MSC3575 (Sliding Sync) add well-known proxy support

[EdGeraghty]

Adds MSC3575 (Sliding Sync) proxy support from well-known. Removes the ability for adding custom sliding sync servers through Labs.

Signed-off-by: Ed Geraghty ed@geraghty.family

[t3chguy]

This seems like it wouldn't work if the value in the well-known was to be updated, as it would only ever be fetched when you enable Sliding Sync and you would not be able to update it AFAICT. This seems like a problem

[EdGeraghty]

This seems like it wouldn't work if the value in the well-known was to be updated, as it would only ever be fetched when you enable Sliding Sync and you would not be able to update it AFAICT. This seems like a problem

Yes. Perhaps adding a setting that it's a proxy and needs to get the client well-known on launch?

[t3chguy]

Yes. Perhaps adding a setting that it's a proxy and needs to get the client well-known on launch?

Yeah this would be a better option, happy for the proxy & "native" support to be removed in favour of the well-known check

[EdGeraghty]

Yes. Perhaps adding a setting that it's a proxy and needs to get the client well-known on launch?

Yeah this would be a better option, happy for the proxy & "native" support to be removed in favour of the well-known check

So should I do away with the dialog altogether, allowing it to be enabled from labs iff ( proxySlidingSyncSupportCheck() || nativeSlidingSyncSupportCheck() ) && slidingSyncHealthCheck()?

I guess I could then add two settings at device level, slidingSyncEnabled and isProxy or similar so it knows whether to do the well-known check on start?

ETA:

Perhaps just set whether or not sliding sync has been enabled, then on open (if there isn't already a proxy url set) have a fail-through from "native" support on the unstable endpoint to a well-known check for the proxy? 🤔

I don't really know how all this stuff is handled in the views and dialogs...

ETA2: Thanks to feedback in the relevant matrix #, I'll:

1. check if the labs flag is enabled; if not bail: no sliding sync
2. check if the labs flag proxy url is set; if so: use sliding sync with that proxy url
3. check if the well-known contains a proxy url; if so: use sliding sync with that proxy url
4. fallback to "native" mode

Falling back in this order means that people with "native" support provided by their homeserver can still choose to use a proxy url...

[EdGeraghty]

🎉

[t3chguy]

LGTM otherwise, thanks for persevering!!

[t3chguy]

Needs sign-off as per https://github.com/element-hq/element-web/blob/develop/CONTRIBUTING.md#sign-off

[richvdh]

this is unsafe. It will cause the Sliding Sync proxy to start polling with our access token, which means that the SS proxy will receive our to-device messages, causing decryption errors.