charles@charles-Aspire-4741:~/mydir/mylib/redis$ ./src/redis-cli
127.0.0.1:6379> lpush list hello
(integer) 1
127.0.0.1:6379> brpop list 0
1) "list"
2) "hello"
127.0.0.1:6379> brpop list 0
//阻塞在这里
/* ---------------------------------------------------- */
//当我在另一个客户端lpush一个元素之后，客户端输出为
127.0.0.1:6379> brpop list 0
1) "list"
2) "world"
(50.60s)//阻塞的时间

//阻塞状态
typedef struct blockingState {
 /* Generic fields. */
 mstime_t timeout;  /* 超时时间 */
 /* REDIS_BLOCK_LIST */
 dict *keys;    /* The keys we are waiting to terminate a blocking
        * operation such as BLPOP. Otherwise NULL. */
 robj *target;   /* The key that should receive the element,
        * for BRPOPLPUSH. */
 /* REDIS_BLOCK_WAIT */
 int numreplicas;  /* Number of replicas we are waiting for ACK. */
 long long reploffset; /* Replication offset to reach. */
} blockingState;
//继续列表
typedef struct readyList {
 redisDb *db;//就绪键所在的数据库
 robj *key;//就绪键
} readyList;
//客户端有关属性
typedef struct redisClient {
 int btype;    /* Type of blocking op if REDIS_BLOCKED. */
 blockingState bpop;  /* blocking state */
}
//服务器有关属性
struct redisServer {
  /* Blocked clients */
 unsigned int bpop_blocked_clients; /* Number of clients blocked by lists */
 list *unblocked_clients; /* list of clients to unblock before next loop */
 list *ready_keys;  /* List of readyList structures for BLPOP & co */
}
//数据库有关属性
typedef struct redisDb {
  //keys->redisCLient映射
  dict *blocking_keys;  /* Keys with clients waiting for data (BLPOP) */
 dict *ready_keys;   /* Blocked keys that received a PUSH */
}redisDB

void brpopCommand(redisClient *c) {
 blockingPopGenericCommand(c,REDIS_TAIL);
}
//++++++++++++++++++++++++++++++++++++++++++++++++++
void blockingPopGenericCommand(redisClient *c, int where) {
 robj *o;
 mstime_t timeout;
 int j;
 if (getTimeoutFromObjectOrReply(c,c->argv[c->argc-1],&timeout,UNIT_SECONDS)
  != REDIS_OK) return;//将超时时间保存在timeout中
 for (j = 1; j < c->argc-1; j++) {
  o = lookupKeyWrite(c->db,c->argv[j]);//在数据库中查找操作的链表
  if (o != NULL) {//如果不为空
   if (o->type != REDIS_LIST) {//不是链表类型
    addReply(c,shared.wrongtypeerr);//报错
    return;
   } else {
    if (listTypeLength(o) != 0) {//链表不为空
     /* Non empty list, this is like a non normal [LR]POP. */
     char *event = (where == REDIS_HEAD) "lpop" : "rpop";
     robj *value = listTypePop(o,where);//从链表中pop出一个元素
     redisAssert(value != NULL);
     //给客户端发送pop出来的元素信息
     addReplyMultiBulkLen(c,2);
     addReplyBulk(c,c->argv[j]);
     addReplyBulk(c,value);
     decrRefCount(value);
     notifyKeyspaceEvent(REDIS_NOTIFY_LIST,event,
          c->argv[j],c->db->id);
     if (listTypeLength(o) == 0) {//如果链表为空，从数据库删除链表
      dbDelete(c->db,c->argv[j]);
      notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,"del",
           c->argv[j],c->db->id);
     }
     /* 省略一部分 */
    }
   }
  }
 }
  /* 如果链表为空，则阻塞客户端 */
  blockForKeys(c, c->argv + 1, c->argc - 2, timeout, NULL);
}

void blockForKeys(redisClient *c, robj **keys, int numkeys, mstime_t timeout, robj *target) {
 dictEntry *de;
 list *l;
 int j;
 c->bpop.timeout = timeout;//超时时间赋值给客户端blockingState属性
 c->bpop.target = target;//这属性适用于brpoplpush命令的输入对象，如果是brpop, //则target为空
 if (target != NULL) incrRefCount(target);//不为空，增加引用计数
 for (j = 0; j < numkeys; j++) {
  /* 将阻塞的key存入c.bpop.keys字典中 */
  if (dictAdd(c->bpop.keys,keys[j],NULL) != DICT_OK) continue;
  incrRefCount(keys[j]);
  /* And in the other "side", to map keys -> clients */
  //将阻塞的key和客户端添加进c->db->blocking_keys
  de = dictFind(c->db->blocking_keys,keys[j]);
  if (de == NULL) {
   int retval;
   /* For every key we take a list of clients blocked for it */
   l = listCreate();
   retval = dictAdd(c->db->blocking_keys,keys[j],l);
   incrRefCount(keys[j]);
   redisAssertWithInfo(c,keys[j],retval == DICT_OK);
  } else {
   l = dictGetVal(de);
  }
  listAddNodeTail(l,c);//添加到阻塞键的客户点链表中
 }
 blockClient(c,REDIS_BLOCKED_LIST);//设置客户端阻塞标志
}

void blockClient(redisClient *c, int btype) {
 c->flags |= REDIS_BLOCKED;//设置标志
 c->btype = btype;//阻塞操作类型
 server.bpop_blocked_clients++;
}

if (listLength(server.ready_keys))
   handleClientsBlockedOnLists();

