/*

 * This must be called ONCE during postmaster or standalone-backend startup

 */

void

StartupXLOG(void)

{

    …

    /*

     * Read control file and check XLOG status looks valid.

     * Note: in most control paths, *ControlFile is already valid and we need

     * not do ReadControlFile() here, but might as well do it to be sure.

     */

    ReadControlFile();                                        

    if (ControlFile->state < DB_SHUTDOWNED ||

        ControlFile->state > DB_IN_PRODUCTION ||

        !XRecOffIsValid(ControlFile->checkPoint.xrecoff))

        ereport(FATAL,(errmsg("control file contains invalid data")));                            

    if (ControlFile->state == DB_SHUTDOWNED)

        ereport(LOG, errmsg("database system was shut down at %s", str_time(ControlFile->time))));

    else if (ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)

        ereport(LOG,(errmsg("database system was shut down in recovery at %s", 
                        str_time(ControlFile->time))));
 

    else if (ControlFile->state == DB_SHUTDOWNING)

        ereport(LOG,(errmsg("database system shutdown was interrupted; last known up at %s",
                        str_time(ControlFile->time))));                    

    else if (ControlFile->state == DB_IN_CRASH_RECOVERY)

        ereport(LOG,

           (errmsg("database system was interrupted while in recovery at %s",

                   str_time(ControlFile->time)),

            errhint("This probably means that some data is corrupted and"

                     you will have to use the last backup for recovery.)));                        

    else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)

        ereport(LOG, (errmsg("database system was interrupted while in recovery at log time %s",

                str_time(ControlFile->checkPointCopy.time)),

                errhint("If this has occurred more than once some data might be corrupted

                    and you might need to choose an earlier recovery target.")));                          

    else if (ControlFile->state == DB_IN_PRODUCTION)

        ereport(LOG,(errmsg("database system was interrupted; last known up at %s",

                      str_time(ControlFile->time))));                        

    /* This is just to allow attaching to startup process with a debugger */

    #ifdef XLOG_REPLAY_DELAY

    if (ControlFile->state != DB_SHUTDOWNED)

        pg_usleep(60000000L);

    #endif                                        

    …

    /*

     * Check whether we need to force recovery from WAL.  If it appears to

     * have been a clean shutdown and we did not have a recovery.conf file,

     * then assume no recovery needed.

     */

    if (XLByteLT(checkPoint.redo, RecPtr))

    {

        …

    }

    else if (ControlFile->state != DB_SHUTDOWNED)

        InRecovery = true;

    else if (InArchiveRecovery)

    {

        /* force recovery due to presence of recovery.conf */

        InRecovery = true;

    }                                        

    /* REDO */

    if (InRecovery)

    {

        …

        /*

         * Update pg_control to show that we are recovering and to show the

         * selected checkpoint as the place we are starting from. We also mark

         * pg_control with any minimum recovery stop point obtained from a

         * backup history file.

         */

        if (InArchiveRecovery)

            ControlFile->state = DB_IN_ARCHIVE_RECOVERY;

        else

        {

            ereport(LOG,

                    (errmsg("database system was not properly shut down; "

                            automatic recovery in progress)));

            ControlFile->state = DB_IN_CRASH_RECOVERY;

        }                                    

        …

    }                                        

    …                                        

    /*

     * Okay, we're officially UP.

     */

    InRecovery = false;                                        

    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

    ControlFile->state = DB_IN_PRODUCTION;

    ControlFile->time = (pg_time_t) time(NULL);

    UpdateControlFile();

    LWLockRelease(ControlFileLock);                                        

    …                                        

}