什么是“a block is a sentinel for a buffer pool watch.”

如下代码为，mysql在做buffer pool load时，如果某一个page既在内存，同时也不符合“a block is a sentinel for a buffer pool watch.”，则跳过此page的加载。

watch_page = buf_page_hash_get_low(buf_pool, page_id);
    if (watch_page && !buf_pool_watch_is_sentinel(buf_pool, watch_page)) {
        /* The page is already in the buffer pool. */
        watch_page = NULL;
        rw_lock_x_unlock(hash_lock);
        if (block) {
            buf_page_mutex_enter(block);
            buf_LRU_block_free_non_file_page(block);
            buf_page_mutex_exit(block);
        }

        bpage = NULL;
        goto func_exit;
    }

/** Determine if a block is a sentinel for a buffer pool watch.
@param[in]    buf_pool    buffer pool instance
@param[in]    bpage        block
@return TRUE if a sentinel for a buffer pool watch, FALSE if not */
ibool
buf_pool_watch_is_sentinel(
    const buf_pool_t*    buf_pool,
    const buf_page_t*    bpage)
{
    /* We must also own the appropriate hash lock. */
    ut_ad(buf_page_hash_lock_held_s_or_x(buf_pool, bpage));
    ut_ad(buf_page_in_file(bpage));

    if (bpage < &buf_pool->watch[0]
        || bpage >= &buf_pool->watch[BUF_POOL_WATCH_SIZE]) {

        ut_ad(buf_page_get_state(bpage) != BUF_BLOCK_ZIP_PAGE
              || bpage->zip.data != NULL);

        return(FALSE);
    }

    ut_ad(buf_page_get_state(bpage) == BUF_BLOCK_ZIP_PAGE);
    ut_ad(!bpage->in_zip_hash);
    ut_ad(bpage->in_page_hash);
    ut_ad(bpage->zip.data == NULL);
    return(TRUE);
}