Heroku doesn't support differentiated Gemfiles

[pvande]

I'm trying to set up dual-booting for my application, using a similar approach to the next_rails gem. Put simply, my application has a Gemfile/Gemfile.lock pair for the "stable" configuration, and a Gemfile.next/Gemfile.next.lock pair for the configuration being validated.

In my local environment, I can switch between these by setting BUNDLE_GEMFILE=Gemfile.next as necessary. In this way, my "next" environment can explore updates to Ruby, Rails, and other dependencies over the long-term without endangering my production app or holding up the development pipeline.

For all appearances, however, this buildpack doesn't play nicely with such a configuration. Bundler version inference, Ruby version inference, and initial gem installation are done (at least in part) based on the Gemfile.lock file, regardless of the value of BUNDLE_GEMFILE, which causes mismatches during build startup.

[schneems]

I previously looked into this in #351 though that was a long time ago and quite a bit has changed.

This fork of the buildpack supports it https://www.fastruby.io/blog/how-to-run-multiple-versions-of-rails-on-heroku.html

[schneems]

I've been thinking that it will be difficult to hunt down all the locations where it's expected that Gemfile/Gemfile.lock combination will be used at the root of the application.

An alternative way forward could be to detect when BUNDLE_GEMFILE is set to something different and then move those files into the root of the app before heroku/ruby runs.

We could make a micro buildpack that does this and not even need to touch heroku/ruby.

[fbuys]

Hey @schneems I would like to attempt the micro buildpack you suggested, do you have any pointers to help me get started?

I have not created a buildpack before, am I over-ambitious?

[fbuys]

Hey @schneems we opened a PR that could potentially enable setting the Gemfile via an ENV value.
When you have a moment could you take a look and see if this could potentially be merged into the official buildpack?
fastruby#2