什么时候调用这些函数呢？值得思考，其实，每次传世壶数据数据到客户端时，都会调用。

/* 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;

    /* If CLIENT_CLOSE_ASAP flag is set, we need not write anything. */
    if (c->flags & CLIENT_CLOSE_ASAP) return C_ERR;

    /* 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->conn) 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 CLIENT_PENDING_READ is set, we're in an IO thread and should
     * not install a write handler. Instead, it will be done by
     * handleClientsWithPendingReadsUsingThreads() upon return.
     */
    if (!clientHasPendingReplies(c) && !(c->flags & CLIENT_PENDING_READ))
            clientInstallWriteHandler(c);

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

/* -----------------------------------------------------------------------------
 * Low level functions to add more data to output buffers.
 * -------------------------------------------------------------------------- */

/* Attempts to add the reply to the static buffer in the client struct.
 * Returns C_ERR if the buffer is full, or the reply list is not empty,
 * in which case the reply must be added to the reply list. */
int _addReplyToBuffer(client *c, const char *s, size_t len) {
    size_t available = sizeof(c->buf)-c->bufpos;

    if (c->flags & CLIENT_CLOSE_AFTER_REPLY) return C_OK;

    /* If there already are entries in the reply list, we cannot
     * add anything more to the static buffer. */
    if (listLength(c->reply) > 0) return C_ERR;

    /* Check that the buffer has enough space available for this string. */
    if (len > available) return C_ERR;

    memcpy(c->buf+c->bufpos,s,len);
    c->bufpos+=len;
    return C_OK;
}

/* Adds the reply to the reply linked list.
 * Note: some edits to this function need to be relayed to AddReplyFromClient. */
void _addReplyProtoToList(client *c, const char *s, size_t len) {
    if (c->flags & CLIENT_CLOSE_AFTER_REPLY) return;

    listNode *ln = listLast(c->reply);
    clientReplyBlock *tail = ln? listNodeValue(ln): NULL;

    /* Note that 'tail' may be NULL even if we have a tail node, because when
     * addReplyDeferredLen() is used, it sets a dummy node to NULL just
     * fo fill it later, when the size of the bulk length is set. */

    /* Append to tail string when possible. */
    if (tail) {
        /* Copy the part we can fit into the tail, and leave the rest for a
         * new node */
        size_t avail = tail->size - tail->used;
        size_t copy = avail >= len? len: avail;
        memcpy(tail->buf + tail->used, s, copy);
        tail->used += copy;
        s += copy;
        len -= copy;
    }
    if (len) {
        /* Create a new node, make sure it is allocated to at
         * least PROTO_REPLY_CHUNK_BYTES */
        size_t size = len < PROTO_REPLY_CHUNK_BYTES? PROTO_REPLY_CHUNK_BYTES: len;
        tail = zmalloc(size + sizeof(clientReplyBlock));
        /* take over the allocation's internal fragmentation */
        tail->size = zmalloc_usable_size(tail) - sizeof(clientReplyBlock);
        tail->used = len;
        memcpy(tail->buf, s, len);
        listAddNodeTail(c->reply, tail);
        c->reply_bytes += tail->size;

        closeClientOnOutputBufferLimitReached(c, 1);
    }
}

/* -----------------------------------------------------------------------------
 * Higher level functions to queue data on the client output buffer.
 * The following functions are the ones that commands implementations will call.
 * -------------------------------------------------------------------------- */

/* 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)
            _addReplyProtoToList(c,obj->ptr,sdslen(obj->ptr));
    } else if (obj->encoding == OBJ_ENCODING_INT) {
        /* For integer encoded strings we just convert it into a string
         * using our optimized function, and attach the resulting string
         * to the output buffer. */
        char buf[32];
        size_t len = ll2string(buf,sizeof(buf),(long)obj->ptr);
        if (_addReplyToBuffer(c,buf,len) != C_OK)
            _addReplyProtoToList(c,buf,len);
    } else {
        serverPanic("Wrong obj->encoding in addReply()");
    }
}

/* Add the SDS 's' string to the client output buffer, as a side effect
 * the SDS string is freed. */
void addReplySds(client *c, sds s) {
    if (prepareClientToWrite(c) != C_OK) {
        /* The caller expects the sds to be free'd. */
        sdsfree(s);
        return;
    }
    if (_addReplyToBuffer(c,s,sdslen(s)) != C_OK)
        _addReplyProtoToList(c,s,sdslen(s));
    sdsfree(s);
}

