Re: Prefer luatex for documentation

[Werner LEMBERG]

> Which brings me to my second point, that I share Jean's concern and
> the reasoning about supporting three TeX engines and appreciated him
> asking if we could drop support for some as pretty much the first
> comment on the merge request, less than a day after it was posted:
> https://gitlab.com/lilypond/lilypond/-/merge_requests/1714#note_1161468272
> (Sadly, this was shot down immediately by Werner in his comment:
> https://gitlab.com/lilypond/lilypond/-/merge_requests/1714#note_1161550648
> )

Mhmm, this was an immediate reaction to 'drop support'.  It took some
time until it emerged that with 'support' you and Jean meant something
completely different than me: You were talking about the LilyPond
infrastructure, I deduced that you were talking about the general
ability to use a given engine to compile the documentation because of
special code necessary in `texinfo.tex`.

>> Which makes me wonder why this [compiling LilyPond with LuaTex in
>> CI] works without installing texlive-luatex?
> 
> Looking at the list of contained files in the texlive-luatex package
> (https://packages.ubuntu.com/bionic/all/texlive-luatex/filelist) it
> seems that this is mostly documentation, packages for lualatex, and
> additional packages for plain luatex. Compiling texinfo.tex with the
> LuaTeX engine apparently doesn't need any of this. And even if it
> did, the package is relatively small and doesn't pull in additional
> dependencies in the way that texlive-xetex does.

Thanks for checking.  Do you imply that using luatex (instead of
XeTeX) would actually make the docker images smaller?

>> And whether we can just *require* LuaTeX and stop looking for
>> pdfTeX and XeTeX altogether?
> 
> I did a few measurements for the case of building the LilyPond
> documentation and, in terms of speed with the "CI configuration" (no
> extractpdfmark and using the Ghostscript API), LuaTeX seems to
> position itself between pdfTeX, which remains the fastest, and
> XeTeX.  So at least in my opinion, this would be a viable path and
> we could just always build with LuaTeX.

Thanks, too, for your (and Jean's) timing tests!  I really didn't
expect that luatex is that fast with `texinfo.tex`, which is a
pleasant surprise – I am used to excruciatingly slow builds of LaTeX
documents and assumed the same for Texinfo documentation.  In
hindsight it is clear that the overhead for LaTeX is caused by loading
OpenType support files (together with reading the database of OpenType
fonts present on a given system), not necessary for `texinfo.tex`.


    Werner