Make it easier to debug the asset pipeline

[wuputah]

Debugging pipeline issues can be hard and devious. If the --dry-run fails, it looks like the pipeline support is not working at all, since there's no output. Here's what's currently required to track down issues:

$ heroku run bash
$ unset DATABASE_URL REDISTOGO_URL REDIS_URL MONGOLAB_URI SENDGRID_USERNAME SENDGRID_PASSWORD
$ time env RAILS_ENV=production DATABASE_URL=postgres://user:pass@127.0.0.1/dbname rake assets:precompile --trace

That said I am not sure what the best solution is, but here's some ideas.

• What if we had a branch of the buildpack with --trace enabled on the --dry-run and the task itself?
• What if we built this into the build pack and then it could be enabled... somehow? It can't be a config var, so... File.exists?(".debug-asset-pipeline")?
• Is there a way we can detect the presence of the pipeline without booting their app, so at least we can say that the --dry-run failed and that's why the pipeline stuff isn't working? Maybe just blindly do this if sprockets is present?

-----> assets:precompile rake task not detected; asset pipeline support disabled

[hone]

I've never been a big fan of the way the rake task detection works. Is there a better way than using --dry-run?

[hone]

This is related to #9.

[wuputah]

A better way? No.
A different way? Yes.
https://gist.github.com/2922172

[wuputah]

Proposal:

• Using something akin to the gist provided, provide better error messaging about why the pipeline failed.
• Run pipeline in a limited environment and with --trace if DEBUG_PIPELINE=1

While a completely separate issue, follow Python's lead and allow users to turn off the pipeline entirely via DISABLE_PIPELINE=1

[hone]

@wuputah how does python do this?

[wuputah]

:s/ @wuputah / @kennethreitz /

[kennethreitz]

I allow users to disable it with DISABLE_COLLECTSTATIC=1, but ideally someone should almost never need to do that.

[wuputah]

@kennethreitz what happens if collectstatic fails (and, of course, is not disabled)?

[kennethreitz]

I do a dry run first. If it fails, nothing happens, the user doesn't see anything about.

If the dry run passes, I run it, if it fails, i tell them the output, and point to documentation on how to debug w/ heroku run.

[wuputah]

In the latter case, does the build still succeed?

[kennethreitz]

Yes. I had it fail at first, but too many people have configuration issues for that to be practical.

People have crazy configurations.

[kennethreitz]

In the long term future, I'd like to remove this feature completely and replace it with "add this oneliner to a build hook (e.g. bin/post_compile)" instead.

But, some things have to happen for that to be a reality.

[wuputah]

Okay, to come back to what I was talking about earlier, I was principally referring to Kenneth's use of DISABLE_COLLECTSTATIC.

[hone]

@wuputah i don't think we should be pushing build stuff through config vars.

[hone]

Should be solved by #34