MySQL半同步复制--reserve_header

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mysql_binlog_send->reset_transmit_packet->RUN_HOOK(binlog_transmit, reserve_header, (thd, flags, packet))--

repl_semi_reserve_header

每次发送event前，如果是半同步都需要添加3个字节。

int ReplSemiSyncMaster::reserveSyncHeader(unsigned char *header,  unsigned long size){  const char *kWho = "ReplSemiSyncMaster::reserveSyncHeader";  function_enter(kWho);  int hlen=0;  //如果slave没有开启半同步，则不需要添加额外字节内容  if (!is_semi_sync_slave())  {    hlen= 0;  }  else  {    /* No enough space for the extra header, disable semi-sync master */    if (sizeof(kSyncHeader) > size)    {      disableMaster();      return 0;    }        /* Set the magic number and the sync status.  By default, no sync     * is required.     */    memcpy(header, kSyncHeader, sizeof(kSyncHeader));    hlen= sizeof(kSyncHeader);  }  return function_exit(kWho, hlen);}

用于在packet的头部预留字节，以维护和slave的交互信息，目前预留3字节
ReplSemiSyncBase:kSyncHeader[2]，这里为在packet头部拷贝2字节，
固定值作为MAGIC NUMBER,只有slave开启semisync才会保留额外的
packet头部比特位，不管master是否开启了semisync
const unsigned char ReplSemiSyncBase::kPacketMagicNum = 0xef;
const unsigned char ReplSemiSyncBase::kSyncHeader[2] ={ReplSemiSyncBase::kPacketMagicNum, 0};

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