Postgresql检查点中如何获取xlog位置（未完工）

CreateCheckPoint：启动检查点     <-------- 本篇涉及
  |
  ...
  XLogBytePosToRecPtr    <-------- 本篇涉及
  ...
  CheckPointGuts：检查点流程入口    
    |
    CheckPointBuffers：刷缓存页入口
      |
      BufferSync：刷缓存页具体函数
        |
        CheckpointWriteDelay：主动延迟函数               
          |
          IsCheckpointOnSchedule：主动延迟函数的判断算法    

/*
 * Converts a "usable byte position" to XLogRecPtr. A usable byte position
 * is the position starting from the beginning of WAL, excluding all WAL
 * page headers.
 */
static XLogRecPtr
XLogBytePosToRecPtr(uint64 bytepos)
{
    uint64      fullsegs;
    uint64      fullpages;
    uint64      bytesleft;
    uint32      seg_offset;
    XLogRecPtr  result;

    fullsegs = bytepos / UsableBytesInSegment;
    bytesleft = bytepos % UsableBytesInSegment;

    if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
    {
        /* fits on first page of segment */
        seg_offset = bytesleft + SizeOfXLogLongPHD;
    }
    else
    {
        /* account for the first page on segment with long header */
        seg_offset = XLOG_BLCKSZ;
        bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

        fullpages = bytesleft / UsableBytesInPage;
        bytesleft = bytesleft % UsableBytesInPage;

        seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
    }

    XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, result);

    return result;
}