puma: BUNDLE_GEMFILE is incorrectly set to parent Gemfile in worker processes (when not setting BUNDLE_GEMFILE explicitly)

Describe the bug

Before #1893, if you used prune_bundler and started bundle exec puma without explicitly setting BUNDLE_GEMFILE, workers would not have a value set for the environment variable BUNDLE_GEMFILE. This is desirable when you want workers to use the Gemfile located in the application directory, especially if that directory is different from the directory that you started the puma master process in (such as with a phased restart).

After #1893, workers have BUNDLE_GEMFILE unexpectedly set to the path of the Gemfile used by the master process. I think this forces bundler in the workers to use that Gemfile instead of the one in the worker’s working directory as it would if the environment variable was unset.

I think the intent of that PR was that if you start your server with BUNDLE_GEMFILE=Gemfile.rails6 bundle exec puma, that workers would have BUNDLE_GEMFILE set to the same value. But it ignores the fact that Bundler itself sets a value for ENV['BUNDLE_GEMFILE'] even if it wasn’t specified at startup time. Workers now end up inheriting this value, when they didn’t before.

One consequence of this bug is that it is no longer possible to delete from your server the Gemfile that was used when you booted the puma master process. If you do, workers fail with the next phased restart. I imagine there are other bugs related to being able to change gems in the Gemfile for new versions of your application, but I haven’t been able to reproduce those.

Puma config:

port 3000
workers 1
prune_bundler
directory '/usr/src/releases/current'

on_worker_boot do
  pp "BUNDLE_GEMFILE: #{ENV['BUNDLE_GEMFILE']}"
end

To Reproduce

I wrote a script that reproduces the error here: https://github.com/cjlarose/puma-phased-restart-could-not-find-gem-errors/tree/regression-1893 (I reused a repo from an unrelated issue)

Before #1893 , workers boot and print BUNDLE_GEMFILE: . After that PR, workers boot and print BUNDLE_GEMFILE: /usr/src/releases/1582850222334837300/Gemfile, referring to the path of the Gemfile that was used by the puma master process.

Expected behavior

If BUNDLE_GEMFILE was unspecified when you started the master process, it should remain unspecified in the worker processes.

Desktop (please complete the following information):

• OS: Linux
• Puma Version 8c9b3ebc7a86bdc5488704a0225b76409b9f555f