Re: how is the WAL receiver process stopped and restarted when the network connection is broken and then restored?

Question-1. How the WAL receiver process exits

===============================================

When the network connection is broken, WAL receiver couldn't communicate with the WAL sender.  For a long time (timer：wal_receiver_timeout), the WAL receiver  gets nothing from the WAL sender, the WAL receiver process exits by calling "ereport(ERROR,...)".  

Calling ereport(ERROR,...) causes the current process exit, but calling ereport(LOG,...) doesn't.

    WalReceiverMain(void)

    {

            len = walrcv_receive(NAPTIME_PER_CYCLE, &buf);

            if (len != 0) 

            {

            }

            else 

            {

                    if (wal_receiver_timeout > 0)

                    {

                            if (now >= timeout)

                                ereport(ERROR,

                                        (errmsg("terminating walreceiver due to timeout")));

                    }

            }

    }

Question-2. How  WAL receiver process starts again

=====================================================

At the Standby side, the startup process is responsible for recovery processing. If streaming replication is configured and the startup process finds that the WAL receiver process is not running, it notify the Postmaster to start the WAL receiver process.Note: This is also how the WAL receiver process starts for the first time!

(1) startup process notify Postmaster to start the WAL receiver by sending a SIGUSR1.

    RequestXLogStreaming()

    {

        if (launch)

            SendPostmasterSignal(PMSignalReason reason=PMSIGNAL_START_WALRECEIVER)

            {

                kill(PostmasterPid, SIGUSR1);  

            }

    }

(2) Postmaster gets SIGUSR1 and starts the WAL receiver process.

    sigusr1_handler(SIGNAL_ARGS)

    {

        WalReceiverPID = StartWalReceiver();

    }