/*
 *    mdopen() -- Open the specified relation.
 *
 * Note we only open the first segment, when there are multiple segments.
 *
 * If first segment is not present, either ereport or return NULL according
 * to "behavior".  We treat EXTENSION_CREATE the same as EXTENSION_FAIL;
 * EXTENSION_CREATE means it's OK to extend an existing relation, not to
 * invent one out of whole cloth.
 */
static MdfdVec *
mdopen(SMgrRelation reln, ForkNumber forknum, ExtensionBehavior behavior)
{
    MdfdVec    *mdfd;
    char       *path;
    File        fd;

    /* No work if already open */
    if (reln->md_fd[forknum])
        return reln->md_fd[forknum];

    path = relpath(reln->smgr_rnode, forknum);

    fd = PathNameOpenFile(path, O_RDWR | PG_BINARY, 0600);

    if (fd < 0)
    {
        /*
         * During bootstrap, there are cases where a system relation will be
         * accessed (by internal backend processes) before the bootstrap
         * script nominally creates it.  Therefore, accept mdopen() as a
         * substitute for mdcreate() in bootstrap mode only. (See mdcreate)
         */
        if (IsBootstrapProcessingMode())
            fd = PathNameOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY, 
0600);
if (fd < 0)
        {
            if (behavior == EXTENSION_RETURN_NULL &&
                FILE_POSSIBLY_DELETED(errno))
            {
                pfree(path);
                return NULL;
            }
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not open file \"%s\": %m", path)));
        }
    }

    pfree(path);

    reln->md_fd[forknum] = mdfd = _fdvec_alloc();

    mdfd->mdfd_vfd = fd;
    mdfd->mdfd_segno = 0;
    mdfd->mdfd_chain = NULL;
    Assert(_mdnblocks(reln, forknum, mdfd) <= ((BlockNumber) RELSEG_SIZE));

    return mdfd;
}

    /* No work if already open */
    if (reln->md_fd[forknum])
        return reln->md_fd[forknum];