Enable nginx buildpack without app config & code changes.

[wrr]

Hi,

I've been working with a fork of heroku-buildpack-nginx with the goal of having an nginx-based buildpack that can be fully enabled with a heroku buidlpack:add command, without requiring any changes in the application config or code. Such a mode turned out to be possible with the following approach:

No Procfile modification

nginx can be started as a background process from a .profile.d script (https://devcenter.heroku.com/articles/buildpack-api#profile-d-scripts). This removes the need to modify the Procfile to launch the start-nginx script (as with the start-nginx script, some care is needed to properly terminate a dyno when either nginx or the web app terminates).

No need to listen on a Unix domain socket

The .profile.d script can remap the $PORT environment variable to any other TCP port and pass the remapped port to the application. nginx can then listen on the original $PORT and proxy the requests to the remapped local port instead of the Unix domain socket. Heroku runtime guarantees that only $PORT is externally accessible, external traffic to any other port is filtered out by a firewall https://devcenter.heroku.com/articles/dynos#common-runtime-networking.

No need to create /tmp/app-initialized when the backend is ready for traffic

All three supported Heroku stacks include the netcat tool that can be used to wait in a loop for the app to start accepting requests on a local TCP port.

I open this issue for a reference if you ever consider supporting such a mode; please feel free to close it. For reference, here is my .profile.d script that implements this mode (it includes code specific to my buildpack, not intended to be generic).