Enable multiple REPLs

[zauster]

Hey,

Coming from R and ESS, I use multiple REPLs quite frequently. So I set
out to do it in Julia as well. This is my first Emacs Lisp PR: So
while the code works (I have been using it for the past week), the
code could definitely need more elegance.

Added functions

• julia-snail--port-in-use-p
• julia-snail--find-free-port
• julia-snail-switch-or-create-repl

When calling julia-snail in a Julia buffer, it now does the following:

• If there is no REPL yet, create one (as before)
• If there is an REPL, let the user choose to create a new REPL or switch to an existing one

When a buffer has already been associated with a REPL and you want to connect it to
a new or other one, call julia-snail-switch-or-create-repl.

What does not work:

• Renaming the REPL buffer after it is created: then the user has to
  manually update the julia-snail-repl-buffer variable too
• Exiting an REPL still leaves the REPL name in the list of REPLs

At the moment, I simply record any new REPL buffers (and their ports)
in a list when they are created. Obviously, this is error-prone, as
this approach fails when a REPL buffer is exited or renamed. So what
would still be nice to have is a way to get the list of active REPL
(and their ports) at a given time. Do you happen to know this could be
done?

[gcv]

Thank you for all this work. I think your approach of keeping track of the running REPLs and their ports is reasonable, but probably needs to be refined by hooking into the events that occur when a REPL buffer is killed or renamed.

I’m concerned that the way you check for open ports by calling ss is very portable. Not sure what the right approach is.

Broadly speaking, you implemented an automatic port choosing mechanism, whereas I tried to avoid doing all that work and forced port selection to be manually done by the user. :) This is a good feature, but I won’t have time for a while to properly review or work on it. Let’s keep this open, and I’ll get to it when I can.

[zauster]

I agree, the port-checking function is quite far from being portable. There is no guarantee that ss is even installed on a Linux system. I didn't find an Elisp-exclusive way to check for occupied ports. AI suggested to use the make-network-process Elisp function (if the connection is refused, that port is already in use), but that seemed a bit over-the-top for me...

[rossviljoen]

Another approach to finding a free port is to use make-network-process with :service 0 (I think :service t would work as well?). This finds a random free port, which you can then extract from the network process. This is used in emacs' own jsonrpc code:

https://github.com/emacs-mirror/emacs/blob/24297f6d9a2fb3bf464f25bdf80c85b13aa04a36/lisp/jsonrpc.el#L1154

The downside is of course that the chosen port is much less predictable than your approach