Trying to fix #23027

There was an error Error: No available formula with the name "brew-cask"

Reference: Homebrew/homebrew-cask#75135 (comment), Homebrew/homebrew-cask#57875 (comment), or maybe rvm/rvm#3995 (comment)

[Edit:] The above seems to be a minor error, is under a set +e and hence is skipped

Investigations

The script seems to be failing on this particular command, originally coming from #17750:

rvm --debug requirements ruby-2.5.0

eventually with this error Requirements installation failed with status: 1..

The last green build has these lines:

==> Upgrading 4 dependents:
libevent 2.1.8 -> 2.1.11_1, sqlite 3.26.0_1 -> 3.31.1, tmux 2.8 -> 3.1b, wget 1.20.1_3 -> 1.20.3_2

Now the failures have these lines:

==> Upgrading 5 dependents:
gnupg 2.2.12 -> 2.2.20, libevent 2.1.8 -> 2.1.11_1, sqlite 3.26.0_1 -> 3.31.1, tmux 2.8 -> 3.1b, wget 1.20.1_3 -> 1.20.3_2

And then later

==> Installing gnupg
==> Pouring gnupg-2.2.20.high_sierra.bottle.tar.gz
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/gpg
Target /usr/local/bin/gpg
already exists. You may want to remove it:
  rm '/usr/local/bin/gpg'

To force the link and overwrite all conflicting files:
  brew link --overwrite gnupg

To list all files that would be deleted:
  brew link --overwrite --dry-run gnupg

Possible conflicting files are:
/usr/local/bin/gpg -> /usr/local/bin/gpg2
/usr/local/bin/gpgconf -> /usr/local/gnupg-2.2/bin/gpgconf
/usr/local/bin/gpgsm -> /usr/local/gnupg-2.2/bin/gpgsm

So looks like somehow an additional dependency about this gnupg package is being pulled in recently, causing the error.

The investigation from this comment on is extremely similar to the issue here.

One thing to note is that the rvm --debug requirements ruby-2.5.0 command used to be under a set +ex before, so it doesn't error out the whole script. After this recent change (#22195) in Mar 2, the command is now under a set -e.

In other words, the command rvm requirements ruby-2.5.0 did pass under a set -e between Mar 2 and May 20.

Fix attempt 1: putting rvm requirements ruby-2.5.0 under a set +e

This will make the build green. But the brew link gnupg error is still there. We just ignored it. Ignoring the error doesn't seem to affect the rest of the build (e.g. rvm install 2.5.0). But, this doesn't exactly feel like the right solution.

Fix attempt 2: just remove the rvm requirements ruby-2.5.0 command

Just removing the rvm requirements ruby-2.5.0 command did not help. Even though we went on to do rvm install 2.5.0 directly, rvm eventually decides to do rvm requirements ruby-2.5.0 anyways, which still leads back to the error with gnupg.

So the problem is still with brew link gnupg.
Another question: what happened between May 20 6:35pm PDT and 8:17pm PDT?

Fix attempt 3: do something special to deal with the error associated with brew upgrade gnupg

Since these Kokoro MacOS workers are images anyways, we can probably just do something special just to deal with the gnupg error. Every run starts fresh anyways. This is borrowed from Alex's attempts in #23048.