#send_file leans on Rack::Sendfile to X-Accel-Redirect the file's pat…

…h, so opening the file to set the response body is wasteful. Set a FileBody wrapper instead that responds to to_path and streams the file if needed.

actionpack/lib/action_controller/metal/data_streaming.rb

@@ -74,7 +74,27 @@ def send_file(path, options = {}) #:doc:
 
         self.status = options[:status] || 200
         self.content_type = options[:content_type] if options.key?(:content_type)
-        self.response_body = File.open(path, "rb")
+        self.response_body = FileBody.new(path)
+      end
+
+      # Avoid having to pass an open file handle as the response body.
+      # Rack::Sendfile will usually intercepts the response and just uses
+      # the path directly, so no reason to open the file.
+      class FileBody #:nodoc:
+        attr_reader :to_path
+
+        def initialize(path)
+          @to_path = path
+        end
+
+        # Stream the file's contents if Rack::Sendfile isn't present.
+        def each
+          File.open(to_path, 'rb') do |file|
+            while chunk = file.read(16384)
+              yield chunk
+            end
+          end
+        end
       end
 
       # Sends the given binary data to the browser. This method is similar to