Require at least a full SDU from the kernel.

Use RecvLowWater to signal that we want to read at least the length of
the SDU.

Author: karknu

network-mux/src/Network/Mux/Bearer/Socket.hs

@@ -123,8 +123,14 @@ socketAsBearer sduSize batchSize readBuffer_m sduTimeout tracer sd =
                    if BL.null availableData
                       then do
                         -- Not data in buffer; read more from socket
+                        when (not waitingOnNxtHeader) $
+                          -- Don't let the kernel wake us up until there is
+                          -- at least l bytes of data.
+                          Socket.setSocketOption sd Socket.RecvLowWater $ fromIntegral l
                         newBuf <- recvFromSocket $ fromIntegral rbSize
                         atomically $ modifyTVar rbVar (`BL.append` newBuf)
+                        when (not waitingOnNxtHeader) $
+                          Socket.setSocketOption sd Socket.RecvLowWater 1
                         recvAtMost waitingOnNxtHeader l
                       else do
                         traceWith tracer $ Mx.TraceRecvEnd $ fromIntegral $ BL.length availableData