Invalidate the cache when the stack changes

[edmorley]

Previously the buildpack did not invalidate the cache when the stack changed, which causes the errors seen in #19. This will be particularly an issue when Heroku-24 is GAed and Heroku-20 deprecated, since there will be a number of apps changing stack for the first time after switching to this buildpack.

Now, the stack version is stored in the cache and if it differs from the current stack version (or the version file is missing since the cache is from an older version of this buildpack), the cache will be purged.

This implementation is an improved version of what's implemented for the generic APT buildpack (I'll be backporting those improvements over to that buildpack shortly):
https://github.com/heroku/heroku-buildpack-apt/blob/4eb4b35d35d0178e5cef73d6998a26ba2c9bbf17/bin/compile#L42-L86

Longer term we should probably re-evaluate whether we need the cache at all, since from my testing locally it offered very little benefit, and if anything causes a number of issues (particularly when several APT-using buildpacks are set on an app at the same time, given the shared Debian archives directory and install strategy).

Fixes #19.
GUS-W-16045454.

[edmorley]

I've tested all four scenarios:

Clean cache build using this branch

$ mkdir chrome-test && cd $_
$ git init && touch foo && git add -A && git commit -m '.'
$ h create --buildpack 'https://github.com/heroku/heroku-buildpack-chrome-for-testing#edmorley/cache-invalidation-stack'
$ git push heroku main
...
remote: -----> Installing Chrome dependencies
remote:        Updating APT package index
...

Warm cache build showing cache reuse

$ mkdir chrome-test && cd $_
$ git init && touch foo && git add -A && git commit -m '.'
$ h create --buildpack 'https://github.com/heroku/heroku-buildpack-chrome-for-testing#edmorley/cache-invalidation-stack'
$ git push heroku main
$ echo >> foo && git commit -am '.'
$ git push heroku main
...
remote: -----> Installing Chrome dependencies
remote:        Reusing APT cache
remote:        Updating APT package index
...

Warm cache build showing cache invalidation on stack change

$ mkdir chrome-test && cd $_
$ git init && touch foo && git add -A && git commit -m '.'
$ h create --stack heroku-22 --buildpack 'https://github.com/heroku/heroku-buildpack-chrome-for-testing#edmorley/cache-invalidation-stack'
$ git push heroku main
$ h stack:set heroku-20
$ echo >> foo && git commit -am '.'
$ git push heroku main
...
remote: -----> Installing Chrome dependencies
remote:        Clearing APT cache since stack changed from heroku-22 to heroku-20
remote:        Updating APT package index
...

Migration of existing cache written by older version of this buildpack

$ mkdir chrome-test && cd $_
$ git init && touch foo && git add -A && git commit -m '.'
$ h create --buildpack https://github.com/heroku/heroku-buildpack-chrome-for-testing
$ git push heroku main
$ h buildpacks:set 'https://github.com/heroku/heroku-buildpack-chrome-for-testing#edmorley/cache-invalidation-stack'
$ echo >> foo && git commit -am '.'
$ git push heroku main
...
remote: -----> Installing Chrome dependencies
remote:        Clearing APT cache since it's missing the stack version metadata file.
remote:        Updating APT package index
...

[mars]

Wonderful, this will avoid so many issues as customers migrate to newer stacks!

[edmorley]

Released in:
https://github.com/heroku/heroku-buildpack-chrome-for-testing/releases/tag/1.0.7

$ heroku buildpacks:publish heroku-community/chrome-for-testing 1.0.7
Publishing buildpack heroku-community/chrome-for-testing... done
Publishing buildpack was successful