Cache asset ids.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@7607 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/CHANGELOG

 *SVN*
 
+* Cache asset ids.  [Jeremy Kemper]
+
 * Optimized named routes respect AbstractRequest.relative_url_root.  #9612 [danielmorrison, Jeremy Kemper]
 
 * Introduce ActionController::Base.rescue_from to declare exception-handling methods. Cleaner style than the case-heavy rescue_action_in_public.  #9449 [norbert]

actionpack/lib/action_view/helpers/asset_tag_helper.rb

@@ -409,9 +409,18 @@ def compute_asset_host(source)
         # Use the RAILS_ASSET_ID environment variable or the source's
         # modification time as its cache-busting asset id.
         def rails_asset_id(source)
-          ENV["RAILS_ASSET_ID"] ||
-            File.mtime(File.join(ASSETS_DIR, source)).to_i.to_s rescue ""
+          if asset_id = ENV["RAILS_ASSET_ID"]
+            asset_id
+          else
+            @@asset_id_cache[source] ||=
+              if File.exist?(path = File.join(ASSETS_DIR, source))
+                File.mtime(path).to_i.to_s
+              else
+                ''
+              end
+          end
         end
+        @@asset_id_cache = {}
 
         # Break out the asset path rewrite so you wish to put the asset id
         # someplace other than the query string.
@@ -451,4 +460,4 @@ def join_asset_file_contents(paths)
         end
     end
   end
-end
\ No newline at end of file
+end