Synapse uses an unstable prefix for thirdparty application service requests

[babolivier]

The appservice code has an APP_SERVICE_PREFIX constant:

synapse/synapse/appservice/api.py

Line 52 in bf72d10

APP_SERVICE_PREFIX = "/_matrix/app/unstable"

Which it then uses for any .../thirdparty/... request:

synapse/synapse/appservice/api.py

Lines 137 to 142 in bf72d10

	uri = "%s%s/thirdparty/%s/%s" % (
	service.url,
	APP_SERVICE_PREFIX,
	kind,
	urllib.parse.quote(protocol),
	)

synapse/synapse/appservice/api.py

Lines 172 to 176 in bf72d10

	uri = "%s%s/thirdparty/protocol/%s" % (
	service.url,
	APP_SERVICE_PREFIX,
	urllib.parse.quote(protocol),
	)

However, the spec says that these endpoints should have the prefix /_matrix/app/v1, not /_matrix/app/unstable. This means that if an application service correctly implements the spec, Synapse will 404 when trying to contact it on these endpoints (when e.g. looking up which third-party protocol an application service uses).

[babolivier]

According to the bridges team, https://github.com/matrix-org/matrix-appservice-bridge supports both endpoints so won't be impacted when we fix this.

https://github.com/mautrix/go and https://github.com/mautrix/python don't seem to support either version of this endpoint.

These are all the appservice frameworks I know, but I don't claim to know all of them, though, so just to be sure we can always implement a temporary fallback to /unstable if the /v1 request fails, with a deprecation notice in the changelog and upgrade notes.

[Half-Shot]

The matrix-bot-sdk is also fine

[S7evinK]

Stumbled over this while trying to update Sytest - matrix-org/sytest#1310

[DMRobertson]

We should Just Do This.