Inability to join Matrix rooms via `seagl.org` aliases

[AndrewKvalheim]

We’ve received reports (example) of vague error messages when people try to join Matrix rooms via the seagl.org aliases.

Reproduction

When trying to join #test-2023-06-13:seagl.org, Synapse logs:

Request failed: PUT matrix://seagl.org/_matrix/federation/v1/send/1685687770792: HttpResponseException('404: Not Found')
Request failed: GET matrix://seagl.org/_matrix/federation/v1/make_join/%21rvWybNVXTWAXMIVoCW%3Aseattlematrix.org/…: HttpResponseException('404: Not Found')
Request failed: PUT matrix://seagl.org/_matrix/federation/v1/send/1685687770793: HttpResponseException('404: Not Found')

Synapse correctly resolved the alias to a room ID—it successfully followed the delegation and queried the proxy—yet still it tried to federate with a nonexistent endpoint.

[AndrewKvalheim]

@‌tulir identified that this is likely due to Synapse unconditionally adding a seagl.org homeserver to the proxy’s response, has proposed the removal of that behavior from future versions of Synapse, and updated mauliasproxy to work around the issue with existing Synapse versions.

[altsalt]

The proposed change was merged, dunno when it will make it into the ansible project.

[strugee]

Doesn't matter - we might have Debian stable users. We need to deploy the updates to the alias that work around stuff for old Synapse versions.

[AndrewKvalheim]

See also @‌csett86’s Synapse adoption graphs