int, int]):
+        if isinstance(exc, errors.NoDiscordChannel):
+            await self.bot.settings.reset(exc.channel_id)
+
     async def status(self) -> dict:
         # FIXME: make something useful here
         return {}

librarian/discord/errors.py

@@ -0,0 +1,8 @@
+class LibrarianException(Exception):
+    pass
+
+
+class NoDiscordChannel(LibrarianException):
+    def __init__(self, channel_id):
+        self.channel_id = channel_id
+        super().__init__(f"Discord channel #{channel_id} does not exist")

tests/discord/cogs/background/test_github.py

@@ -1,6 +1,8 @@
+import collections
 import random
 
 import discord as discord_py
+import discord.errors as discord_errors
 import pytest
 
 from librarian.storage.models import (
@@ -240,3 +242,31 @@ async def test__sort_for_updates__existing_pull(
             assert done[0].number == expected[0].number
             assert done[1] == expected[1]
             assert done[2].id == expected[2].id
+
+    async def test__exception_handling_no_channel(
+        self, client, storage, existing_pulls, mocker, codes_by_titles
+    ):
+        monitor = github.MonitorPulls(client)
+        monitor.CUTOFF_PULL_NUMBER = 0
+
+        p = next(iter(
+            _
+            for _ in existing_pulls if
+            (
+                codes_by_titles[_["title"]] and
+                _["state"] == "open"
+            )
+        ))
+        language = custom.Language(codes_by_titles[p["title"]])
+        await client.settings.update(123, 1234, [language.name, language.code])
+        storage.pulls.save_from_payload(p)
+        pp = storage.pulls.by_number(p["number"])
+
+        response = collections.namedtuple("Response", "status reason")(404, "testing stuff")
+        client.fetch_channel = mocker.AsyncMock(side_effect=discord_errors.NotFound(response, "error"))
+        client.settings.reset = mocker.AsyncMock()
+        await monitor.sort_for_updates([pp])
+
+        client.settings.reset.assert_called()
+        args, _ = client.settings.reset.call_args
+        assert args == (123,)