PG服务进程（Postgres）——BeginReportingGUCOptions向客户端汇报GUC

BeginReportingGUCOptions函数利用ParameterStatus 消息向客户端启动对标记为 GUC_REPORT 的变量更改的自动报告。这是在后端启动完成时执行的。

void BeginReportingGUCOptions(void){
  int      i;
  /* Don't do anything unless talking to an interactive frontend of protocol 3.0 or later. 除非与协议 3.0 或更高版本的交互式前端交谈，否则不要做任何事情。 */
  if (whereToSendOutput != DestRemote || PG_PROTOCOL_MAJOR(FrontendProtocol) < 3)
    return;
  reporting_enabled = true;
  /* Transmit initial values of interesting variables */
  for (i = 0; i < num_guc_variables; i++) {
    struct config_generic *conf = guc_variables[i];
    if (conf->flags & GUC_REPORT) ReportGUCOption(conf);
  }
}

ReportGUCOption向客户端发送消息。

/* ReportGUCOption: if appropriate, transmit option value to frontend */
static void ReportGUCOption(struct config_generic *record) {
  if (reporting_enabled && (record->flags & GUC_REPORT)) {
    char     *val = _ShowOption(record, false);
    StringInfoData msgbuf;
    pq_beginmessage(&msgbuf, 'S');
    pq_sendstring(&msgbuf, record->name);
    pq_sendstring(&msgbuf, val);
    pq_endmessage(&msgbuf);
    pfree(val);
  }
}

CommandDest 是一种识别所需目的地的简单方法。 总有一天，这可能需要改进。注意：只有值 DestNone、DestDebug、DestRemote 对于全局变量 whereToSendOutput 是合法的。 其他值可用作各个命令的目的地。

/* ----------------
 *   CommandDest is a simplistic means of identifying the desired
 *   destination.  Someday this will probably need to be improved.
 *
 * Note: only the values DestNone, DestDebug, DestRemote are legal for the
 * global variable whereToSendOutput.   The other values may be used
 * as the destination for individual commands.
 * ----------------
 */
typedef enum {
  DestNone,          /* results are discarded */
  DestDebug,          /* results go to debugging output */
  DestRemote,          /* results sent to frontend process 结果发送给前端处理 */
  DestRemoteExecute,      /* sent to frontend, in Execute command */
  DestRemoteSimple,      /* sent to frontend, w/no catalog access */
  DestSPI,          /* results sent to SPI manager 结果发送给SPI管理器 */
  DestTuplestore,        /* results sent to Tuplestore 结果发送给Tuplestore */
  DestIntoRel,        /* results sent to relation (SELECT INTO) 结果发送到relation表 */
  DestCopyOut,        /* results sent to COPY TO code 结果发送到COPY TO */
  DestSQLFunction,      /* results sent to SQL-language func mgr 结果发送到SQL-language func mgr */
  DestTransientRel,      /* results sent to transient relation 结果发送到transient relation表 */
  DestTupleQueue        /* results sent to tuple queue 结果发送给Tuple队列 */
} CommandDest;