Add percent_free property

Signed-off-by: Clinton Wolfe <clintoncwolfe@gmail.com>

docs/resources/filesystem.md.erb

@@ -26,6 +26,7 @@ A `filesystem` resource block declares tests for disk space in a partition:
     describe filesystem('/') do
       its('size') { should be >= 32000 }
       its('free') { should be >= 5000000 }
+      its('percent_free') { should be >= 20 }
       its('type') { should cmp 'ext4' }
     end
 
@@ -34,6 +35,7 @@ where
 * `filesystem('/')` states that the resource will look at the root (/) partition.
 * `size` is the total partition size and is measured in kilobytes (KB).
 * `free` is the available space on the partition and is measured in kilobytes (KB).
+* `percent_free` is the percentage of available free space, and ranges from 0 to 100.
 
 <br>
 
@@ -65,6 +67,12 @@ The following examples show how to use this InSpec audit resource.
 
 <br>
 
+### Test if the /var partition has sufficient free space
+
+    describe filesystem('/var') do
+      its('percent_free') { should be >= 20 }
+    end
+
 ## Matchers
 
 For a full list of available matchers, please visit our [matchers page](https://www.inspec.io/docs/reference/matchers/).

lib/resources/filesystem.rb

@@ -9,11 +9,13 @@ module Inspec::Resources
         its('size') { should be >= 32000 }
         its('free') { should be >= 3200 }
         its('type') { should cmp 'ext4' }
+        its('percent_free') { should be >= 20 }
       end
       describe filesystem('c:') do
         its('size') { should be >= 90 }
         its('free') { should be >= 3200 }
         its('type') { should cmp 'NTFS' }
+        its('percent_free') { should be >= 20 }
       end
     "
     attr_reader :partition
@@ -54,6 +56,11 @@ module Inspec::Resources
       info[:free]
     end
 
+    def percent_free
+      info = @fsman.info(@partition)
+      100 * info[:free] / info[:size]
+    end
+
     def type
       info = @fsman.info(@partition)
       info[:type]

test/unit/resources/filesystem_test.rb

@@ -4,13 +4,14 @@ require 'inspec/resource'
 describe 'Inspec::Resources::FileSystemResource' do
   # arch linux
   it 'verify filesystem on linux' do
-    resource = MockLoader.new(:ubuntu1404).load_resource('filesystem','/') 
+    resource = MockLoader.new(:ubuntu1404).load_resource('filesystem','/')
     _(resource.size).must_be :>=, 1
     _(resource.name).must_equal '/'
     _(resource.type).must_equal 'ext4'
     _(resource.free).must_be :>=, 1
+    _(resource.percent_free).must_equal 68
   end
-  
+
   # arch windows
   it 'verify filesystem on windows' do
     resource = MockLoader.new(:windows).load_resource('filesystem','c:')
@@ -18,6 +19,7 @@ describe 'Inspec::Resources::FileSystemResource' do
     _(resource.name).must_equal 'c:'
     _(resource.type).must_equal 'NTFS'
     _(resource.free).must_be :>=, 1
+    _(resource.free).must_equal 50000 # Test fixture reports size 100 with 50000 free
   end
 
   # unsuported os