write a custom buildpack

[chadwhitacre]

We want to install sass on Heroku as part of #1245. We should write a custom buildpack for this. This seems like a good idea in general because the generic Python buildpack does a lot more than we need it to, adding time and complexity to our deploy process (#985). Let's bring that under control for master as it stands today and then we can add sass to it on #1245.

[chadwhitacre]

I'm scrapping this approach. I was hoping we could use the same build system we use locally (i.e., make build) but it's not that straightforward, because making a virtualenv fixes a directory name that is temporary. Once Heroku builds the app in the temporary location it tars it up and unpackages it elsewhere (once per dyno). There's a (somewhat beta) --relocatable option to virtualenv, and of course we could iterate further in the direction of heroku-buildpack-python, which doesn't use a virtualenv at all. But then @btubbs pointed out the {pre,post}_compile hooks that are available and that turns out to be a much easier way to install sass, which is the actual goal here.

[patcon]

Python-newbie question: Can we ditch the virtualenv jazz if we use vagrant? Does virtualenv not play nice with heroku?

[btubbs]

The Heroku buildpack uses virtualenv to build the environment that you're deployed into. Using it locally makes your environment closer to production. It also lets you install packages as an unprivileged user instead of needing to be root.

[patcon]

Ah ok, I think I saw this way back when first looking into how python works, and thought it was no longer being used on heroku :)
https://github.com/heroku/heroku-buildpack-python/blob/master/bin/compile#L119-L123

But would it be more consistent if we somehow ran the heroku-buildpack-python locally in vagrant? Is that something people do?

[btubbs]

It's not normal for people to do that, because the Heroku buildpack is hardcoded to run your application at /app in the filesystem, which isn't a particularly developer-friendly directory.

[patcon]

@btubbs Worst case scenario, we could create /etc/profile.d/login-dir.sh so that vagrant ssh cd's you right there. Or am I misunderstanding? I mean, there might be plenty of good reasons not to try to run a buildpack via vagrant, but not sure if /app should be it :)