/**********************************************************************//**
Allocates a single free page from a space. */
static __attribute__((nonnull))
void
fsp_alloc_from_free_frag(
/*=====================*/
    fsp_header_t*    header,    /*!< in/out: tablespace header */
    xdes_t*        descr,    /*!< in/out: extent descriptor */
    ulint        bit,    /*!< in: slot to allocate in the extent */
    mtr_t*        mtr)    /*!< in/out: mini-transaction */
{
    ulint        frag_n_used;

    ut_ad(xdes_get_state(descr, mtr) == XDES_FREE_FRAG);
    ut_a(xdes_get_bit(descr, XDES_FREE_BIT, bit, mtr));
    xdes_set_bit(descr, XDES_FREE_BIT, bit, FALSE, mtr);

    /* Update the FRAG_N_USED field */
    frag_n_used = mtr_read_ulint(header + FSP_FRAG_N_USED, MLOG_4BYTES,
                     mtr);
    frag_n_used++;
    mlog_write_ulint(header + FSP_FRAG_N_USED, frag_n_used, MLOG_4BYTES,
             mtr);
    if (xdes_is_full(descr, mtr)) {
        /* The fragment is full: move it to another list */
        flst_remove(header + FSP_FREE_FRAG, descr + XDES_FLST_NODE,mtr); //移除node到 FSP_FULL_FRAGE 详见
        xdes_set_state(descr, XDES_FULL_FRAG, mtr);
        flst_add_last(header + FSP_FULL_FRAG, descr + XDES_FLST_NODE,mtr);
        mlog_write_ulint(header + FSP_FRAG_N_USED,frag_n_used - FSP_EXTENT_SIZE, MLOG_4BYTES,mtr);
    }
}