https://github.com/redis/redis/blob/5.0/src/networking.c#L190-L211

/* Add the object 'obj' string representation to the client output buffer. */
void addReply(client *c, robj *obj) {
    if (prepareClientToWrite(c) != C_OK) return;

    if (sdsEncodedObject(obj)) {
        if (_addReplyToBuffer(c,obj->ptr,sdslen(obj->ptr)) != C_OK)
            _addReplyStringToList(c,obj->ptr,sdslen(obj->ptr));
    } else if (obj->encoding == OBJ_ENCODING_INT) {
        char buf[32];
        size_t len = ll2string(buf,sizeof(buf),(long)obj->ptr);
        if (_addReplyToBuffer(c,buf,len) != C_OK)
            _addReplyStringToList(c,buf,len);
    } else {
        serverPanic("Wrong obj->encoding in addReply()");
    }
}

在函数的开头，会通过prepareClientToWrite(c)判断是否需要将数据写入客户端的 output buffer 中。我们看下什么条件下数据会被写入客户端的 output buffer 中，即返回 C_OK。

/* This function is called every time we are going to transmit new data
 * to the client. The behavior is the following:
 *
 * If the client should receive new data (normal clients will) the function
 * returns C_OK, and make sure to install the write handler in our event
 * loop so that when the socket is writable new data gets written.
 *
 * If the client should not receive new data, because it is a fake client
 * (used to load AOF in memory), a master or because the setup of the write
 * handler failed, the function returns C_ERR.
 *
 * The function may return C_OK without actually installing the write
 * event handler in the following cases:
 *
 * 1) The event handler should already be installed since the output buffer
 *    already contains something.
 * 2) The client is a slave but not yet online, so we want to just accumulate
 *    writes in the buffer but not actually sending them yet.
 *
 * Typically gets called every time a reply is built, before adding more
 * data to the clients output buffers. If the function returns C_ERR no
 * data should be appended to the output buffers. */
int prepareClientToWrite(client *c) {
    /* If it's the Lua client we always return ok without installing any
     * handler since there is no socket at all. */
    if (c->flags & (CLIENT_LUA|CLIENT_MODULE)) return C_OK;

    /* CLIENT REPLY OFF / SKIP handling: don't send replies. */
    if (c->flags & (CLIENT_REPLY_OFF|CLIENT_REPLY_SKIP)) return C_ERR;

    /* Masters don't receive replies, unless CLIENT_MASTER_FORCE_REPLY flag
     * is set. */
    if ((c->flags & CLIENT_MASTER) &&
        !(c->flags & CLIENT_MASTER_FORCE_REPLY)) return C_ERR;

    if (c->fd <= 0) return C_ERR; /* Fake client for AOF loading. */

    /* Schedule the client to write the output buffers to the socket, unless
     * it should already be setup to do so (it has already pending data). */
    if (!clientHasPendingReplies(c)) clientInstallWriteHandler(c);

    /* Authorize the caller to queue in the output buffer of this client. */
    return C_OK;
}

/* Return true if the specified client has pending reply buffers to write to
 * the socket. */
int clientHasPendingReplies(client *c) {
    return c->bufpos || listLength(c->reply);
}

void clientInstallWriteHandler(client *c) {
    /* Schedule the client to write the output buffers to the socket only
     * if not already done and, for slaves, if the slave can actually receive
     * writes at this stage. */
    if (!(c->flags & CLIENT_PENDING_WRITE) &&
        (c->replstate == REPL_STATE_NONE ||
         (c->replstate == SLAVE_STATE_ONLINE && !c->repl_put_online_on_ack)))
    {
        c->flags |= CLIENT_PENDING_WRITE;
        listAddNodeHead(server.clients_pending_write,c);
    }
}

由于函数默认返回C_OK，我们只需要看哪几类情况返回的不是C_OK，即C_ERR，数据就不会被写入到客户端的 output buffer 中。
返回C_ERR的情况：

• 客户端是个 fake client（用于加载 AOF 文件）
• 客户端是一个 master
• slave 的状态为 SLAVE_STATE_ONLINE 且其回调函数失败（(c->replstate == SLAVE_STATE_ONLINE && !c->repl_put_online_on_ack)），或slave 的状态为 REPL_STATE_NONE

If the client should not receive new data, because it is a fake client (used to load AOF in memory), a master or because the setup of the write handler failed, the function returns C_ERR.

在 master 保存和发送 rdb 文件时，slave 的状态是以下几种，所以在这期间的写命令都会保存在 slave 的 output buffer。由于没有设置回调函数，数据并不会发送到 slave 上，仅存储在 master 为 slave 创建的 output buffer 内。

#define SLAVE_STATE_WAIT_BGSAVE_START 6 /* We need to produce a new RDB file. */
#define SLAVE_STATE_WAIT_BGSAVE_END 7 /* Waiting RDB file creation to finish. */
#define SLAVE_STATE_SEND_BULK 8 /* Sending RDB file to slave. */

那么何时才会从 output buffer 中“刷入”slave 呢？直到 master 将 rdb 文件完全发送给 slave 后，master 会在 sendBulkToSlave函数中进行相关操作。以下源码见：https://github.com/redis/redis/blob/5.0/src/replication.c#L876-L930

void sendBulkToSlave(aeEventLoop *el, int fd, void *privdata, int mask) {
    // 此处省略部分源码

    // rdb 文件已完全发送给 slave 
    if (slave->repldboff == slave->repldbsize) {
        close(slave->repldbfd);
        slave->repldbfd = -1;
        aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE);
        putSlaveOnline(slave);
    }
}

void putSlaveOnline(client *slave) {
    slave->replstate = SLAVE_STATE_ONLINE;
    slave->repl_put_online_on_ack = 0;
    slave->repl_ack_time = server.unixtime; /* Prevent false timeout. */
    if (aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE,
        sendReplyToClient, slave) == AE_ERR) {
        serverLog(LL_WARNING,"Unable to register writable event for replica bulk transfer: %s", strerror(errno));
        freeClient(slave);
        return;
    }
    refreshGoodSlavesCount();
    serverLog(LL_NOTICE,"Synchronization with replica %s succeeded",
        replicationGetSlaveName(slave));
}

此处会将 slave 状态改为 SLAVE_STATE_ONLINE，并将repl_put_online_on_ack置为0，（有没有很熟悉，对了，就是上面clientInstallWriteHandler中判断的内容）。同时也会设置回调函数sendReplyToClient，将此前 master 为 slave 创建的 output buffer 中的写操作全部发送到 slave 上。同时 slave 状态的变更，会使得后续 master 上的写操作可以正常的 push 到 slave 上了（直接，无需走 output buffer）。

总结

本次我们通过 client-output-buffer-limit参数，了解了其使用场景，并重点就主从同步时 output buffer 写入情况进行了源码的简单分析。今天的学习就到这里，我们改天接着肝。

参考内容