Post-build Clean Buildpack

A simple buildpack to run after all other buildpacks have completed, which removes a set of files defined in .slug-post-clean, so that they are not included in the finished slug.

Rationale

While this may seem to duplicate functionality provided by Heroku's .slugignore, there is a key difference: .slugignore'd files are removed after the repo is cloned, but before any buildpack is run. They can therefore not be involved in the build process itself.

However, it is not uncommon for there to exist files in the repo that are necessary for the build, but are not required at runtime. There may also be installable build dependencies that are not runtime dependencies.

In our case, a complex front-end build involves significant CSS, JS and image assets, along with a large installation of node modules, all of which are used only for building the production assets, but then remain part of the slug.

Usage

You will need to use the heroku-buildpack-multi buildpack, and add your existing buildpack(s) to the .buildpacks file in your application repository. The post-build-clean buildpack must be last in the buildpack order:

# .buildpacks
https://github.com/heroku/heroku-buildpack-nodejs
https://github.com/heroku/heroku-buildpack-ruby
https://github.com/Lostmyname/heroku-buildpack-post-build-clean

The .slug-post-clean file supports single-file and single-directory declarations only, e.g.:

some_huge_file.psd
some/nested/directory
why_does_this_app_even_contain_a.tiff

I might expand it to support file globs, but for the moment it's not necessary and the testing implications give me the willies.