//db.c
void dbAdd(redisDb *db, robj *key, robj *val) {
 sds copy = sdsdup(key->ptr);
 int retval = dictAdd(db->dict, copy, val);//将数据添加进数据库
 redisAssertWithInfo(NULL,key,retval == REDIS_OK);
 //判断是否为链表类型，如果是，调用有链表已经ready函数
 if (val->type == REDIS_LIST) signalListAsReady(db, key);
 if (server.cluster_enabled) slotToKeyAdd(key);
 }
//t_list.c
void signalListAsReady(redisDb *db, robj *key) {
 readyList *rl;
 /* 没有客户端阻塞在这个键上，则直接返回. */
 if (dictFind(db->blocking_keys,key) == NULL) return;
 /* 这个键已近被唤醒了，所以没必要重新入队 */
 if (dictFind(db->ready_keys,key) != NULL) return;
 /* Ok, 除了上述两情况，把这个键放入server.ready_keys */
 rl = zmalloc(sizeof(*rl));
 rl->key = key;
 rl->db = db;
 incrRefCount(key);
 listAddNodeTail(server.ready_keys,rl);//添加链表末尾
 /* We also add the key in the db->ready_keys dictionary in order
  * to avoid adding it multiple times into a list with a simple O(1)
  * check. */
 incrRefCount(key);
 //同时将这个阻塞键放入db->ready_keys
 redisAssert(dictAdd(db->ready_keys,key,NULL) == DICT_OK);
}

void handleClientsBlockedOnLists(void) {
 while(listLength(server.ready_keys) != 0) {
  list *l;
  /* 将server.ready_keys赋给一个新的list,再将server.ready_keys清空 */
  l = server.ready_keys;
  server.ready_keys = listCreate();
  /* 迭代每一个就绪的每一个readyList */
  while(listLength(l) != 0) {
   listNode *ln = listFirst(l);//获取第一个就绪readyList
   readyList *rl = ln->value;
   /* 从rl所属的数据库中删除rl */
   dictDelete(rl->db->ready_keys,rl->key);
   /* 查询rl所属的数据库查找rl->key ,给阻塞客户端回复rl->key链表中的第一个元素*/
   robj *o = lookupKeyWrite(rl->db,rl->key);
   if (o != NULL && o->type == REDIS_LIST) {
    dictEntry *de;
    /* 在rl->db->blocking_keys查找阻塞在rl->key的客户端链表 */
    de = dictFind(rl->db->blocking_keys,rl->key);
    if (de) {
     list *clients = dictGetVal(de);//转换为客户端链表
     int numclients = listLength(clients);
     while(numclients--) {//给每个客户端发送消息
      listNode *clientnode = listFirst(clients);
      redisClient *receiver = clientnode->value;//阻塞的客户端
      robj *dstkey = receiver->bpop.target;//brpoplpush命令目的链表
      int where = (receiver->lastcmd &&
          receiver->lastcmd->proc == blpopCommand) "htmlcode">

int serveClientBlockedOnList(redisClient *receiver, robj *key, robj *dstkey, redisDb *db, robj *value, int where)
{
 robj *argv[3];
 if (dstkey == NULL) {
  /* Propagate the [LR]POP operation. */
  argv[0] = (where == REDIS_HEAD) "color: #ff0000">通过超时时间解阻塞


如果链表一直没有数据插入，那么客户端将会一直阻塞下去，这肯定是不行的，所以brpop还支持超时阻塞，即阻塞时间超过一定值之后，服务器返回一个空值，这样客户端就解脱阻塞了。
对于时间超时，都放在了100ms执行一次的时间事件中；超时解脱阻塞函数也在serverCron中；在serverCron->clientsCron->clientsCronHandleTimeout


int clientsCronHandleTimeout(redisClient *c, mstime_t now_ms) {
 time_t now = now_ms/1000;
 //..........
 else if (c->flags & REDIS_BLOCKED) {
  /* Blocked OPS timeout is handled with milliseconds resolution.
   * However note that the actual resolution is limited by
   * server.hz. */
  if (c->bpop.timeout != 0 && c->bpop.timeout < now_ms) {
   /* Handle blocking operation specific timeout. */
   replyToBlockedClientTimedOut(c);
   unblockClient(c);
  }
 }
 //.............


把这个函数不相干的代码删除，主要部分先判断这个客户端是否阻塞，如果是，超时时间是否到期，如果是，则调用replyToBlockedClientTimedOut给客户端回复一个空回复，以及接触客户端阻塞。
总结
链表消息队列实现暂时分析到这了，大家都学会了吗？希望这篇文章给大家能带来一定的帮助，如果有疑问可以留言交流。