title: Rails: Asset precompilation with 'digest' activated fails in jRuby 1.7.6
body: <p>Using Ruby on Rails 3.1.14 in conjunction with jRuby 1.7.6, I cannot execute</p>
  
  <pre><code>RAILS_ENV=production RAILS_ENV=production /path/to/jruby -S rake --trace assets:precompile:all
  </code></pre>
  
  <p>when the following option is set:</p>
  
  <pre><code># config/environments/production.rb
  config.assets.digest = false
  </code></pre>
  
  <p>It results in the following stack trace:</p>
  
  <pre><code>** Invoke assets:precompile:all (first_time)
  ** Execute assets:precompile:all
  ** Invoke assets:precompile:primary (first_time)
  ** Invoke assets:environment (first_time)
  ** Execute assets:environment
  ** Invoke environment (first_time)
  ** Execute environment
  ** Invoke tmp:cache:clear (first_time)
  ** Execute tmp:cache:clear
  ** Execute assets:precompile:primary
  rake aborted!
  No such file or directory - file:/Users/myhome/.m2/repository/org/jruby/jruby-complete/1.7.6/jruby-complete-1.7.6.jar!/META-INF/jruby.home/bin/jruby
  org/jruby/RubyKernel.java:1790:in `_exec_internal'
  file:/Users/myhome/.m2/repository/org/jruby/jruby-complete/1.7.6/jruby-complete-1.7.6.jar!/jruby/kernel19/kernel.rb:25:in `exec'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/actionpack-3.2.14/lib/sprockets/assets.rake:12:in `ruby_rake_task'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/actionpack-3.2.14/lib/sprockets/assets.rake:66:in `(root)'
  org/jruby/RubyProc.java:271:in `call'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/task.rb:236:in `execute'
  org/jruby/RubyArray.java:1613:in `each'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/task.rb:231:in `execute'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/task.rb:175:in `invoke_with_call_chain'
  jar:file:/Users/myhome/.m2/repository/org/jruby/jruby-complete/1.7.6/jruby-complete-1.7.6.jar!/META-INF/jruby.home/lib/ruby/1.9/monitor.rb:211:in `mon_synchronize'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/task.rb:168:in `invoke_with_call_chain'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/task.rb:161:in `invoke'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:149:in `invoke_task'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:106:in `top_level'
  org/jruby/RubyArray.java:1613:in `each'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:106:in `top_level'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:115:in `run_with_threads'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:100:in `top_level'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:78:in `run'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:165:in `standard_exception_handling'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/lib/rake/application.rb:75:in `run'
  /Users/myhome/Desktop/testapp/vendor/bundle/gems/rake-10.1.0/bin/rake:33:in `(root)'
  org/jruby/RubyKernel.java:1101:in `load'
  /Users/myhome/Desktop/testapp/vendor/bundle/bin/rake:23:in `(root)'
  Tasks: TOP =&gt; assets:precompile:all
  </code></pre>
  
  <p>The problem seems to happen at the following line:</p>
  
  <pre><code># actionpack-3.2.14/lib/sprockets/assets.rake:12
  fork ? ruby(*args) : Kernel.exec(FileUtils::RUBY, *args)
  </code></pre>
  
  <p>It seems quite clear to me that this <code>exec</code> statement can't work with jRuby. This line is only executed if <code>digest</code> is set to <code>true</code>.</p>
  
  <p>Is this a rails bug? I though I'd ask here before submitting a bug report.</p>