How to debug custom Heroku buildpack: release script not being driven - heroku

Question

I am developing an Eclipse Virgo buildpack but when I try it with Heroku and push a trivial app, detection fails:
-----> Fetching custom git buildpack... done
! Heroku push rejected, no Cedar-supported app detected
The buildpack's detect script works fine locally in the root directory of the app:
$[...]virgo-buildpack/bin/detect .
Virgo Web
Any tips on how to debug this? I tried having the detect script write to stdout and stderr, but the output doesn't appear under "heroku logs".
I am running Mac OS X 10.8.2, Ruby 1.9.3p374, and gems 1.8.23.
Update: following the first two answers, I used a bash detect script to install the missing gem before driving a detect.rb script. I also changed the buildpack scripts to write to standard output.
The heroku push gets a lot further, but still fails. I can see the compile script exiting, but the release bash script has an echo command right at the start and this does not appear in the output.
So it seems that the release script is not being called.
The output is as follows (# indicates comments to avoid clutter):
-----> Fetching custom git buildpack... done
# detect script enter and exit
Virgo Web app detected
# compile script enter and exit
-----> Discovering process types
Procfile declares types
-> (none)
# detect script enter and exit (again)
Virgo Web -> web
-----> Compiled slug size: 60.4MB
-----> Launching... ! Heroku push rejected, failure releasing code
heroku logs --tail simply shows:
2013-03-06T10:53:48+00:00 heroku[slugc]: Slug compilation started
2013-03-06T10:54:27+00:00 heroku[slugc]: Slug compilation failed: failure releasing code

Answer 1

For a custom buildpack, the detect script doesn't really need to be that complicated because users will be specifying it manually with BUILDPACK_URL anyway. A sophisticated detect script really only makes sense for the default Heroku buildpacks because they need to detect if the buildpack applies for any given app.
If you want to completely punt on detection, you can get away with something like this:
#!/usr/bin/env bash
# bin/detect <build-dir>
echo "Vergo"
exit 0
Once you get that working (and work out any issues with compile and release), you can make it a little more intelligent. For example, you could detect if a certain file pattern exists:
#!/usr/bin/env bash
# bin/detect <build-dir>
if [ -f $1/index.jsp ]; then
echo "Vergo" && exit 0
else
echo "no" && exit 1
fi
Also, speaking of simplicity, it looks like you are basing your buildpack on the Ruby buildpack, which is one of the most complicated ones out there. You may want to take a look at the list of third-party buildpacks to see how most are done in simple bash.

Answer 2

Currnetly there are a few tools to make this much easier.
This build pack https://github.com/kr/heroku-buildpack-inline (A build pack to help construct buildpack)
heroku run bash Is your best friend.
Push your build pack on to the platform using the inline buildpack so that you can get good logging, and use heroku run bash so that you can actaully tinker with your buildpack in the environment that you will be using it in.