Avoid unnecessary empty data frames at end of stream

Currently, if a stream terminates with an empty data frame, we will send that
empty data frame even if it is succeeded by trailers that are marked end of
stream. That empty data frame is unnecessary. We should only send it if the
payload length is non-zero and we are not sending trailers.

Network/HTTP2/H2/Sender.hs

@@ -282,17 +282,23 @@ frameSender
             _
             reqflush = do
                 let buf = confWriteBuffer `plusPtr` off
-                    off' = off + frameHeaderLength + datPayloadLen
                 (mtrailers, flag) <- do
                     Trailers trailers <- tlrmkr Nothing
                     if null trailers
                         then return (Nothing, setEndStream defaultFlags)
                         else return (Just trailers, defaultFlags)
-                fillFrameHeader FrameData datPayloadLen streamNumber flag buf
+                -- Avoid sending an empty data frame before trailers at the end
+                -- of a stream
+                off' <-
+                  if datPayloadLen /= 0 || isNothing mtrailers then do
+                    decreaseWindowSize ctx strm datPayloadLen
+                    fillFrameHeader FrameData datPayloadLen streamNumber flag buf
+                    return $ off + frameHeaderLength + datPayloadLen
+                  else
+                    return off
                 off'' <- handleTrailers mtrailers off'
                 _ <- sync Nothing
                 halfClosedLocal ctx strm Finished
-                decreaseWindowSize ctx strm datPayloadLen
                 if reqflush
                     then do
                         flushN off''