I had a php app on heroku with the default buildpack (apache), and then decided to change it to the php-fpm + nginx buildpack (https://github.com/iphoting/heroku-buildpack-php-tyler/).
I issued the change command:
heroku config:set BUILDPACK_URL=https://github.com/iphoting/heroku-buildpack-php-tyler.git
And pushed with:
git push heroku master
The slug then compiles, but all the binaries fail to execute with "command not found", because PATH config variable which should be updated with bin/release isn't updated.
In comparison, when creating an app from scratch with this buildpack with
heroku create -b https://github.com/iphoting/heroku-buildpack-php-tyler.git
Everything runs as expected (and PATH is present and updated).
My assumption is that bin/release was not run after changing the config variable BUILDPACK_URL, and therefore the PATH variable isn't set.
In order to make the app work, I had to manually add the PATH config variable. Did anybody else ever encounter this? Is this the expected behavior? By my understanding, bin/release should always run after a slug compile?
EDIT: corrected the git url to the correct ".git" one
You are correct in your assumption, the addons and config_vars properties of bin/release are only taken from a buildpack on an app's first deploy. See https://devcenter.heroku.com/articles/buildpack-api#binrelease for more details.
Heroku is moving over to a new system for a buildpack to add config vars that will work beyond an app's first deploy: https://devcenter.heroku.com/articles/labs-dot-profile-d
If a buildpack copies a .profile.d/buildpack_name.sh into the app, that file will be sourced during application boot. This can be used to set up things like the PATH.