---------------------------------------------------------------------------------------------
                     old process                         new process
send upgrade signal ---> |                                    |
                         |    ----------- fork ----------->   |
                         |                                    | start listen and init finish
                         |    <- send init finish signal --   |
         close listening |                                    |
                         |    send connection state data ->   |
                         |                                    | accept and init finish
                         |    <---- send finish signal ----   |
                    exit |                                    |
---------------------------------------------------------------------------------------------
Use a domain socket to transfer connection state data