Locally-rejected invites don't get passed on to the client over SSS

[richvdh]

Description

[in fairness I have not validated if this is a server-side or client-side problem]

Background: When a user rejects an invite, we generate a "leave" event to update their membership state. For an invite received from a remote server, we normally ask the remote server to generate the leave event for us. However, there is a fallback case for when the remote server is unreachable: we will generate an "outlier" event locally, which is then only sent to the invited user.

It appears that such locally-generated leave events are not passed down SSS.

In other words: this is a regression of matrix-org/synapse#2181, but applies only to SSS.

Steps to reproduce

• Receive an invite from a user on a remote server
• The remote server goes offline
• On (say) Element-Web, reject the invite
• On EX, observe that you still have a pending invite

Homeserver

sw1v.org

Synapse Version

v1.115.0

Installation Method

pip (from PyPI)

Database

postgres. no. yes. yes.

Workers

Single process

Platform

Configuration

Relevant log output

-

Anything else that would be useful to know?

[Andoriax]

I have something like this going on with EX, but it cleared up after rejecting Invites in Element then logging out on EX and back in on EX to find the Invites list updated and cleared up,

[HarHarLinks]

I was having this issue for a long time now that an (abusive) invite is sitting in my EXA inbox, which is not there in any of my many other incl non-Element clients and while in principle I know enough workarounds to get rid of it, my goal is to help uncover and keep track of flaws impacting "Matrix 2.0" users with this.

I've had the time to investigate a bit and the invite is definitely coming down synapse's SSS endpoint. Sadly it's almost impossible to check the stable sync for it manually, but I tested that a client doing a stable initial sync does not receive it.

I'm using the mjolnir Antispam module off the shelf from the matrix-docker-ansible-deploy playbook with #matrix-org-coc-bl:matrix.org and CME.

My suspicion is currently that I'm hitting this bug or it's equivalent with the module API.