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@@ -8,6 +8,8 @@ module ActionController
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           # Development mode callbacks
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           before_dispatch :reload_application
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           after_dispatch :cleanup_application
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+
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+          ActionView::Helpers::AssetTagHelper.cache_asset_timestamps = false
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         end
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         if defined?(ActiveRecord)

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@@ -468,6 +468,22 @@ module ActionView
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         tag("img", options)
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       end
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+      def self.cache_asset_timestamps
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+        @@cache_asset_timestamps
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+      end
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+
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+      # You can enable or disable the asset tag timestamps cache.
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+      # With the cache enabled, the asset tag helper methods will make fewer
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+      # expense file system calls. However this prevents you from modifying
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+      # any asset files while the server is running.
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+      #
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+      #   ActionView::Helpers::AssetTagHelper.cache_asset_timestamps = false
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+      def self.cache_asset_timestamps=(value)
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+        @@cache_asset_timestamps = value
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+      end
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+
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+      @@cache_asset_timestamps = true
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+
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       private
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         # Add the the extension +ext+ if not present. Return full URLs otherwise untouched.
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         # Prefix with <tt>/dir/</tt> if lacking a leading +/+. Account for relative URL
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@@ -526,18 +542,28 @@ module ActionView
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           end
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         end
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+        @@asset_timestamps_cache = {}
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+        @@asset_timestamps_cache_guard = Mutex.new
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+
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         # Use the RAILS_ASSET_ID environment variable or the source's
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         # modification time as its cache-busting asset id.
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         def rails_asset_id(source)
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           if asset_id = ENV["RAILS_ASSET_ID"]
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             asset_id
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           else
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-            path = File.join(ASSETS_DIR, source)
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-
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-            if File.exist?(path)
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-              File.mtime(path).to_i.to_s
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+            if @@cache_asset_timestamps && (asset_id = @@asset_timestamps_cache[source])
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+              asset_id
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             else
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-              ''
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+              path = File.join(ASSETS_DIR, source)
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+              asset_id = File.exist?(path) ? File.mtime(path).to_i.to_s : ''
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+
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+              if @@cache_asset_timestamps
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+                @@asset_timestamps_cache_guard.synchronize do
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+                  @@asset_timestamps_cache[source] = asset_id
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+                end
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+              end
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+
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+              asset_id
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             end
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           end
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         end