[WIP] Add Presence module

[S7evinK]

WIP of a presence implementation. (#602)

Todo:

• Implement GetPresence
• Add m.presence to gomatrixserverlib
• Implement /sync
• Implement federationapi & sender

Questions:

• Where to store the data? (memory is not possible, as we need to store e.g. status_msg)
• Add a new component? (would make sense for the future, scaling wise [IMHO])

Pull Request Checklist

• I have added any new tests that need to pass to sytest-whitelist as specified in docs/sytest.md
• Pull request includes a sign off

[r3k2]

When will this be on master?

[S7evinK]

@r3k2 There are still some missing pieces (e.g. database), which I didn't start yet.

[S7evinK]

@kegsay @neilalexander
Opinions on where to store the data?

syncapi (same as receipts) - would require an inthttp package for getting the presence from the clientapi
eduserver - would require a new database

The other idea would be, as mentioned, a new component with the known pros/cons.

[kegsay]

Adding components is extremely costly and incurs IPC costs on any public API. We generally want to stick logic to orthogonal scaling points. Presence will scale based on the number of users independent of rooms, which is the same as the user API, so that feels like the most logical place to put this.

[S7evinK]

Unfortunately still stuck with the following tests. full-api mode is a bit worse, probably just error handling, didn't have a look at that yet.

Presence: 67% (10/15 tests)
    × Newly joined room includes presence in incremental sync
    × Get presence for newly joined members in incremental sync
    × User sees their own presence in a sync
    × User sees updates to presence from other users in the incremental sync.
    × New federated private chats get full presence information (SYN-115)

[thinkwelltwd]

I'm doing some due diligence research about moving to a multi-homeserver Matrix deployment. For performance benefits, I'd be very glad to deploy Dendrite, but for the lack of presence. Is there any sort of ETA (ie 1-6 months) for this PR, or should I just plan to go with Synapse?

[S7evinK]

As there have been many changes since I've started working on this, and I now know better (hopefully), I'll close this for now. But definitely will revisit this, unfortunately no ETA on this.