Memcached源碼分析——process_command函數解析

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/**
 * process_command 在memcached中是用來處理用戶發送的命令的，
 * 包括get set，add，delete，replace，stats，flush_all等經常使用的和不經常使用的命令，全在這進行處理的。
 * 一共有兩個參數，conn *c 和字符串指針 char *command ；
 * 關於conn結構體就先不說了，反正它是整個memcached中最重要的結構體就好了，等之後研究明白了再說，先從總體上領會精神吧。
 * 這裏我想說一下的是， memcached 和 redis 在處理命令上的想法仍是有很大差異的，
 * 在 redis 裏面，你要是想看一下一共支持多少命令，每一個命令對應的函數，很方便，都在一個名叫 redisCommandTable 的結構體數組裏面，一目瞭然；
 * 可是 memcached 卻不是這樣，我剛開始也是按照看 redis 源碼的方式去找 memcached 中的，可是找了好久也沒有發現，緣由就是做者把支持的全部命令都散落在下面這個函數中了。
 * 先說一下 memcached 是怎麼從 command 字符串中分解出具體的命令和對應的參數的。
 * 在函數中用到了一個結構體數組： tokens[MAX_TOKENS] ，它實際上是用來存放分析完的 command 結果用的，分析工做在函數 tokenize_command 中進行。
 * 如：有一個命令 "get aaaaaaaaaa" ，分析完之後存在tokens中的就是
 * tokens[3] = {{value:"get",length:3},{value:"aaaaaaaaaa",length:10},{value:NULL,length:0}};
 * 函數 tokenize_command 返回的是一個int型數據 ntokens，記錄了 tokens 的大小，表示從 command 命令中分解出了幾條數據，
 * 固然 ntokens 的值會比實際中 command 中包含的數據多1，由於字符串結尾的'\0'也要佔同樣。
 * 
 * 下面說說以 "get aaaaaaaaaa" 命令爲例，具體的命令分析和函數調用過程：
 * 當一條命令 "get aaaaaaaaaa" 傳到 process_command 中以後，先調用負責解析命令的函數tokenize_command，
 * 將解析後的命令存儲在tokens數組中，結果如上面的tokens[3]，並返回ntokens，說明command中包含幾個字段（這裏獲得的是3），
 * 而後根據字段的數目進行判斷應該到哪一個條件語句去進行比對，當確認以後，就會跳到對應的條件語句中
 * 因此這裏應該到 tokenize_command 下面的第一個if語句中，而後使用tokens[0].value，也就是tokens數組中存儲的get命令和字符串"get"進行比較，
 * 匹配，則調用對應的函數，這裏調用process_get_command(c, tokens, ntokens, false);
 * 而後 process_command 的使命就結束了。
 * 如下代碼在memcached-1.4.22/memcached.c
 */
static void process_command(conn *c, char *command) {

    token_t tokens[MAX_TOKENS];
    size_t ntokens;
    int comm;

    assert(c != NULL);

    MEMCACHED_PROCESS_COMMAND_START(c->sfd, c->rcurr, c->rbytes);

    if (settings.verbose > 1)
        fprintf(stderr, "<%d %s\n", c->sfd, command);

    /*
     * for commands set/add/replace, we build an item and read the data
     * directly into it, then continue in nread_complete().
     */

    c->msgcurr = 0;
    c->msgused = 0;
    c->iovused = 0;
    if (add_msghdr(c) != 0) {
        out_of_memory(c, "SERVER_ERROR out of memory preparing response");
        return;
    }

    ntokens = tokenize_command(command, tokens, MAX_TOKENS);
    if (ntokens >= 3 &&
        ((strcmp(tokens[COMMAND_TOKEN].value, "get") == 0) ||
         (strcmp(tokens[COMMAND_TOKEN].value, "bget") == 0))) {

        process_get_command(c, tokens, ntokens, false);

