Generate Gemfile for plugin runner dynamically

Add a template and method for generating the Gemfile needed for
stand alone staging script

Test plan:
- New unit test passes
- Existing unit tests pass

Change-Id: Iab3a188e6b79cbf6843fb536d68b62b3e9beb9c4

stager/assets/plugin_runner_gemfile.erb

@@ -0,0 +1,12 @@
+source :rubygems
+
+gem 'vcap_logging', '>= 0.1.1'
+gem 'vcap_cloud_controller_ipc'
+
+<% for plugin_gem in plugin_gems %>
+<% if plugin_gem["version"] %>
+gem '<%= plugin_gem["name"] %>', '= <%= plugin_gem["version"] %>'
+<% else %>
+gem '<%= plugin_gem["name"] %>'
+<% end %>
+<% end %>

stager/lib/vcap/stager/plugin_runner.rb

@@ -1,3 +1,4 @@
+require 'erb'
 require 'fileutils'
 
 require 'vcap/cloud_controller/ipc'
@@ -16,6 +17,8 @@ module Stager
 # Responsible for orchestrating the execution of all staging plugins selected
 # by the user.
 class VCAP::Stager::PluginRunner
+  GEMFILE_TEMPLATE_PATH  = File.join(VCAP::Stager::ASSET_DIR, 'plugin_runner_gemfile.erb')
+
   class << self
     attr_reader :registered_plugins
 
@@ -28,6 +31,24 @@ def register_plugins(*plugins)
     def reset_registered_plugins
       @registered_plugins = []
     end
+
+    # Generates a Gemfile that should be used in conjuction with running the
+    # standalone script using 'bundle exec'. We assume that every plugin has
+    # already been installed locally on the system (therefore its individual
+    # dependencies have been satisfied) and rely on Bundler to resolve possible
+    # conflicts in plugin dependencies.
+    #
+    # @param  dest_path    String  Where to place the generated Gemfile
+    # @param  plugin_gems  Array   Plugins to be loaded
+    #                              [{'name' => GEM_NAME, 'version' => GEM_VERSION}]
+    def generate_standalone_gemfile(dest_path, plugins)
+      plugin_gems = plugins.map {|p| p['gem'] }
+      template = File.read(GEMFILE_TEMPLATE_PATH)
+      renderer = ERB.new(template)
+      gemfile_contents = renderer.result(binding())
+      File.open(dest_path, 'w+') {|f| f.write(gemfile_contents) }
+      dest_path
+    end
   end
 
   # @param source_dir      String  Directory containing application source

stager/lib/vcap/stager/task.rb

@@ -279,5 +279,4 @@ def run_logged(command, expected_exitstatus=0, timeout=nil)
 
     ret
   end
-
 end

stager/spec/unit/plugin_runner_spec.rb

@@ -4,6 +4,20 @@
 require 'tmpdir'
 
 describe VCAP::Stager::PluginRunner do
+  describe '.generate_standalone_gemfile' do
+    it 'should write out a Gemfile containing all plugins to be run' do
+      tmpdir = Dir.mktmpdir
+      gemfile_path = File.join(tmpdir, 'Gemfile')
+      plugins = [{'gem' => {'name' => 'test1'}},
+                 {'gem' => {'name' => 'test2', 'version' => '0.0.1'}}]
+      VCAP::Stager::PluginRunner.generate_standalone_gemfile(gemfile_path, plugins)
+      File.exist?(gemfile_path).should be_true
+      gemfile_contents = File.read(gemfile_path)
+      gemfile_contents.match(/^gem 'test1'$/).should be_true
+      gemfile_contents.match(/^gem 'test2', '= 0.0.1'$/).should be_true
+    end
+  end
+
   describe '#run_plugins' do
     before :each do
       @src_dir = Dir.mktmpdir