Synapse does not correctly enforce the spec's regex for client_secret

[anoadragon453]

The spec states that we should enforce a regex on client_secret. Synapse currently does not do this.

As a result of this, FluffyChat has started sending client_secret parameters that do not comply with the spec: https://gitlab.com/ChristianPauly/fluffychat/issues/160 Specifically, it sends client_secret values with : in them.

#6767 is written to allow client_secret to take : values, while otherwise respecting the specification. This will allow older versions of FluffyChat to continue working while a fix in put in place and people upgrade.

Eventually we will remove : from the client_secret regex, and will use this as a tracking issue for that.

[anoadragon453]

FluffyChat has fixed the bug 🎉 Now to wait for a majority of their users to update and we'll do the same on our side. They have metrics of client version numbers through those that installed via the OpenStore (most people I suspect).

[anoadragon453]

A couple months ago Christian Pauly of Fluffychat mentioned that most users had now updated to a non-affected version, so I think it's safe to now remove the : requirement (the last person to use : as part of their client_secret on matrix.org was back in April 2020).