趁着年轻

1.4.20 版本在处理 lru_crawler crawl 2 指令的时候，lru_crawler_crawl函数竟然存在变量未定义的bug，从而导致错误的清理了不应该的slabs槽位的数据。

看 lru_crawler_crawl 函数的代码：

enum crawler_result_type lru_crawler_crawl(char *slabs) {
    //对参数指定的slabs的类进行爬虫扫描，告诉后台线程item_crawler_thread 去清理过期数据
    char *b = NULL;
    uint32_t sid = 0;
    uint8_t tocrawl[POWER_LARGEST];//需要清理扫描的大小类
    //注意上面变了数组没有初始化，下面肯定有问题的。
    if (pthread_mutex_trylock(&lru_crawler_lock) != 0) {
        return CRAWLER_RUNNING;
    }
    pthread_mutex_lock(&cache_lock);

    if (strcmp(slabs, "all") == 0) {
        for (sid = 0; sid < LARGEST_ID; sid++) {
            tocrawl[sid] = 1;
        }
    } else {
        for (char *p = strtok_r(slabs, ",", &b);
             p != NULL;
             p = strtok_r(NULL, ",", &b)) {

            if (!safe_strtoul(p, &sid) || sid < POWER_SMALLEST
                    || sid > POWER_LARGEST) {
                pthread_mutex_unlock(&cache_lock);
                pthread_mutex_unlock(&lru_crawler_lock);
                return CRAWLER_BADCLASS;
            }
            tocrawl[sid] = 1;//为什么不初始化？如果不是0怎么办
        }
    }

    for (sid = 0; sid < LARGEST_ID; sid++) {
        if (tocrawl[sid] != 0 && tails[sid] != NULL) {
            if (settings.verbose > 2)
                fprintf(stderr, "Kicking LRU crawler off for slab %d\n", sid);
            crawlers[sid].nbytes = 0;
            crawlers[sid].nkey = 0;
            crawlers[sid].it_flags = 1; /* For a crawler, this means enabled. */
            crawlers[sid].next = 0;
            crawlers[sid].prev = 0;
            crawlers[sid].time = 0;
            crawlers[sid].remaining = settings.lru_crawler_tocrawl;//这次要清理这么多
            crawlers[sid].slabs_clsid = sid;
            crawler_link_q((item *)&crawlers[sid]);
            crawler_count++;
        }
    }
    pthread_mutex_unlock(&cache_lock);
    pthread_cond_signal(&lru_crawler_cond);//唤起item_crawler_thread
    STATS_LOCK();
    stats.lru_crawler_running = true;
    STATS_UNLOCK();
    pthread_mutex_unlock(&lru_crawler_lock);
    return CRAWLER_OK;
}

如果安装的是这个memcached版本，最好不要轻易使用crawler指令。
查了一下最新的代码，最近的1.4.24版本已经修复了这个问题，增加了如下代码即可：

memset(tocrawl, 0, sizeof(uint8_t) * MAX_NUMBER_OF_SLAB_CLASSES);