Don't write data to disconnected http clients

This is a fix for trac #9410

src/twisted/web/http.py

@@ -1078,6 +1078,13 @@ def write(self, data):
         if self.finished:
             raise RuntimeError('Request.write called on a request after '
                                'Request.finish was called.')
+
+        if self._disconnected:
+            # Don't attempt to write any data to a disconnected client.
+            # The RuntimeError exception will be thrown as usual when 
+            # request.finish is called
+            return
+
         if not self.startedWriting:
             self.startedWriting = 1
             version = self.clientproto

src/twisted/web/test/test_http.py

@@ -2932,6 +2932,19 @@ def test_finishAfterConnectionLost(self):
         self.assertRaises(RuntimeError, req.finish)
 
 
+    def test_writeAfterConnectionLost(self):
+        """
+        Calling L{Request.write} after L{Request.connectionLost} has been
+        called should not throw an exception. L{RuntimeError} will be raised
+        when finish is called on the request.
+        """
+        channel = DummyChannel()
+        req = http.Request(channel, False)
+        req.connectionLost(Failure(ConnectionLost("The end.")))
+        req.write(b'foobar')
+        self.assertRaises(RuntimeError, req.finish)
+
+
     def test_reprUninitialized(self):
         """
         L{Request.__repr__} returns the class name, object address, and