/* This low level function just adds whatever protocol you send it to the
 * client buffer, trying the static buffer initially, and using the string
 * of objects if not possible.
 *
 * It is efficient because does not create an SDS object nor an Redis object
 * if not needed. The object will only be created by calling
 * _addReplyProtoToList() if we fail to extend the existing tail object
 * in the list of objects. */
void addReplyProto(client *c, const char *s, size_t len) {
    if (prepareClientToWrite(c) != C_OK) return;
    if (_addReplyToBuffer(c,s,len) != C_OK)
        _addReplyProtoToList(c,s,len);
}

/* Low level function called by the addReplyError...() functions.
 * It emits the protocol for a Redis error, in the form:
 *
 * -ERRORCODE Error Message<CR><LF>
 *
 * If the error code is already passed in the string 's', the error
 * code provided is used, otherwise the string "-ERR " for the generic
 * error code is automatically added.
 * Note that 's' must NOT end with \r\n. */
void addReplyErrorLength(client *c, const char *s, size_t len) {
    /* If the string already starts with "-..." then the error code
     * is provided by the caller. Otherwise we use "-ERR". */
    if (!len || s[0] != '-') addReplyProto(c,"-ERR ",5);
    addReplyProto(c,s,len);
    addReplyProto(c,"\r\n",2);
}

/* Do some actions after an error reply was sent (Log if needed, updates stats, etc.) */
void afterErrorReply(client *c, const char *s, size_t len) {
    /* Increment the global error counter */
    server.stat_total_error_replies++;
    /* Increment the error stats
     * If the string already starts with "-..." then the error prefix
     * is provided by the caller ( we limit the search to 32 chars). Otherwise we use "-ERR". */
    if (s[0] != '-') {
        incrementErrorCount("ERR", 3);
    } else {
        char *spaceloc = memchr(s, ' ', len < 32 ? len : 32);
        if (spaceloc) {
            const size_t errEndPos = (size_t)(spaceloc - s);
            incrementErrorCount(s+1, errEndPos-1);
        } else {
            /* Fallback to ERR if we can't retrieve the error prefix */
            incrementErrorCount("ERR", 3);
        }
    }

    /* Sometimes it could be normal that a slave replies to a master with
     * an error and this function gets called. Actually the error will never
     * be sent because addReply*() against master clients has no effect...
     * A notable example is:
     *
     *    EVAL 'redis.call("incr",KEYS[1]); redis.call("nonexisting")' 1 x
     *
     * Where the master must propagate the first change even if the second
     * will produce an error. However it is useful to log such events since
     * they are rare and may hint at errors in a script or a bug in Redis. */
    int ctype = getClientType(c);
    if (ctype == CLIENT_TYPE_MASTER || ctype == CLIENT_TYPE_SLAVE || c->id == CLIENT_ID_AOF) {
        char *to, *from;

        if (c->id == CLIENT_ID_AOF) {
            to = "AOF-loading-client";
            from = "server";
        } else if (ctype == CLIENT_TYPE_MASTER) {
            to = "master";
            from = "replica";
        } else {
            to = "replica";
            from = "master";
        }

        if (len > 4096) len = 4096;
        char *cmdname = c->lastcmd ? c->lastcmd->name : "<unknown>";
        serverLog(LL_WARNING,"== CRITICAL == This %s is sending an error "
                             "to its %s: '%.*s' after processing the command "
                             "'%s'", from, to, (int)len, s, cmdname);
        if (ctype == CLIENT_TYPE_MASTER && server.repl_backlog &&
            server.repl_backlog_histlen > 0)
        {
            showLatestBacklog();
        }
        server.stat_unexpected_error_replies++;
    }
}

/* The 'err' object is expected to start with -ERRORCODE and end with \r\n.
 * Unlike addReplyErrorSds and others alike which rely on addReplyErrorLength. */
void addReplyErrorObject(client *c, robj *err) {
    addReply(c, err);
    afterErrorReply(c, err->ptr, sdslen(err->ptr)-2); /* Ignore trailing \r\n */
}

/* See addReplyErrorLength for expectations from the input string. */
void addReplyError(client *c, const char *err) {
    addReplyErrorLength(c,err,strlen(err));
    afterErrorReply(c,err,strlen(err));
}

/* See addReplyErrorLength for expectations from the input string. */
/* As a side effect the SDS string is freed. */
void addReplyErrorSds(client *c, sds err) {
    addReplyErrorLength(c,err,sdslen(err));
    afterErrorReply(c,err,sdslen(err));
    sdsfree(err);
}

