Could not detect rake tasks

I started getting this strange error all of a sudden yesterday. Heroku confirmed making an update to the Ruby buildpack...

It has to do with the Rakefile. Does your Rakefile require any files? Does it require your app files? If so, then the app should not raise exceptions when it is loaded without any config vars set.

It's counter-intuitive because the app never runs without the config vars set.

In my case, the Sinatra app was looking for database urls in the init file:

uri = URI.parse( ENV[ "REDISTOGO_URL" ])

This will raise an exception if there are no env vars set.

You may have the same issue with other database URL's, such as Mongo or Postgres.

So, protect against missing env vars:

if  ENV[ "REDISTOGO_URL" ]
  uri = URI.parse( ENV[ "REDISTOGO_URL" ])
  ...

You can check if it will work before pushing to Heroku by running bundle exec rake -P

Also, make sure all your tests pass after updating your init. Remove any cached init state by restarting Spork or similar.

Reference: Show Rakefile errors in Ruby deploys

Effective December 05, 2013, Heroku added debug output to deploys using the Ruby build pack.

The error is triggered:

• if the assets:precompile task does not exist or
• if there is an error in the Rakefile.
  • Two common causes of errors in the Rakefile are referencing a dependency not available in production (such as rspec) or
  • expecting an environment variable to be present.

It's counter-intuitive because the app never runs without config vars, but Heroku's build process executes without config vars.

As per the error message, ensure you can run bundle exec rake -P RAILS_ENV=production with no environment variables present before pushing to Heroku (e.g. comment out your environment variables while running the aforementioned command).

Also, rest assured that rake's -P switch is harmless, so you can run it as much as you want until you fix this issue. This switch is used to display a list of all tasks and their immediate prerequisites. See Rake Command Line Usage if you want to double-check. The output may have over 200 lines, and look something like this:

rake about
    environment
rake assets:clean
    environment
rake assets:clobber
    environment
rake assets:environment
rake assets:precompile
    environment
rake db:_dump
...
rake tmp:pids:clear
rake tmp:sessions:clear
rake tmp:sockets:clear

you can also try enabling user-env-compile

heroku labs:enable user-env-compile