Function shiftReplicationId

app/redis-6.2.6/src/replication.c:1403–1415

Use the current replication ID / offset as secondary replication * ID, and change the current one in order to start a new history. * This should be used when an instance is switched from slave to master * so that it can serve PSYNC requests performed using the master * replication ID. */

 * so that it can serve PSYNC requests performed using the master
 * replication ID. */
void shiftReplicationId(void) {
    memcpy(server.replid2,server.replid,sizeof(server.replid));
    /* We set the second replid offset to the master offset + 1, since
     * the slave will ask for the first byte it has not yet received, so
     * we need to add one to the offset: for example if, as a slave, we are
     * sure we have the same history as the master for 50 bytes, after we
     * are turned into a master, we can accept a PSYNC request with offset
     * 51, since the slave asking has the same history up to the 50th
     * byte, and is asking for the new bytes starting at offset 51. */
    server.second_replid_offset = server.master_repl_offset+1;
    changeReplicationId();
    serverLog(LL_WARNING,"Setting secondary replication ID to %s, valid up to offset: %lld. New replication ID is %s", server.replid2, server.second_replid_offset, server.replid);
}

/* ----------------------------------- SLAVE -------------------------------- */