/* See addReplyErrorLength for expectations from the formatted string.
 * The formatted string is safe to contain \r and \n anywhere. */
void addReplyErrorFormat(client *c, const char *fmt, ...) {
    va_list ap;
    va_start(ap,fmt);
    sds s = sdscatvprintf(sdsempty(),fmt,ap);
    va_end(ap);
    /* Trim any newlines at the end (ones will be added by addReplyErrorLength) */
    s = sdstrim(s, "\r\n");
    /* Make sure there are no newlines in the middle of the string, otherwise
     * invalid protocol is emitted. */
    s = sdsmapchars(s, "\r\n", "  ",  2);
    addReplyErrorLength(c,s,sdslen(s));
    afterErrorReply(c,s,sdslen(s));
    sdsfree(s);
}

void addReplyStatusLength(client *c, const char *s, size_t len) {
    addReplyProto(c,"+",1);
    addReplyProto(c,s,len);
    addReplyProto(c,"\r\n",2);
}

void addReplyStatus(client *c, const char *status) {
    addReplyStatusLength(c,status,strlen(status));
}

void addReplyStatusFormat(client *c, const char *fmt, ...) {
    va_list ap;
    va_start(ap,fmt);
    sds s = sdscatvprintf(sdsempty(),fmt,ap);
    va_end(ap);
    addReplyStatusLength(c,s,sdslen(s));
    sdsfree(s);
}

/* Sometimes we are forced to create a new reply node, and we can't append to
 * the previous one, when that happens, we wanna try to trim the unused space
 * at the end of the last reply node which we won't use anymore. */
void trimReplyUnusedTailSpace(client *c) {
    listNode *ln = listLast(c->reply);
    clientReplyBlock *tail = ln? listNodeValue(ln): NULL;

    /* Note that 'tail' may be NULL even if we have a tail node, because when
     * addReplyDeferredLen() is used */
    if (!tail) return;

    /* We only try to trim the space is relatively high (more than a 1/4 of the
     * allocation), otherwise there's a high chance realloc will NOP.
     * Also, to avoid large memmove which happens as part of realloc, we only do
     * that if the used part is small.  */
    if (tail->size - tail->used > tail->size / 4 &&
        tail->used < PROTO_REPLY_CHUNK_BYTES)
    {
        size_t old_size = tail->size;
        tail = zrealloc(tail, tail->used + sizeof(clientReplyBlock));
        /* take over the allocation's internal fragmentation (at least for
         * memory usage tracking) */
        tail->size = zmalloc_usable_size(tail) - sizeof(clientReplyBlock);
        c->reply_bytes = c->reply_bytes + tail->size - old_size;
        listNodeValue(ln) = tail;
    }
}

/* Adds an empty object to the reply list that will contain the multi bulk
 * length, which is not known when this function is called. */
void *addReplyDeferredLen(client *c) {
    /* Note that we install the write event here even if the object is not
     * ready to be sent, since we are sure that before returning to the
     * event loop setDeferredAggregateLen() will be called. */
    if (prepareClientToWrite(c) != C_OK) return NULL;
    trimReplyUnusedTailSpace(c);
    listAddNodeTail(c->reply,NULL); /* NULL is our placeholder. */
    return listLast(c->reply);
}

void setDeferredReply(client *c, void *node, const char *s, size_t length) {
    listNode *ln = (listNode*)node;
    clientReplyBlock *next, *prev;

    /* Abort when *node is NULL: when the client should not accept writes
     * we return NULL in addReplyDeferredLen() */
    if (node == NULL) return;
    serverAssert(!listNodeValue(ln));

    /* Normally we fill this dummy NULL node, added by addReplyDeferredLen(),
     * with a new buffer structure containing the protocol needed to specify
     * the length of the array following. However sometimes there might be room
     * in the previous/next node so we can instead remove this NULL node, and
     * suffix/prefix our data in the node immediately before/after it, in order
     * to save a write(2) syscall later. Conditions needed to do it:
     *
     * - The prev node is non-NULL and has space in it or
     * - The next node is non-NULL,
     * - It has enough room already allocated
     * - And not too large (avoid large memmove) */
    if (ln->prev != NULL && (prev = listNodeValue(ln->prev)) &&
        prev->size - prev->used > 0)
    {
        size_t len_to_copy = prev->size - prev->used;
        if (len_to_copy > length)
            len_to_copy = length;
        memcpy(prev->buf + prev->used, s, len_to_copy);
        prev->used += len_to_copy;
        length -= len_to_copy;
        if (length == 0) {
            listDelNode(c->reply, ln);
            return;
        }
        s += len_to_copy;
    }

