Large Output Shell Stall

Previously if a large amount of output was received during a shell call, the
buildpack would stall because the process' output buffer would fill to
capacity before the process would complete.  This change assigns the output to
a variable during execution, ensuring that the process' output buffers do not
fill during execution.

[resolves #507]

lib/java_buildpack/util/shell.rb

@@ -28,10 +28,13 @@ module Shell
       # @return [Void]
       def shell(*args)
         Open3.popen3(*args) do |_stdin, stdout, stderr, wait_thr|
+          out = stdout.gets nil
+          err = stderr.gets nil
+
           unless wait_thr.value.success?
             puts "\nCommand '#{args.join ' '}' has failed"
-            puts "STDOUT: #{stdout.gets nil}"
-            puts "STDERR: #{stderr.gets nil}"
+            puts "STDOUT: #{out}"
+            puts "STDERR: #{err}"
 
             raise
           end

spec/java_buildpack/util/shell_spec.rb

@@ -16,6 +16,7 @@
 require 'spec_helper'
 require 'console_helper'
 require 'java_buildpack/util/shell'
+require 'timeout'
 
 describe JavaBuildpack::Util::Shell do
   include described_class
@@ -29,4 +30,8 @@
     expect { shell 'false' }.to raise_error
   end
 
+  it 'handles a large amount of output' do
+    Timeout.timeout(2) { shell "cat /dev/urandom | env LC_CTYPE=C tr -dc 'a-zA-Z0-9' | fold -w 1000000 | head -n 1" }
+  end
+
 end