/**********************************************************************//**
Creates a new segment.
@return the block where the segment header is placed, x-latched, NULL
if could not create segment because of lack of space */
UNIV_INTERN
buf_block_t*
fseg_create_general(
/*================*/
    ulint    space,    /*!< in: space id */
    ulint    page,    /*!< in: page where the segment header is placed: if
            this is != 0, the page must belong to another segment,
            if this is 0, a new page will be allocated and it
            will belong to the created segment */
    ulint    byte_offset, /*!< in: byte offset of the created segment header
            on the page */
    ibool    has_done_reservation, /*!< in: TRUE if the caller has already
            done the reservation for the pages with
            fsp_reserve_free_extents (at least 2 extents: one for
            the inode and the other for the segment) then there is
            no need to do the check for this individual
            operation */
    mtr_t*    mtr)    /*!< in: mtr */
{
    ulint        flags;
    ulint        zip_size;
    fsp_header_t*    space_header;
    fseg_inode_t*    inode; //typedef byte fseg_inode_t;
    ib_id_t        seg_id;
    buf_block_t*    block    = ; /* remove warning */
    fseg_header_t*    header    = ; /* remove warning */
    rw_lock_t*    latch;
    ibool        success;
    ulint        n_reserved;
    ulint        i;

    ut_ad(mtr);
    ut_ad(byte_offset + FSEG_HEADER_SIZE
          <= UNIV_PAGE_SIZE - FIL_PAGE_DATA_END);

    latch = fil_space_get_latch(space, &flags);
    zip_size = dict_table_flags_to_zip_size(flags);

    ) {
        block = buf_page_get(space, zip_size, page, RW_X_LATCH, mtr);
        header = byte_offset + buf_block_get_frame(block);
    }

    ut_ad(!mutex_own(&kernel_mutex)
          || mtr_memo_contains(mtr, latch, MTR_MEMO_X_LOCK));

    mtr_x_lock(latch, mtr);

    ) {
        /* This thread did not own the latch before this call: free
        excess pages from the insert buffer free list */

        if (space == IBUF_SPACE_ID) {
            ibuf_free_excess_pages();
        }
    }

    if (!has_done_reservation) {
        success = fsp_reserve_free_extents(&n_reserved, space, ,
                           FSP_NORMAL, mtr);
        if (!success) {
            return(NULL);
        }
    }

    space_header = fsp_get_space_header(space, zip_size, mtr);//详见

    inode = fsp_alloc_seg_inode(space_header, mtr);//申请inode entry 详见    if (inode == NULL) {

        goto funct_exit;
    }

    /* Read the next segment id from space header and increment the
    value in space header */

    seg_id = mach_read_from_8(space_header + FSP_SEG_ID);//设置下一下seg id

    mlog_write_ull(space_header + FSP_SEG_ID, seg_id + , mtr);
            *     *#define FSEG_FULL (12 + 2 * FLST_BASE_NODE_SIZE)        *     */
    mlog_write_ull(inode + FSEG_ID, seg_id, mtr);
    mlog_write_ulint(inode + FSEG_NOT_FULL_N_USED, , MLOG_4BYTES, mtr);

    flst_init(inode + FSEG_FREE, mtr); //初始化inode中的seg list 详见
    flst_init(inode + FSEG_NOT_FULL, mtr);
    flst_init(inode + FSEG_FULL, mtr);

    mlog_write_ulint(inode + FSEG_MAGIC_N, FSEG_MAGIC_N_VALUE,
             MLOG_4BYTES, mtr);

    //#define FSEG_FRAG_ARR_N_SLOTS (FSP_EXTENT_SIZE / 2) 64/2=32
    ; i < FSEG_FRAG_ARR_N_SLOTS; i++) {
        fseg_set_nth_frag_page_no(inode, i, FIL_NULL, mtr); //设置frag 碎片 详见
    }

    ) {
        block = fseg_alloc_free_page_low(space, zip_size,
                         inode, , FSP_UP, mtr, mtr);

        if (block == NULL) {

            fsp_free_seg_inode(space, zip_size, inode, mtr);

            goto funct_exit;
        }

        ut_ad(rw_lock_get_x_lock_count(&block->);

        header = byte_offset + buf_block_get_frame(block);
        mlog_write_ulint(buf_block_get_frame(block) + FIL_PAGE_TYPE,
                 FIL_PAGE_TYPE_SYS, MLOG_2BYTES, mtr);
    }
        //设置fset_header信息
    mlog_write_ulint(header + FSEG_HDR_OFFSET,page_offset(inode), MLOG_2BYTES, mtr);

    mlog_write_ulint(header + FSEG_HDR_PAGE_NO,page_get_page_no(page_align(inode)),MLOG_4BYTES, mtr);

    mlog_write_ulint(header + FSEG_HDR_SPACE, space, MLOG_4BYTES, mtr);

funct_exit:
    if (!has_done_reservation) {

        fil_space_release_free_extents(space, n_reserved);
    }

    return(block);
}