bug#62116: RFE: eglot: support window.showDocument LSP RPC

["João Távora" <joaotavora@gmail.com>]

[ Cc'ing Dmitry mostly because of the question in the last paragraph ]

On Mon, May 15, 2023 at 11:45 AM João Távora <joaotavora@gmail.com> wrote:

> I'm working on this change to jsonrpc.el.  This will solve the Eglot
> showDocument
> problem by allowing the find-file _and_ the return code calculation to be async.

Continuing this soliloquy :-) I'm putting this more ambitious change
on the back burner.  It's not immediately clear how to implement this
cleanly.  The best I can think of is to change jsonrpc.el to allow the
request handler to return a function  that is then passed a "success"
and an "error" callbacks.  But not only is this not spectacularly clean,
it  doesn't really solve the "nested" request problem by itself.
Furthermore there aren't any other clients for these kinds of requests
yet, and there seems to be ongoing work for a future/promises system
which could make this cleaner.

So for window/showDocument specifically, I went with the much simpler
check of file-readable-p before reporting success (or failure).

I also addressed Alan's report of a problem with the widening.  I
used xref--goto-pos, which I normally wouldn't do, but that
internal function semantics is exactly what was needed here.
Maybe Dmitry can agree to export it?

João