[ci] installing 'basictex' from homebrew failing in macOS builds

[jameslamb]

Opening an issue for this because the problem has been going on for a few days.

This was first noticed two days ago, in #4147 (review). R-package jobs on GitHub Actions (for example, this one) are failing with the following errors from R CMD check.

* checking PDF version of manual ... WARNING
LaTeX errors when creating PDF version.
This typically indicates Rd problems.
* checking PDF version of manual without hyperrefs or index ... ERROR
Re-running with no redirection of stdout/stderr.
Hmm ... looks like a package
Error in texi2dvi(file = file, pdf = TRUE, clean = clean, quiet = quiet,  : 
  pdflatex is not available
Error in texi2dvi(file = file, pdf = TRUE, clean = clean, quiet = quiet,  : 
  pdflatex is not available
Error in running tools::texi2pdf()
You may want to clean up by 'rm -Rf /var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T//RtmppA4pCU/Rd2pdf86631a98164'
* checking for non-standard things in the check directory ... NOTE

I believe the root case is that installing basictex is failing.

==> Downloading http://mirror.ctan.org/systems/mac/mactex/mactex-basictex-20200407.pkg
==> Downloading from https://ctan.math.illinois.edu/systems/mac/mactex/mactex-basictex-20200407.pkg
curl: (22) The requested URL returned error: 404 
Error: Download failed on Cask 'basictex' with message: Download failed: http://mirror.ctan.org/systems/mac/mactex/mactex-basictex-20200407.pkg
sudo: tlmgr: command not found
sudo: tlmgr: command not found

This happens at

LightGBM/.ci/test_r_package.sh

Line 86 in d517ba1

brew install --cask basictex

More Context

This is not about the ctan.math.illinois.edu mirror specifically. brew is picking a random mirror, and I've seen others mentioned in other builds, and locally on my own Mac.

It looks like these failures started around the same time as this relevant-looking PR was merged: Homebrew/homebrew-cask#103001. It's interesting that version 20200407 is mentioned in those logs, when according that to that PR the new version should be 2021.0325.

[github-actions]

This issue has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.