    } else if ((ntokens == 6 || ntokens == 7) &&
               ((strcmp(tokens[COMMAND_TOKEN].value, "add") == 0 && (comm = NREAD_ADD)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "set") == 0 && (comm = NREAD_SET)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "replace") == 0 && (comm = NREAD_REPLACE)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "prepend") == 0 && (comm = NREAD_PREPEND)) ||
                (strcmp(tokens[COMMAND_TOKEN].value, "append") == 0 && (comm = NREAD_APPEND)) )) {

        process_update_command(c, tokens, ntokens, comm, false);

    } else if ((ntokens == 7 || ntokens == 8) && (strcmp(tokens[COMMAND_TOKEN].value, "cas") == 0 && (comm = NREAD_CAS))) {

        process_update_command(c, tokens, ntokens, comm, true);

    } else if ((ntokens == 4 || ntokens == 5) && (strcmp(tokens[COMMAND_TOKEN].value, "incr") == 0)) {

        process_arithmetic_command(c, tokens, ntokens, 1);

    } else if (ntokens >= 3 && (strcmp(tokens[COMMAND_TOKEN].value, "gets") == 0)) {

        process_get_command(c, tokens, ntokens, true);

    } else if ((ntokens == 4 || ntokens == 5) && (strcmp(tokens[COMMAND_TOKEN].value, "decr") == 0)) {

        process_arithmetic_command(c, tokens, ntokens, 0);

    } else if (ntokens >= 3 && ntokens <= 5 && (strcmp(tokens[COMMAND_TOKEN].value, "delete") == 0)) {

        process_delete_command(c, tokens, ntokens);

    } else if ((ntokens == 4 || ntokens == 5) && (strcmp(tokens[COMMAND_TOKEN].value, "touch") == 0)) {

        process_touch_command(c, tokens, ntokens);

    } else if (ntokens >= 2 && (strcmp(tokens[COMMAND_TOKEN].value, "stats") == 0)) {

        process_stat(c, tokens, ntokens);

    } else if (ntokens >= 2 && ntokens <= 4 && (strcmp(tokens[COMMAND_TOKEN].value, "flush_all") == 0)) {
        time_t exptime = 0;

        set_noreply_maybe(c, tokens, ntokens);

        pthread_mutex_lock(&c->thread->stats.mutex);
        c->thread->stats.flush_cmds++;
        pthread_mutex_unlock(&c->thread->stats.mutex);

        if (!settings.flush_enabled) {
            // flush_all is not allowed but we log it on stats
            out_string(c, "CLIENT_ERROR flush_all not allowed");
            return;
        }

        if(ntokens == (c->noreply ? 3 : 2)) {
            settings.oldest_live = current_time - 1;
            item_flush_expired();
            out_string(c, "OK");
            return;
        }

        exptime = strtol(tokens[1].value, NULL, 10);
        if(errno == ERANGE) {
            out_string(c, "CLIENT_ERROR bad command line format");
            return;
        }

        /*
          If exptime is zero realtime() would return zero too, and
          realtime(exptime) - 1 would overflow to the max unsigned
          value.  So we process exptime == 0 the same way we do when
          no delay is given at all.
        */
        if (exptime > 0)
            settings.oldest_live = realtime(exptime) - 1;
        else /* exptime == 0 */
            settings.oldest_live = current_time - 1;
        item_flush_expired();
        out_string(c, "OK");
        return;

    } else if (ntokens == 2 && (strcmp(tokens[COMMAND_TOKEN].value, "version") == 0)) {

        out_string(c, "VERSION " VERSION);

    } else if (ntokens == 2 && (strcmp(tokens[COMMAND_TOKEN].value, "quit") == 0)) {

        conn_set_state(c, conn_closing);

    } else if (ntokens == 2 && (strcmp(tokens[COMMAND_TOKEN].value, "shutdown") == 0)) {

        if (settings.shutdown_command) {
            conn_set_state(c, conn_closing);
            raise(SIGINT);
        } else {
            out_string(c, "ERROR: shutdown not enabled");
        }

