1 files changed, 21 insertions, 0 deletions
diff --git a/actionpack/lib/action_controller/metal/live.rb b/actionpack/lib/action_controller/metal/live.rb
index aea00849d3..efeeefda9d 100644
--- a/actionpack/lib/action_controller/metal/live.rb
+++ b/actionpack/lib/action_controller/metal/live.rb
@@ -1,4 +1,5 @@
 require 'action_dispatch/http/response'
+require 'delegate'
 
 module ActionController
   module Live
@@ -30,6 +31,26 @@ module ActionController
         end
       end
 
+      class Header < DelegateClass(Hash)
+        def initialize(response, header)
+          @response = response
+          super(header)
+        end
+
+        def []=(k,v)
+          if @response.committed?
+            raise ActionDispatch::IllegalStateError, 'header already sent'
+          end
+
+          super
+        end
+      end
+
+      def initialize(status = 200, header = {}, body = [])
+        header = Header.new self, header
+        super(status, header, body)
+      end
+
       private
 
       def build_buffer(response, body)