postgresql socket读数据返回-1

    {
        n = secure_raw_read(port, ptr, len);   // pg的socket读是非阻塞读，所以返回-1不影响，后面等到socket可读之后继续读。见下文socket返回值解释
        waitfor = WL_SOCKET_READABLE;
    }

    /* In blocking mode, wait until the socket is ready */
    if (n < 0 && !port->noblock && (errno == EWOULDBLOCK || errno == EAGAIN))   // 非阻塞读，且errono==EAGAIN，所以就是每次处理完一个SQL语句后，会继续读下一个SQL，发现读不到，进到这里，通过latch设置等待客户端可读事件
    {
        WaitEvent    event;

        Assert(waitfor);

        ModifyWaitEvent(FeBeWaitSet, 0, waitfor, NULL);

        WaitEventSetWait(FeBeWaitSet, -1 /* no timeout */ , &event, 1,
                         WAIT_EVENT_CLIENT_READ);

        /*
         * If the postmaster has died, it's not safe to continue running,
         * because it is the postmaster's job to kill us if some other backend
         * exits uncleanly.  Moreover, we won't run very well in this state;
         * helper processes like walwriter and the bgwriter will exit, so
         * performance may be poor.  Finally, if we don't exit, pg_ctl will be
         * unable to restart the postmaster without manual intervention, so no
         * new connections can be accepted.  Exiting clears the deck for a
         * postmaster restart.
         *
         * (Note that we only make this check when we would otherwise sleep on
         * our latch.  We might still continue running for a while if the
         * postmaster is killed in mid-query, or even through multiple queries
         * if we never have to wait for read.  We don't want to burn too many
         * cycles checking for this very rare condition, and this should cause
         * us to exit quickly in most cases.)
         */
        if (event.events & WL_POSTMASTER_DEATH)
            ereport(FATAL,
                    (errcode(ERRCODE_ADMIN_SHUTDOWN),
                     errmsg("terminating connection due to unexpected postmaster exit")));

        /* Handle interrupt. */
        if (event.events & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            ProcessClientReadInterrupt(true);

            /*
             * We'll retry the read. Most likely it will return immediately
             * because there's still no data available, and we'll wait for the
             * socket to become ready again.
             */
        }
        goto retry;
    }