Fix installed? check for gem resource

The gem resource used to determine if a gem is installed based on the exit
status of the `gem` command, however that command will return zero
if the package was found or not. This patch checks to ensure that the
`gem list` command actually includes the gem name or is empty to
determine if the gem is in fact installed.

If the gem command returns something other than a `0` exit code, then
it'll skip the resource.

Signed-off-by: Keith Walters <keith.walters@cattywamp.us>

lib/resources/gem.rb

@@ -39,16 +39,16 @@ module Inspec::Resources
       return @info if defined?(@info)
 
       cmd = inspec.command("#{@gem_binary} list --local -a -q \^#{@package_name}\$")
-      @info = {
-        installed: cmd.exit_status.zero?,
-        type: 'gem',
-      }
-      return @info unless @info[:installed]
+      return {} unless cmd.exit_status.zero?
 
       # extract package name and version
       # parses data like winrm (1.3.4, 1.3.3)
       params = /^\s*([^\(]*?)\s*\((.*?)\)\s*$/.match(cmd.stdout.chomp)
-      return {} if params.nil?
+      @info = {
+        installed: !params.nil?,
+        type: 'gem',
+      }
+      return @info unless @info[:installed]
 
       versions = params[2].split(',')
       @info[:name] = params[1]

test/helper.rb

@@ -174,6 +174,7 @@ class MockLoader
       'rpm -qia curl' => cmd.call('rpm-qia-curl'),
       'pacman -Qi curl' => cmd.call('pacman-qi-curl'),
       'brew info --json=v1 curl' => cmd.call('brew-info--json-v1-curl'),
+      'gem list --local -a -q ^not-installed$' => cmd.call('gem-list-local-a-q-not-installed'),
       'gem list --local -a -q ^rubocop$' => cmd.call('gem-list-local-a-q-rubocop'),
       '/opt/ruby-2.3.1/embedded/bin/gem list --local -a -q ^pry$' => cmd.call('gem-list-local-a-q-pry'),
       '/opt/chef/embedded/bin/gem list --local -a -q ^chef-sugar$' => cmd.call('gem-list-local-a-q-chef-sugar'),

test/unit/resources/gem_test.rb

@@ -7,6 +7,11 @@ require 'helper'
 require 'inspec/resource'
 
 describe 'Inspec::Resources::Gem' do
+  it 'verify gem is not installed' do
+    resource = load_resource('gem', 'not-installed')
+    _(resource.installed?).must_equal false
+  end
+
   it 'verify gem package detail parsing' do
     resource = load_resource('gem', 'rubocop')
     pkg = {