为什么PostgreSQL启动后有个UDP监听localhost

用于pgstat进程发送和接收统计信息用的，见代码：
src/backend/postmaster/pgstat.c

/* ----------
 * pgstat_init() -
 *
 *      Called from postmaster at startup. Create the resources required
 *      by the statistics collector process.  If unable to do so, do not
 *      fail --- better to let the postmaster start with stats collection
 *      disabled.
 * ----------
 */
void
pgstat_init(void)
{
        ACCEPT_TYPE_ARG3 alen;
        struct addrinfo *addrs = NULL,
                           *addr,
                                hints;
        int                     ret;
        fd_set          rset;
        struct timeval tv;
        char            test_byte;
        int                     sel_res;
        int                     tries = 0;

#define TESTBYTEVAL ((char) 199)

        /*
         * This static assertion verifies that we didn't mess up the calculations
         * involved in selecting maximum payload sizes for our UDP messages.
         * Because the only consequence of overrunning PGSTAT_MAX_MSG_SIZE would
         * be silent performance loss from fragmentation, it seems worth having a
         * compile-time cross-check that we didn't.
         */
        StaticAssertStmt(sizeof(PgStat_Msg) <= PGSTAT_MAX_MSG_SIZE,
                                   "maximum stats message size exceeds PGSTAT_MAX_MSG_SIZE");

        /*
         * Create the UDP socket for sending and receiving statistic messages
         */
        hints.ai_flags = AI_PASSIVE;
        hints.ai_family = AF_UNSPEC;
        hints.ai_socktype = SOCK_DGRAM;
        hints.ai_protocol = 0;
        hints.ai_addrlen = 0;
        hints.ai_addr = NULL;
        hints.ai_canonname = NULL;
        hints.ai_next = NULL;
        ret = pg_getaddrinfo_all("localhost", NULL, &hints, &addrs);
        if (ret || !addrs)
        {
                ereport(LOG,
                                (errmsg("could not resolve \"localhost\": %s",
                                                gai_strerror(ret))));
                goto startup_failed;
        }