bug#73681: Maybe partly undo the patch on Elisp comp-el-to-eln-filename

[Liliana Marie Prikler]

Am Dienstag, dem 08.10.2024 um 12:41 +0200 schrieb Martin Edström:
> It comes as part of the package. I don't want to assume that it has
> been compiled, since it's fairly performance-sensitive. That's why
> I'll either use a previously existing compiled object or make a new
> one.
Could you leave that decision to the user?

> […]
> 
> So let's ignore my package, it is just an example of a downstream use
> of `comp-el-to-eln-filename` that relied on its hashing
> functionality.
> 
> Let's just discuss that function.
> 
> I have to point out that the emacs `load-path` does not include any
> native paths.  When I inspect the value on my non-Guix emacs, I see
> no references to .../eln-cache/..., just references to directories
> where there are .elc and .el files.
There is a separate load path for natively compiled files, called
`native-comp-eln-load-path'.

> I infer that Emacs starts with finding a library in load-path, then
> converts the path with `comp-el-to-eln-filename`, and checks if that
> file exists, then loads it.
> 
> And crucially, it is not just about the filepath, the function hashes
> the file contents as well. That ensures that the output path is
> always different if the source file changes.
I think relying on such implementation details is perhaps permitted if
it's inside of Emacs itself, but even then it clashes with our
expectation that Emacs be graftable.

> Since Guix has a patch that removes this effect, it seems like a
> package could be upgraded many, many times, without the .eln path
> ever changing, and so the user would stay on that very outdated file.
> 
> Is that not a regression/bug?
The way our load paths are set up, it is actually the opposite (which
still is a bug, just not the one reported).  While `guix upgrade` or a
command to the similar effect will swap out the .eln under the hood,
the `.el` and `.elc` files stay stable – remember what I wrote in the
previous message about that having caused issues with byte compilation?

We also get a similar-looking bug if our packages aren't actually
native-compiled, but Emacs itself vendors them.  That is resolved by
dropping those .eln-files from the Emacs package.

Cheers