Redis的pubsub.c是对pubsub功能的实现。
Redis的pub/sub是一个稳定快速的消息系统。
基于 ziplist 和 dict 实现。

主要包含了消息的发布、订阅、取消订阅功能。

在server.h有如下属性：

 /* Pubsub */
    dict *pubsub_channels;  /* Map channels to list of subscribed clients */
    list *pubsub_patterns;  /* A list of pubsub_patterns */

基于这两个属性，
client用pubsub_channels来保存client和channel列表之间的关系。
server用pubsub_channels来保存channel和client列表之间的关系。

本质上，pub/sub就是在操作这几个属性的值。

1. 消息发布

//publish命令 publish channel message
void publishCommand(client *c) {
    int receivers = pubsubPublishMessage(c->argv[1],c->argv[2]);
    if (server.cluster_enabled)
        clusterPropagatePublish(c->argv[1],c->argv[2]);
    else
        forceCommandPropagation(c,PROPAGATE_REPL);
    addReplyLongLong(c,receivers);
}

/* Publish a message */
//发布消息到指定channel
int pubsubPublishMessage(robj *channel, robj *message) {
    int receivers = 0;
    dictEntry *de;
    listNode *ln;
    listIter li;

    /* Send to clients listening for that channel */
    //查找订阅了channel的clients
    de = dictFind(server.pubsub_channels,channel);
    if (de) {
        list *list = dictGetVal(de);
        listNode *ln;
        listIter li;

        listRewind(list,&li);
        //遍历所有client
        while ((ln = listNext(&li)) != NULL) {
            client *c = ln->value;

            addReply(c,shared.mbulkhdr[3]);
            addReply(c,shared.messagebulk);
            addReplyBulk(c,channel);
            addReplyBulk(c,message);
            receivers++;
        }
    }
    /* Send to clients listening to matching channels */
    //查找匹配pattern的channel对应的clients
    if (listLength(server.pubsub_patterns)) {
        listRewind(server.pubsub_patterns,&li);
        channel = getDecodedObject(channel);
        //遍历所有client
        while ((ln = listNext(&li)) != NULL) {
            pubsubPattern *pat = ln->value;

            if (stringmatchlen((char*)pat->pattern->ptr,
                                sdslen(pat->pattern->ptr),
                                (char*)channel->ptr,
                                sdslen(channel->ptr),0)) {
                addReply(pat->client,shared.mbulkhdr[4]);
                addReply(pat->client,shared.pmessagebulk);
                addReplyBulk(pat->client,pat->pattern);
                addReplyBulk(pat->client,channel);
                addReplyBulk(pat->client,message);
                receivers++;
            }
        }
        decrRefCount(channel);
    }
    return receivers;
}

2. 消息订阅

//subscribe命令 subscribe channel [channel...]
void subscribeCommand(client *c) {
    int j;
    //获取所有的channel，然后执行订阅操作
    for (j = 1; j < c->argc; j++)
        pubsubSubscribeChannel(c,c->argv[j]);
    c->flags |= CLIENT_PUBSUB;
}

/* Subscribe a client to a channel. Returns 1 if the operation succeeded, or
 * 0 if the client was already subscribed to that channel. */
//client订阅channel
int pubsubSubscribeChannel(client *c, robj *channel) {
    dictEntry *de;
    //一个channel可以被多个client订阅
    //一个client可以订阅多个channel
    list *clients = NULL;
    int retval = 0;

    /* Add the channel to the client -> channels hash table */
    //c->pubsub_channels，保存的是一个client订阅的channel列表
    //将该channel加入到client的pubsub_channels中
    if (dictAdd(c->pubsub_channels,channel,NULL) == DICT_OK) {
        retval = 1;
        incrRefCount(channel);
        //通过server.pubsub_channels保存一个channel被订阅的client列表
        //查找该chanel的client列表
        de = dictFind(server.pubsub_channels,channel);
        if (de == NULL) {
            clients = listCreate();
            dictAdd(server.pubsub_channels,channel,clients);
            incrRefCount(channel);
        } else {
            clients = dictGetVal(de);
        }
        //将client加入到channel的pubsub_channels中
        listAddNodeTail(clients,c);
    }
    /* Notify the client */
    addReply(c,shared.mbulkhdr[3]);
    addReply(c,shared.subscribebulk);
    addReplyBulk(c,channel);
    addReplyLongLong(c,clientSubscriptionsCount(c));
    return retval;
}

