函数buf_LRU_get_free_only

/******************************************************************//**
Returns a free block from the buf_pool.  The block is taken off the
free list.  If it is empty, returns NULL.
@return    a free control block, or NULL if the buf_block->free list is empty */
UNIV_INTERN
buf_block_t*
buf_LRU_get_free_only(
/*==================*/
    buf_pool_t*    buf_pool)
{
    buf_block_t*    block;

    ut_ad(buf_pool_mutex_own(buf_pool));

    block = (buf_block_t*) UT_LIST_GET_FIRST(buf_pool->free);

    if (block) {

        ut_ad(block->page.in_free_list);
        ut_d(block->page.in_free_list = FALSE);
        ut_ad(!block->page.in_flush_list);
        ut_ad(!block->page.in_LRU_list);
        ut_a(!buf_page_in_file(&block->page));
        UT_LIST_REMOVE(list, buf_pool->free, (&block->page));

        mutex_enter(&block->mutex);

        buf_block_set_state(block, BUF_BLOCK_READY_FOR_USE);
        UNIV_MEM_ALLOC(block->frame, UNIV_PAGE_SIZE);

        ut_ad(buf_pool_from_block(block) == buf_pool);

        mutex_exit(&block->mutex);
    }

    return(block);
}