bug#54459: 29.0.50; Simplify connection-local variables (feature request)

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi:

I use tramp very often and I find myself using this sort of snippet very
often:

(defvar-local gtags--executable 'unknown)

(defun gtags--executable ()
  (cond
   ((not (eq gtags--executable 'unknown)) ;; the value is already cached
    gtags--executable)
   ((file-remote-p default-directory)
    (with-connection-local-variables
     (if (boundp 'gtags--executable-connection)
         (setq-local gtags--executable gtags--executable-connection)

       ;; Else search and set as connection local for next uses.
       (setq-local gtags--executable (executable-find "gtags" t))
       (let* ((host (file-remote-p default-directory 'host))
              (symvars (intern (concat host "-gtags--vars")))) ;; profile name

         (connection-local-set-profile-variables
          symvars
          `((gtags--executable-connection . ,gtags--executable)))
         (connection-local-set-profiles `(:machine ,host) symvars))

       gtags--executable)))
   (t     ;; use default value (searched locally)
    (setq gtags--executable gtags-executable))))

The idea is that when using tramp with company it doesn't call
executable-find constantly in the remote buffers.

There are 3 issues that could be solved/improved:

1) If another buffer of a file in the same remote host is already open,
then when the function is called in that buffer for the first time it
will use the value cached in gtags--executable-connection. That's why in
the code I need to create a buffer local variable... Could we provide a
way to update all of them immediately?

2) There are functions to set connection-local profile variables
(connection-local-set-profile-variables) But so far there are not any
clean way to append variables to an existing profile; so a call to
connection-local-set-profile-variables deleted any previous variable,
actually the user is enforced to create a unique name for every profile
every time it needs something like this.

3) In the ideal world it may be possible to add an optional parameter to
tell executable-find to cache the found values per host and executable
name... so it could avoid the extra remote search... I understand this
shouldn't be enabled by default, but many packages may find it
useful. executable-find could be smart enough to remember some of the
previous searches at least in remote hosts.

Does it makes sense?



In GNU Emacs 29.0.50 (build 133, x86_64-pc-linux-gnu, GTK+ Version 3.24.33, cairo version 1.17.4)
 of 2022-03-18 built on Ergus
Repository revision: 9bcf58d27c7c9ebe3bdd4d0564808767420bd44c
Repository branch: feature/completions-customs
System Description: Arch Linux

Configured using:
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Load-path shadows:
~/gits/emacs_clones/composable/composable-mark hides /home/ergo/.config/emacs/elpa/composable-20201024.1458/composable-mark
~/gits/emacs_clones/composable/composable hides /home/ergo/.config/emacs/elpa/composable-20201024.1458/composable
/home/ergo/.config/emacs/elpa/transient-20220314.1605/transient hides /home/ergo/.local/share/emacs/29.0.50/lisp/transient

Features:
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Memory information:
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