Add SSE change notifications

[robertbak]

Hi,
I'm not sure if this makes sense for the main server, but I wanted to add it for myself and making a pull request just in case it's interesting. It's (behind a flag, disabled by default), allows any client to listen to version changes for a given client_id, and get instant notifications. I don't see a reason to add more events here, but just in case it's a "version" event.

curl -N -i -H 'X-Client-Id: CLIENT_ID'  https://taskchampion_sync_server.example.com/v1/client/events

will get you an SSE event

event: version
data: {"clientId":"00000000-0000-0000-0000-000000000000","versionId":"00000000-0000-0000-0000-000000000001"}

This could be made more robust by adding :keep-alive as heart-beat, but just wanted a minimal version that doesn't do anything fancy.

I've been using this for the weekend, and it seems like there's no obvious issues.

The only downside (why it's behind a flag) is that it leaks versionIds if you know a clientId. You don't really need the versionId for basic task sync calling, so we could just remove it and keep just the clientId in the version event, which is straight forward

pub(crate) struct ChangeEvent {
    pub(crate) client_id: ClientId,
    pub(crate) version_id: VersionId,
}

[djmitche]

(Just from reading the description)

This seems like a fine thing to add!

Why not use websockets instead of SSE? I thought SSE was deprecated but apparently not, so consider this a curiosity :)

You're correct that the version ID isn't actually useful to a listener -- the only useful version for the listener is the child of the version they have. So yeah, why not avoid leaking that information!

[robertbak]

Oh, I started with sockets, but wanted to keep it as minimal as possible, and since it doesn't need two-way communication, decided to just use SSE, since it's one way.

I think once you move to sockets, it might make sense to do more with them? I'm not against websockets, just didn't want to introduce them without having a reason to.

btw. appreciate this server so much, I'm working on a static web app, with a wasm version inside and it's such a nice way to get other people to use my taskwarrior setup (still a bit raw around the edges, but getting there) and this has taken care of 90% of the work.

ps. removed the versions_ids, it's much simpler now, and as useful ;)