3. 取消订阅

//ubsubscribe命令 unsubscribe channel [channel...]
void unsubscribeCommand(client *c) {
    //取消订阅所有channel
    if (c->argc == 1) {
        pubsubUnsubscribeAllChannels(c,1);
    } else {
        int j;
        //取消订阅指定channel
        for (j = 1; j < c->argc; j++)
            pubsubUnsubscribeChannel(c,c->argv[j],1);
    }
    if (clientSubscriptionsCount(c) == 0) c->flags &= ~CLIENT_PUBSUB;
}

/* Unsubscribe a client from a channel. Returns 1 if the operation succeeded, or
 * 0 if the client was not subscribed to the specified channel. */
//取消订阅channel
int pubsubUnsubscribeChannel(client *c, robj *channel, int notify) {
    dictEntry *de;
    list *clients;
    listNode *ln;
    int retval = 0;

    /* Remove the channel from the client -> channels hash table */
    incrRefCount(channel); /* channel may be just a pointer to the same object
                            we have in the hash tables. Protect it... */
    //从client的pubsub_channels中删除channel
    if (dictDelete(c->pubsub_channels,channel) == DICT_OK) {
        retval = 1;
        /* Remove the client from the channel -> clients list hash table */
        de = dictFind(server.pubsub_channels,channel);
        serverAssertWithInfo(c,NULL,de != NULL);
        clients = dictGetVal(de);
        ln = listSearchKey(clients,c);
        serverAssertWithInfo(c,NULL,ln != NULL);
        //从channel的pubsub_channels中删除client
        listDelNode(clients,ln);
        //如果channel的订阅者为空，则移除对象
        if (listLength(clients) == 0) {
            /* Free the list and associated hash entry at all if this was
             * the latest client, so that it will be possible to abuse
             * Redis PUBSUB creating millions of channels. */
            dictDelete(server.pubsub_channels,channel);
        }
    }
    /* Notify the client */
    if (notify) {
        addReply(c,shared.mbulkhdr[3]);
        addReply(c,shared.unsubscribebulk);
        addReplyBulk(c,channel);
        addReplyLongLong(c,dictSize(c->pubsub_channels)+
                       listLength(c->pubsub_patterns));

    }
    decrRefCount(channel); /* it is finally safe to release it */
    return retval;
}

/* Unsubscribe from all the channels. Return the number of channels the
 * client was subscribed to. */
//取消订阅所有的channel
int pubsubUnsubscribeAllChannels(client *c, int notify) {
    dictIterator *di = dictGetSafeIterator(c->pubsub_channels);
    dictEntry *de;
    int count = 0;

    //遍历client对应的pubsub_channels dict
    while((de = dictNext(di)) != NULL) {
        robj *channel = dictGetKey(de);
        //取消订阅
        count += pubsubUnsubscribeChannel(c,channel,notify);
    }
    /* We were subscribed to nothing? Still reply to the client. */
    if (notify && count == 0) {
        addReply(c,shared.mbulkhdr[3]);
        addReply(c,shared.unsubscribebulk);
        addReply(c,shared.nullbulk);
        addReplyLongLong(c,dictSize(c->pubsub_channels)+
                       listLength(c->pubsub_patterns));
    }
    dictReleaseIterator(di);
    return count;
}

4. 基于正则匹配订阅

//psubscribe命令 psubscribe pattern [pattern...]
void psubscribeCommand(client *c) {
    int j;
    //正则匹配订阅channel
    for (j = 1; j < c->argc; j++)
        pubsubSubscribePattern(c,c->argv[j]);
    c->flags |= CLIENT_PUBSUB;
}

