Support multiple sinks at once (`tee` logging)

[teh-cmc]

Something along the lines of this pseudo code:

struct LogSinkBundle(Vec<Box<dyn LogSink>>)

impl LogSink for LogSinkBundle { ... }

also need to update CLI args helpers for both Rust and Python to support mixing --save with anything else.

[emilk]

It is very useful to be able to dynamically add and remove sinks at runtime.

For instance:

• you start rerun with no sinks
• something happens, so you now start dumping to an .rrd on disk
• you hit a rare corner case happens, so you call spawn and start streaming to the viewer, then stop that stream once you reached the end of the corner case

This mean we need an API along the following lines:

rerun = rr.new_recording(app_id=…, …)
rerun.add_sink(file_path="foo.rrd")
viewer_sink_id = rerun.add_sink(spawn=True)
…
rerun.remove_sink(viewer_sink_id)

When no sinks are attached, rerun.log(…) should be a no-op

[emilk]

An interesting case to consider is streaming to an .rrd and calling spawn. We could set up the native viewer to be able to stream-load an .rrd file that is being written to, requiring no extra WebSocket connection. That would also mean the rerun library without the WebSocket feature (and huge dependency tree) could still support spawn (see #4788)

• Viewer should be able to stream in .rrd files whilst they're still being written to #4056

[vmayoral]

Adding to the thread another potential use case of interest: #6685

In a nutshell: Periodic calls to rr.save() to generate smaller/manageable log files. This is particular relevant in robotic systems generating tons of data from depth sensors and multiple cameras.

[vmayoral]

Hello all, any updates on this topic? Rationale at #1998 (comment) makes a lot of sense to me as well. Thanks!