system alerts keeps reinviting mozilla users despite being empty

[ara4n]

if mozilla users part the system alerts room, they keep getting reinvited to it (perhaps every time the server restarts)? despite the room being empty

[babolivier]

Are we sure it's inviting these users to the same room they left?

From looking at the code, what's the most likely to be happening is that something triggers a server notice to be sent every now and then. In this case, and if a user has left the previous room, Synapse will create a new room and invite the user to it. FWIW the sending of a notice can be triggered either if the user hasn't accepted the latest ToS, if the server's MAU is reached, or if someone sent a notice via the admin API.

I created an account on the homeserver to investigate further when that happens next.

[babolivier]

Hmm, I just witnessed it happen with my account, and the System Alerts user isn't sending any message to the new room (which it shouldn't do). I'll investigate further.

[babolivier]

Quick update on this issue, I haven't seen it happen on my account over the past month or so. I'll wait until the end of this month, and if it still doesn't happen, I'll close this issue (unless I'm told people are still getting bitten by this, which would mean that I'm just not trying to repro it the right way).

[babolivier]

This happened to me on the 23rd (Monday last week) on vector.modular.im.

After digging through the logs, the database and the code, I believe that the issue is that maybe_send_server_notice_to_user, which checks whether a user should be notified about MAU, creates a room and invites the user to it before it knows whether it should notify the user.

The fix should be fairly simple (i.e. introduce a flag to get_notice_room_for_user that make it skip the invite part), but first I want to make sure this is not a symptom of something else, since afaict this bug should happen every time a user syncs.

[babolivier]

Right, turns out that get_notice_room_for_user is cached (with a @cachedInlineCallbacks() decorator), which never expires except when the homeserver restarts (which clears all of the caches), which explains why it isn't run on every sync (but, presumably, the first one since the latest restart).

[babolivier]

Fixed in #7199