/* Subscribe a client to a pattern. Returns 1 if the operation succeeded, or 0 if the client was already subscribed to that pattern. */
//正则匹配订阅channel
int pubsubSubscribePattern(client *c, robj *pattern) {
    int retval = 0;

    //正则匹配 list match
    //client的pubsub_patterns存储正则匹配channel的规则pattern
    if (listSearchKey(c->pubsub_patterns,pattern) == NULL) {
        retval = 1;
        //pubsubPattern存储client及其对应的pattern
        pubsubPattern *pat;
        listAddNodeTail(c->pubsub_patterns,pattern);
        incrRefCount(pattern);
        pat = zmalloc(sizeof(*pat));
        pat->pattern = getDecodedObject(pattern);
        pat->client = c;
        //将pubsubPattern存入server的pubsub_patterns
        listAddNodeTail(server.pubsub_patterns,pat);
    }
    /* Notify the client */
    addReply(c,shared.mbulkhdr[3]);
    addReply(c,shared.psubscribebulk);
    addReplyBulk(c,pattern);
    addReplyLongLong(c,clientSubscriptionsCount(c));
    return retval;
}

5. 基于正则匹配取消订阅

//punsubscribe命令 punsubscribe pattern [pattern...]
void punsubscribeCommand(client *c) {
    //取消订阅所有
    if (c->argc == 1) {
        pubsubUnsubscribeAllPatterns(c,1);
    } else {
        int j;
        //取消订阅符合指定正则的channel
        for (j = 1; j < c->argc; j++)
            pubsubUnsubscribePattern(c,c->argv[j],1);
    }
    if (clientSubscriptionsCount(c) == 0) c->flags &= ~CLIENT_PUBSUB;
}

/* Unsubscribe a client from a channel. Returns 1 if the operation succeeded, or
 * 0 if the client was not subscribed to the specified channel. */
//正则匹配取消订阅pattern
int pubsubUnsubscribePattern(client *c, robj *pattern, int notify) {
    listNode *ln;
    //存储client及对应的pattern
    pubsubPattern pat;
    int retval = 0;

    incrRefCount(pattern); /* Protect the object. May be the same we remove */
    //查找客户端对应的pattern是否存在
    if ((ln = listSearchKey(c->pubsub_patterns,pattern)) != NULL) {
        retval = 1;
        //存在则删除
        listDelNode(c->pubsub_patterns,ln);
        pat.client = c;
        pat.pattern = pattern;
        //查找服务端pattern中对应的client是否存在
        ln = listSearchKey(server.pubsub_patterns,&pat);
        //存在则删除
        listDelNode(server.pubsub_patterns,ln);
    }
    /* Notify the client */
    if (notify) {
        addReply(c,shared.mbulkhdr[3]);
        addReply(c,shared.punsubscribebulk);
        addReplyBulk(c,pattern);
        addReplyLongLong(c,dictSize(c->pubsub_channels)+
                       listLength(c->pubsub_patterns));
    }
    decrRefCount(pattern);
    return retval;
}

/* Unsubscribe from all the patterns. Return the number of patterns the
 * client was subscribed from. */
//取消订阅所有匹配正则的channel
int pubsubUnsubscribeAllPatterns(client *c, int notify) {
    listNode *ln;
    listIter li;
    int count = 0;

    listRewind(c->pubsub_patterns,&li);

    //遍历client对应的pubsub_channels dict
    while ((ln = listNext(&li)) != NULL) {
        robj *pattern = ln->value;
        //取消正则订阅
        count += pubsubUnsubscribePattern(c,pattern,notify);
    }
    if (notify && count == 0) {
        /* We were subscribed to nothing? Still reply to the client. */
        addReply(c,shared.mbulkhdr[3]);
        addReply(c,shared.punsubscribebulk);
        addReply(c,shared.nullbulk);
        addReplyLongLong(c,dictSize(c->pubsub_channels)+
                       listLength(c->pubsub_patterns));
    }
    return count;
}