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Re: PACKAGE-FEATURES, and hot update of Emacs packages

From: Tim Cross
Subject: Re: PACKAGE-FEATURES, and hot update of Emacs packages
Date: Sun, 28 Nov 2021 12:04:53 +1100
User-agent: mu4e 1.7.5; emacs 28.0.60

Phil Sainty <psainty@orcon.net.nz> writes:

> On 2021-11-27 22:31, Qiantan Hong wrote:
>> Then to really update p, one just need to
>> (mapc (lambda (feature) (unload-feature feature t)) (package-features p))
>> (package-reinstall p)
>> (require p)
> Unloading a feature will trash the user's configuration, and so when
> you load the updated library only the default config will be set --
> unless the user used `eval-after-load' (which certainly can't be
> assumed).  As such, I don't think that could work as a reliable "hot
> update" process.
> My approach to this general issue (in libraries I've written) has been
> to detect at load time if the user had an older version of the library
> loaded, and to perform any appropriate in-place upgrades.  E.g.:
> https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/so-long.el?id=aebba085cba1#n2012
> This does require that the code knows both the previously-loaded and
> newly-loaded version numbers, which can be tricky if the previous
> version of the code wasn't storing its version; so I think it would be
> great if Emacs *automatically* tracked this information for packages
> (or any library that supplies a version) so that authors could write
> such update code without needing to have previously prepared for it.
> I don't think this currently happens.  I can see the functions
> `package-get-version' and `package-desc-version' (which can be used
> with `package-alist'), but haven't spotted anything which is storing
> the version of a package that is/was loaded (and I don't think this
> information is as easily accessible when loading byte-compiled files,
> as version comments don't appear in the .elc file).
> I suppose (package-desc-version (package-load-descriptor DIR)) can be
> used for ELPA packages installed in the standard way, so maybe there's
> scope for using that; but I suspect that'd be too brittle an approach,
> as there are many non-standard ways of loading things.

While I like the idea of being able to use package version to help
manage this, the problem is there is no standardisation of package
version formats. This makes calculation of which version is before/after
another version unreliable.

Personally, I wish the package systems used a consistent semantic
versioning approach. In addition to determining which package version is
later than another version, it would also provide the ability to have
more fine grain control over applying updates i.e. could set the system
to automatically apply minor patch updates, but not major version
updates that may contain breaking changes. 

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