缓存主机

从机因为某些原因，譬如网络延迟（PING 超时，ACK 超时等），可能会断开与主机的连接。这时候，从机会尝试保存与主机连接的信息，譬如全局积压空间数据偏移量等，以便下一次的部分同步，并且从机会再一次尝试连接主机。注意一点，如果断开的时间足够长， 部分同步肯定会失败的。

void freeClient(redisClient *c) {
    listNode *ln;

    /* If this is marked as current client unset it */
    if (server.current_client == c) server.current_client = NULL;

    // 如果此机为从机，已经连接主机，可能需要保存主机状态信息，以便进行 PSYNC
    /* If it is our master that's beging disconnected we should make sure
     * to cache the state to try a partial resynchronization later.
     *
     * Note that before doing this we make sure that the client is not in
     * some unexpected state, by checking its flags. */
    if (server.master && c->flags & REDIS_MASTER) {
        redisLog(REDIS_WARNING,"Connection with master lost.");
        if (!(c->flags & (REDIS_CLOSE_AFTER_REPLY|
                          REDIS_CLOSE_ASAP|
                          REDIS_BLOCKED|
                          REDIS_UNBLOCKED)))
        {
            replicationCacheMaster(c);
            return;
        }
    }
    ......
}

// 为了实现部分同步，从机会保存主机的状态信息后才会断开主机的连接，主机状态信息
// 保存在 server.cached_master
// 会在 freeClient() 中调用，保存与主机连接的状态信息，以便进行 PSYNC
void replicationCacheMaster(redisClient *c) {
    listNode *ln;

    redisAssert(server.master != NULL && server.cached_master == NULL);
    redisLog(REDIS_NOTICE,"Caching the disconnected master state.");

    // 从客户端列表删除主机的信息
    /* Remove from the list of clients, we don't want this client to be
     * listed by CLIENT LIST or processed in any way by batch operations. */
    ln = listSearchKey(server.clients,c);
    redisAssert(ln != NULL);
    listDelNode(server.clients,ln);

    // 保存主机的状态信息
    /* Save the master. Server.master will be set to null later by
     * replicationHandleMasterDisconnection(). */
    server.cached_master = server.master;

    // 注销事件，关闭连接
    /* Remove the event handlers and close the socket. We'll later reuse
     * the socket of the new connection with the master during PSYNC. */
    aeDeleteFileEvent(server.el,c->fd,AE_READABLE);
    aeDeleteFileEvent(server.el,c->fd,AE_WRITABLE);
    close(c->fd);

    /* Set fd to -1 so that we can safely call freeClient(c) later. */
    c->fd = -1;

    // 修改连接的状态，设置 server.master = NULL
    /* Caching the master happens instead of the actual freeClient() call,
     * so make sure to adjust the replication state. This function will
     * also set server.master to NULL. */
    replicationHandleMasterDisconnection();
}