Stream trace entries to the inspector

[malted]

Fixes

bombadil/lib/bombadil-cli/src/inspect_server.rs

Line 62 in 90f9bef

// TODO: this should be streamed line-by-line over a websocket or SSE

Thinking of using WS instead of SSE to anticipate 2 way communication in the future (step, play/pause etc?).

Would you prefer just tailing the trace file for now, or setting up a socket between the processes and hooking into TraceWriter::write?

[owickstrom]

Right, I'm not sure 2-way communication is coming, but in any case we might go with a WS connection anyway. Could even make sense to communicate with a binary protocol at some point, but not to start with. The way I've thought about this is that bombadil test spawns a web server much like bombadil inspect does, but with another WS endpoint directly hooked up to the MainObserver, i.e. both writing through the trace writer but also pushing updates to the web server over a broadcast channel (I think this answers your second question).

For the frontend, we could either:

1. pass a query param like &streaming=1 when opening/printing the URL, to have it use the WS rather than batch JSON API, or
2. always use a WS connection, even in after-the-fact bombadil inspect mode, which would probably require a protocol around the trace entries to mark the end of a test (probably makes sense anyway!)

I'm leaning towards (2) to keep the API surface and dimensionality as small as possible. Maybe the always-WS plan would still require the frontend to know if it's really streaming live updates or if it's just being fed old results, but at least there's only one way to get the data.

WDYT?