PostgreSQL 恢复模式错误日志增强 - 提供正在恢复的WAL（XLOG）文件位置

背景

当数据库异常停库，再次启动时。又或者由于进程CRASH后自动重启时。需要进入恢复模式，恢复完成后，数据库才能正常交互。

在启动过程中，正在恢复时，如果此时连接数据库，会报错如下：

the database system is starting up  

用户只看这个信息，并不知道数据库还要启动多久，现在已经恢复到什么状态了？

内核层面可以稍作改进，报错时，同时输出正在恢复的WAL位点，以及距离最后一个文件的WAL位点差多少MB没有恢复。这样用户大概就还知道还需要多久。

src/backend/postmaster/postmaster.c

/*  
 * Read a client's startup packet and do something according to it.  
 *  
 * Returns STATUS_OK or STATUS_ERROR, or might call ereport(FATAL) and  
 * not return at all.  
 *  
 * (Note that ereport(FATAL) stuff is sent to the client, so only use it  
 * if that's what you want.  Return STATUS_ERROR if you don't want to  
 * send anything to the client, which would typically be appropriate  
 * if we detect a communications failure.)  
 */  
static int  
ProcessStartupPacket(Port *port, bool SSLdone)  
{  
  
.......  
  
        /*  
         * If we're going to reject the connection due to database state, say so  
         * now instead of wasting cycles on an authentication exchange. (This also  
         * allows a pg_ping utility to be written.)  
         */  
        switch (port->canAcceptConnections)  
        {  
                case CAC_STARTUP:  
                        ereport(FATAL,  
                                        (errcode(ERRCODE_CANNOT_CONNECT_NOW),  
                                         errmsg("the database system is starting up")));  
                        break;  
                case CAC_SHUTDOWN:  
                        ereport(FATAL,  
                                        (errcode(ERRCODE_CANNOT_CONNECT_NOW),  
                                         errmsg("the database system is shutting down")));  
                        break;  
                case CAC_RECOVERY:  
                        ereport(FATAL,  
                                        (errcode(ERRCODE_CANNOT_CONNECT_NOW),  
                                         errmsg("the database system is in recovery mode")));  
                        break;  
                case CAC_TOOMANY:  
                        ereport(FATAL,  
                                        (errcode(ERRCODE_TOO_MANY_CONNECTIONS),  
                                         errmsg("sorry, too many clients already")));  
                        break;  
                case CAC_WAITBACKUP:  
                        /* OK for now, will check in InitPostgres */  
                        break;  
                case CAC_OK:  
                        break;  
        }  
  
        return STATUS_OK;  
}  

src/backend/access/transam/xlog.c

/*  
 * This must be called ONCE during postmaster or standalone-backend startup  
 */  
void  
StartupXLOG(void)  
{  

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