Re: Mechanisms to persist information

[Jonathan Leech-Pepin <jonathan.leechpepin@gmail.com>]

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On 18 February 2016 at 13:54, Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Clément Pit--Claudel <clement.pit@gmail.com>
> > Date: Thu, 18 Feb 2016 13:25:49 -0500
> >
> > One thing that I would find convenient as a package developer is a
> notion of package-local storage.
>
> We will find a lot of missing infrastructure as we begin using
> package.el more and more.  This is just the tip of the iceberg.
>
> FWIW, I think we should first design the package system we want to
> support, and only after that talk about stuff like saving
> package-specific settings.  But I have no illusions that this will be
> accepted, or that the discussions will indeed be deferred.
>
> > * Save the customization through Custom
> > * Create my own file in user-emacs-directory
> >
> > The first one is unsuitable for persisted information that shouldn't be
> presented to the user (for example, if my package collects statistics, I
> don't want to change the custom file constantly and add large amounts of
> data to it). The second one in not uniform across packages, and forces me
> to invent my own storage format (some packages store a lisp form, other
> store a series of command that are just executed upon loading the file
> (viper, history), others use json or csv-like formats (as was suggested for
> package.el)).
>
> Personally, I see nothing wrong with each package using its own
> storage format.  We don't require package authors to use the same
> coding style and the same abstractions, so why should storage be any
> different?
>
> > In both cases, removing a package won't remove the storage that it uses
> (either in Custom or in separate files in .emacs.d). This is especially
> problematic when trying out packages: I launch a package to try it out, it
> initializes its backing store (often with a file, sometimes with an entry
> in custom-set-variables), then I remove it (if I don't like it), and yet
> the backing store is not removed. Take viper as an example (though viper is
> bundled with Emacs right now): launching it and allowing it to persist its
> settings creates a file ~/.emacs.d/viper.
> >
> > It would be nice to have a uniform way to persist package-specific
> information; ideally one that would collaborate with package.el, so that
> removing a package would remove its stored state. One would need to figure
> out how to handle settings persisted through custom as well, but a simple
> key-value store that plays well with package.el would be a great start.
>
> Removing auxiliary files doesn't require those files to be in the same
> format, it can be solved by having each package produce a record of
> those files that the manager can use to delete them.
>
>
A possible implementation for auxiliary files that would reduce the need
for tracking the files could be something akin to:

    (defcustom user-package-data-directory
      (expand-file-name "package-data"
                        user-emacs-directory)
      "Directory for storing persistent package-related data.

    Installation of a package creates a subdirectory for local persistent
    data that will be removed on package uninstallation."
      :type 'directory)

Then part of package installation process would become:

    (make-directory (expand-file-name packagename
                                      user-package-data-directory))

Package removal would simply remove that directory (and it's contents).

Regards,
Jonathan

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