MSC4417: Application service url previews

[Half-Shot]

Tracking issue for matrix-org/matrix-spec-proposals#4417

Context

This is the implementation for the application service API handler that requests URL previews from integrations when they offer an enhancement to do URL Previews. As with other AS APIs, they register their intent to handle a regex and the request is passed down to them. They may respond with either a 200 response with the valid OpenGraph data, or any other response to be skipped. ASes may also mark themselves as exclusive to disallow other ASes or the homeserver from attempting to resolve a URL.

Current design in Synapse

This is implemented in the appservice/api.py layer, inside the Appservice handler, and ultimately media/url_previewer.py

Usage

Pre-requisites:

• An integration supporting MSC4417, Hookshot has some experimental support for this

The feature is configured using:

experimental_features:
	msc4417_enabled: true

url_preview_enabled: true

Caveats to be aware of

• /_matrix/client/v1/media/preview_url should clarify what status codes / errors may be returned matrix-org/matrix-spec#2327 is an omission in the current spec. The implementation may return a 404 instead, as the spec is vague.

How do we know when to remove this

We can remove this when one of the following is true:

• MSC4417 passes spec review and lands in the spec.
• The integrations team (@Half-Shot is the contact point) decides they no longer need this.

[sandhose]

I'm not super duper confident that it's the role of an AS to do such a thing, and would rather see this being developed as a Synapse module API instead… like it's unclear to me if we're choosing to use ASes for URL previews, why did we chose to do module API callbacks for event spam checks and not a standard AS API?

[Half-Shot]

IMO I actually think the event spam checks shouldn't have ended up as a module at all, especially given things like mjolnir have spun out as effectively more of an integration.

Synapse modules aren't portable to other homeserver implementations, and it means if we were to start wanting to integrate with other platforms we'd need them to start asking them to publish Synapse modules. Appservice APIs have actually been hugely helpful in the past with things like the Gitter and Rocket.Chat projects in the past because it became easier to integrate the projects when they were speaking just REST rather than writing Python at each other.

Happy to chat through this more, but I see modules as not the way forward for deeply linking into other platforms.

[Half-Shot]

As per #19458, we're pausing this for the moment.