bug#63891: 29.0.91; customize-save-variable should not save all variables if a custom file exists

[Jimmy Wong]

Yes sorry, I mean customize-save-variable, custom-save-variable doesn’t exist. There’s a branch in customize-save-variable that saves all previously updated variables to the custom file if it exists.

I just took a look at cus-edit.el, there appears to be no function that can surgically serialize just one variable value to the custom file. Fixing this the right way will probably involve changing all the places that call custom-save-all and still arguably result in a breaking change.

On 4 Jun 2023 at 1:55 PM +0100, Eli Zaretskii <eliz@gnu.org>, wrote:

From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
Date: Sun, 04 Jun 2023 13:36:30 +0100


As a discussion from bug #63300, it appears this long standing
undocumented behavior of `custom-save-variable` is coming into conflict
with the introduction of `connection-local-*` variables being user
customizable and the fact that Tramp in Emacs 29 sets them on
load. Here's a scenario where the combination of these behaviors results
in one too many surprises:

0. (setf custom-file "~/.emacs.d/custom.el")
1. M-x load-library tramp (or install a package that transitively
requires tramp, without the user knowning)
2. Now `connection-local-profile-alist` and
`connection-local-criteria-alist` are set by
`hack-connection-local-variables-apply`.
3. M-x list-packages
4. Installs a new package
5. Now in addition to `package-selected-packages` being updated, 2
gigantic variables are also saved. Since these connection-local
variables are highly machine, application and connection dependent,
saving them into the custom file will make it very annoying to be shared
across multiple machines. This violates the principle of least
astonishment.

I think the connection-local variables should be simple variables,
initialized from corresponding user options. Then Tramp could hack
the variables without fear of clobbering user customizations.

Michael, can this be done on emacs-29 safely enough?

Expectation:

`custom-save-variable` should only save the value of one variable
regardless of whether a custom file exists.

How is custom-save-variable involved in the above scenario?

And what is custom-save-variable? did you mean customize-save-variable
instead? That one does save just one variable, the one you type at
the prompt.