We currently face an issue in all packages requiring bunlder/setup and trying to run the tests for Ruby 2.7 and 3.0. The problem is that the first tests will create Gemfile.lock (or gemfile/gemfile-*.lock) using Ruby 2.7 and the next run for Ruby 3 will report e.g.:

Failure/Error: require 'bundler/setup' # Set up gems listed in the Gemfile.

Bundler::GemNotFound:
  Could not find racc-1.4.16 in any of the sources

or

/usr/share/rubygems-integration/all/gems/bundler-2.2.27/lib/bundler/definition.rb:496:in `materialize':
  Could not find rexml-3.2.3.1 in any of the sources (Bundler::GemNotFound)

Both bugs #996207 and #996302 are incarnations of this issue. The fix is as easy as making sure that the .lock files are removed before each run. This can be done in e.g. debian/ruby-tests.rake as very first task:

File.delete("Gemfile.lock") if File.exist?("Gemfile.lock")

In another case the .lock file is created by the tests in gemfiles/. While the first examples could actually be solved by gem2deb removing Gemfile.lock on its own, I’m not quite sure how to handle the last case using packaging tools.

The interesting part is that we will unlikely be confronted with this issue anytime soon again. It seems very specific to the Ruby 3.0 transition.

Update

After talking to Antonio he added some code to gem2deb-test-runner to moving Gemfile.lock files out of the way. The tool already did this in an autopkgtest environment. In the upcoming 1.7 release it will do it in general and this will fix some more FTBFSes, e.g. #998497 and #996141 - originally reported against ruby-voight-kampff and ruby-bootsnap.