don't automatically dispatch to a busy session

[simonpcouch]

I just ran into a (simplified) situation where I had 2 R sessions running, both configured with mcp_session() via .Rprofile.

• Session 1), the first one to be connected with mcp_session(), is running long-running code and has been / will be busy for hours
• Session 2) is mostly idle

If I explicitly ask to list_r_sessions() and then select 2), the server runs fine. Otherwise, the request is automatically dispatched to 1), where it fails.

Notably, when I list_r_sessions(), 1) doesn't even show up—this makes sense to me and doesn't require any changes—but I think our default session should change from:

• Before: The lowest-numbered session (or the server itself if there is none), to
• After: The lowest-numbered session that is actually responsive, e.g. the smallest index that would appear in list_r_session() if called (or the server itself if there is none)

[shikokuchuo]

Catering for this scenario possibly adds more complexity than it's worth...

It seems brittle in that even if we do this check and one of the processes responds, the process could become busy again straight away.

I think the auto-selection makes sense if we assume that most people have one R session open and it then 'just works'. For other situations, then I think we'd want the user to make the selection explicitly. Perhaps in that case (where there is ambiguity), the default should be to run in the server itself.