    if (ln->next != NULL && (next = listNodeValue(ln->next)) &&
        next->size - next->used >= length &&
        next->used < PROTO_REPLY_CHUNK_BYTES * 4)
    {
        memmove(next->buf + length, next->buf, next->used);
        memcpy(next->buf, s, length);
        next->used += length;
        listDelNode(c->reply,ln);
    } else {
        /* Create a new node */
        clientReplyBlock *buf = zmalloc(length + sizeof(clientReplyBlock));
        /* Take over the allocation's internal fragmentation */
        buf->size = zmalloc_usable_size(buf) - sizeof(clientReplyBlock);
        buf->used = length;
        memcpy(buf->buf, s, length);
        listNodeValue(ln) = buf;
        c->reply_bytes += buf->size;

        closeClientOnOutputBufferLimitReached(c, 1);
    }
}

/* Populate the length object and try gluing it to the next chunk. */
void setDeferredAggregateLen(client *c, void *node, long length, char prefix) {
    serverAssert(length >= 0);

    /* Abort when *node is NULL: when the client should not accept writes
     * we return NULL in addReplyDeferredLen() */
    if (node == NULL) return;

    char lenstr[128];
    size_t lenstr_len = sprintf(lenstr, "%c%ld\r\n", prefix, length);
    setDeferredReply(c, node, lenstr, lenstr_len);
}

void setDeferredArrayLen(client *c, void *node, long length) {
    setDeferredAggregateLen(c,node,length,'*');
}

void setDeferredMapLen(client *c, void *node, long length) {
    int prefix = c->resp == 2 ? '*' : '%';
    if (c->resp == 2) length *= 2;
    setDeferredAggregateLen(c,node,length,prefix);
}

void setDeferredSetLen(client *c, void *node, long length) {
    int prefix = c->resp == 2 ? '*' : '~';
    setDeferredAggregateLen(c,node,length,prefix);
}

void setDeferredAttributeLen(client *c, void *node, long length) {
    serverAssert(c->resp >= 3);
    setDeferredAggregateLen(c,node,length,'|');
}

void setDeferredPushLen(client *c, void *node, long length) {
    serverAssert(c->resp >= 3);
    setDeferredAggregateLen(c,node,length,'>');
}

/* Add a double as a bulk reply */
void addReplyDouble(client *c, double d) {
    if (isinf(d)) {
        /* Libc in odd systems (Hi Solaris!) will format infinite in a
         * different way, so better to handle it in an explicit way. */
        if (c->resp == 2) {
            addReplyBulkCString(c, d > 0 ? "inf" : "-inf");
        } else {
            addReplyProto(c, d > 0 ? ",inf\r\n" : ",-inf\r\n",
                              d > 0 ? 6 : 7);
        }
    } else {
        char dbuf[MAX_LONG_DOUBLE_CHARS+3],
             sbuf[MAX_LONG_DOUBLE_CHARS+32];
        int dlen, slen;
        if (c->resp == 2) {
            dlen = snprintf(dbuf,sizeof(dbuf),"%.17g",d);
            slen = snprintf(sbuf,sizeof(sbuf),"$%d\r\n%s\r\n",dlen,dbuf);
            addReplyProto(c,sbuf,slen);
        } else {
            dlen = snprintf(dbuf,sizeof(dbuf),",%.17g\r\n",d);
            addReplyProto(c,dbuf,dlen);
        }
    }
}

void addReplyBigNum(client *c, const char* num, size_t len) {
    if (c->resp == 2) {
        addReplyBulkCBuffer(c, num, len);
    } else {
        addReplyProto(c,"(",1);
        addReplyProto(c,num,len);
        addReply(c,shared.crlf);
    }
}

/* Add a long double as a bulk reply, but uses a human readable formatting
 * of the double instead of exposing the crude behavior of doubles to the
 * dear user. */
void addReplyHumanLongDouble(client *c, long double d) {
    if (c->resp == 2) {
        robj *o = createStringObjectFromLongDouble(d,1);
        addReplyBulk(c,o);
        decrRefCount(o);
    } else {
        char buf[MAX_LONG_DOUBLE_CHARS];
        int len = ld2string(buf,sizeof(buf),d,LD_STR_HUMAN);
        addReplyProto(c,",",1);
        addReplyProto(c,buf,len);
        addReplyProto(c,"\r\n",2);
    }
}