Regenerator cache clearing tests

Two tests for the regenerator cache clearing changes:

1. Intrusively test that the regnerator.clear_cache actually clears the cache ( in test/test_regenerator.rb )
2. Test that incremental building regenerates files that have changed that previously were unchanged ( in test/test_site.rb )

test/test_regenerator.rb

@@ -75,6 +75,15 @@ class TestRegenerator < JekyllUnitTest
       assert @regenerator.cache[@path]
     end
 
+    should "clear the cache on clear_cache" do
+      # @path will be in the cache because the
+      # site will have processed it
+      assert @regenerator.cache[@path]
+
+      @regenerator.clear_cache
+      assert_equal  @regenerator.cache, {}
+    end
+
     should "write to the metadata file" do
       @regenerator.clear
       @regenerator.add(@path)

test/test_site.rb

@@ -459,5 +459,43 @@ class TestSite < JekyllUnitTest
       end
     end
 
+    context "incremental build" do
+      setup do
+        @site = Site.new(site_configuration({
+          'full_rebuild' => false
+        }))
+        @site.read
+      end
+
+      should "build incrementally" do
+        contacts_html = @site.pages.find { |p| p.name == "contacts.html" }
+        @site.process
+
+        source = @site.in_source_dir(contacts_html.path)
+        dest = File.expand_path(contacts_html.destination(@site.dest))
+        mtime1 = File.stat(dest).mtime.to_i # first run must generate dest file
+
+        # need to sleep because filesystem timestamps have best resolution in seconds
+        sleep 1
+        @site.process
+        mtime2 = File.stat(dest).mtime.to_i
+        assert_equal mtime1, mtime2 # no modifications, so remain the same
+
+        # simulate file modification by user
+        FileUtils.touch source
+
+        sleep 1
+        @site.process
+        mtime3 = File.stat(dest).mtime.to_i
+        refute_equal mtime2, mtime3 # must be regenerated 
+
+        sleep 1
+        @site.process
+        mtime4 = File.stat(dest).mtime.to_i
+        assert_equal mtime3, mtime4 # no modifications, so remain the same
+      end
+
+    end
+
   end
 end