Description

I'll try to do this as exact as I'm able to in reverse.

After upgrading our matrix server running on ubuntu 18.04 to 1.11.1+bionic1 we did not receive any messages from servers running on older instances (only in 1:1 chats, groups have been working fine).

We are explicitly talking with a another company running a matrix instance and a freelancer and his own setup.

Both problems were solved after upgrading their versions to the matrix synapse 1.11.1 version. But since at least one party was running 1.11.0+bionic1 before, it looks like the bug has been introduced in the patch release.

All messages and presence updates going from 1.11.1+bionic1 to the other sides went through without problems. But nothing the other way around.

One major pain point was that we discovered this by getting suspicious that some people did not respond in time and thus tried other communication channels. So the users where totally unaware of this.

A last note: As mentioned bringing all instances to 1.11.1 (or the latest docker image) solved it, no message went missing! As soon as the other parties wrote a new message all the missing ones got redelivered.

Edit: As far as I can tell it did not matter on which side the 1:1 chatroom has been initially created.

Steps to reproduce

(I did not re-test this, but this should result in the problematic setup)

• Connect two matrix servers running 1.11.0+bionic1
• Create two 1:1 chats, one initialized by each side
• Upgrade on instance to 1.11.1+bionic1

You should now only be able to send 1:1 messages from 1.11.1+bionic1 to 1.11.0+bionic1 but not the other way around (At least using any riot the user won't get any feedback about this. I'll write an issue for that project later).

I could not find any suspicious log entries on our 1.11.1 server. But I've requested logs from the other side, maybe we can find something there.

Version information

• apt package installed: matrix-synapse-py3 1.11.1+bionic1 amd64
• Not exactly sure about the other sides. One had matrix-synapse-py3 1.11.0+bionic1 apt
• The other party had the docker container setup using matrixdotorg/synapse (version before upgrade not known)