    } else if (ntokens > 1 && strcmp(tokens[COMMAND_TOKEN].value, "slabs") == 0) {
        if (ntokens == 5 && strcmp(tokens[COMMAND_TOKEN + 1].value, "reassign") == 0) {
            int src, dst, rv;

            if (settings.slab_reassign == false) {
                out_string(c, "CLIENT_ERROR slab reassignment disabled");
                return;
            }

            src = strtol(tokens[2].value, NULL, 10);
            dst = strtol(tokens[3].value, NULL, 10);

            if (errno == ERANGE) {
                out_string(c, "CLIENT_ERROR bad command line format");
                return;
            }

            rv = slabs_reassign(src, dst);
            switch (rv) {
            case REASSIGN_OK:
                out_string(c, "OK");
                break;
            case REASSIGN_RUNNING:
                out_string(c, "BUSY currently processing reassign request");
                break;
            case REASSIGN_BADCLASS:
                out_string(c, "BADCLASS invalid src or dst class id");
                break;
            case REASSIGN_NOSPARE:
                out_string(c, "NOSPARE source class has no spare pages");
                break;
            case REASSIGN_SRC_DST_SAME:
                out_string(c, "SAME src and dst class are identical");
                break;
            }
            return;
        } else if (ntokens == 4 &&
            (strcmp(tokens[COMMAND_TOKEN + 1].value, "automove") == 0)) {
            process_slabs_automove_command(c, tokens, ntokens);
        } else {
            out_string(c, "ERROR");
        }
    } else if (ntokens > 1 && strcmp(tokens[COMMAND_TOKEN].value, "lru_crawler") == 0) {
        if (ntokens == 4 && strcmp(tokens[COMMAND_TOKEN + 1].value, "crawl") == 0) {
            int rv;
            if (settings.lru_crawler == false) {
                out_string(c, "CLIENT_ERROR lru crawler disabled");
                return;
            }

            rv = lru_crawler_crawl(tokens[2].value);
            switch(rv) {
            case CRAWLER_OK:
                out_string(c, "OK");
                break;
            case CRAWLER_RUNNING:
                out_string(c, "BUSY currently processing crawler request");
                break;
            case CRAWLER_BADCLASS:
                out_string(c, "BADCLASS invalid class id");
                break;
            }
            return;
        } else if (ntokens == 4 && strcmp(tokens[COMMAND_TOKEN + 1].value, "tocrawl") == 0) {
            uint32_t tocrawl;
             if (!safe_strtoul(tokens[2].value, &tocrawl)) {
                out_string(c, "CLIENT_ERROR bad command line format");
                return;
            }
            settings.lru_crawler_tocrawl = tocrawl;
            out_string(c, "OK");
            return;
        } else if (ntokens == 4 && strcmp(tokens[COMMAND_TOKEN + 1].value, "sleep") == 0) {
            uint32_t tosleep;
            if (!safe_strtoul(tokens[2].value, &tosleep)) {
                out_string(c, "CLIENT_ERROR bad command line format");
                return;
            }
            if (tosleep > 1000000) {
                out_string(c, "CLIENT_ERROR sleep must be one second or less");
                return;
            }
            settings.lru_crawler_sleep = tosleep;
            out_string(c, "OK");
            return;
        } else if (ntokens == 3) {
            if ((strcmp(tokens[COMMAND_TOKEN + 1].value, "enable") == 0)) {
                if (start_item_crawler_thread() == 0) {
                    out_string(c, "OK");
                } else {
                    out_string(c, "ERROR failed to start lru crawler thread");
                }
            } else if ((strcmp(tokens[COMMAND_TOKEN + 1].value, "disable") == 0)) {
                if (stop_item_crawler_thread() == 0) {
                    out_string(c, "OK");
                } else {
                    out_string(c, "ERROR failed to stop lru crawler thread");
                }
            } else {
                out_string(c, "ERROR");
            }
            return;
        } else {
            out_string(c, "ERROR");
        }
    } else if ((ntokens == 3 || ntokens == 4) && (strcmp(tokens[COMMAND_TOKEN].value, "verbosity") == 0)) {
        process_verbosity_command(c, tokens, ntokens);
    } else {
        out_string(c, "ERROR");
    }
    return;
}

 

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