bug#54405: 27.1; How to reset connection-local variable

[David Landell]

Hi,
Is it possible to reset a connection local variable to the default 
in a buffer when default-directory changes from remote to local?

I have attached an example ert test to demonstrate the issue. I would
kind of expect the supplied code to work but I may be doing something
wrong and there might be some other way of doing this. Couldn't find
anything in the manual though.

To run the code: emacs -Q  -l ./connection-local.el 

The problem is a real world problem from my package rg.el
(https://github.com/dajva/rg.el). It is similar to the grep.el package
with the added possibility to rerun a searches with modified parameters
from the results buffer. So a typical use case would be to do a search
on a remote host and then change the default-directory in the results
buffer to do the same search on the local computer. The connection local
is used to store the path to the ripgrep executable that is driving the
search.

Best regards,
David Landell

connection-local.el
Description: Code to reproduce

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo 
version 1.16.0)
 of 2020-09-19 built on lgw01-amd64-021
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Linux Mint 20.2

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Ran 1 tests, 0 results were as expected, 1 unexpected
Making completion list...

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 --disable-silent-rules '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/27.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/27.1/site-lisp:/usr/share/emacs/site-lisp
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-fstack-protector-strong
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 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro
 -no-pie''

Configured features:
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INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
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Important settings:
  value of $LC_MONETARY: sv_SE.UTF-8
  value of $LC_NUMERIC: sv_SE.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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Memory information:
((conses 16 76214 9618)
 (symbols 48 9041 1)
 (strings 32 26513 2007)
 (string-bytes 1 933622)
 (vectors 16 15642)
 (vector-slots 8 191670 10146)
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 (buffers 1000 15))