[core-updates] texlive 2021 bootstrap problem

[Thiago Jung Bauermann]

Hello,

In an attempt to verify whether bug 48064¹ (”texlive-* packages fail to 
build non-deterministically”) is fixed by the current TeX Live version, I’ve 
been working on updating it to version 2021.1².

Unfortunately I ran into a bootstrapping issue: texlive-latex-base now 
depends on texlive-latex-l3kernel, otherwise its build fails with:

LaTeX2e <2020-10-01> patch level 4
! LaTeX requires expl3.

The problem is that if I add texlive-latex-l3kernel to the propagated-
inputs list of texlive-latex-base, then a dependency loop is formed since 
texlive-latex-l3kernel uses the texlive-build-system, which adds texlive-
latex-base as an implicit input. And if I understand thing correctly, 
l3kernel does indeed need some form of TeX or LaTeX in order to run
`source/latex/l3kernel/l3.ins`.

I tried to “cheat” and added the build argument
`#:texlive-latex-base ,texlive-bin` to texlive-latex-l3kernel (since 
texlive-bin doesn’t depend on texlive-latex-base), but then I get an error 
about not being able to find `luatex.fmt`.

Looking around the web, I found this answer:

> To answer your question about deactivation ... the L3 programming layer 
> is part of the core LaTeX format since release 2019-10-01 and it can't be 
> deactivated. At the moment it is there to speed up processing of packages 
> written in expl3, e.g., xparse, fontspec, and many others, but going 
> forward core parts of LaTeX will make direct use of it.
> 
> Deactivation is only possible by using a release prior to 2019-10 (even 
> rollback via "latexrelease" will load).
— https://tex.stackexchange.com/a/538716

Does anyone have an idea how to break this dependency loop? Do we need to 
have a package definition for a pre-2019-10 version of LaTeX just to build 
texlive-latex-l3kernel, and then use that as a dependency for the actual 
texlive-latex-base package?

The only alternative I can think of is trying to build a minimal version of 
2021.1 LaTeX which somehow doesn’t use expl3 so that it can be used to 
build texlive-latex-l3kernel. But that is above my very superficial 
understanding of TeX and LaTeX. And the quote I pasted above doesn’t look 
very encouraging.

If anyone wants to have a look at what I have so far, I pushed my patch to 
branch “bug-48064-texlive-2021” at https://gitlab.com/bauermann/guix .

-- 
Thanks,
Thiago


¹ https://issues.guix.gnu.org/48064
² Well, almost. In order to have a stable URL to fetch from, I’m using the 
latest version available in the historic repository, which is slightly 
older than 2021.1.