• Redis源码解析:19Hiredis异步API代码解析


             Hiredis中的异步API函数需要与事件库(libevent,libev, ev)一起工作。因为事件循环的机制,异步环境中的命令是自动管道化的。因为命令是异步发送的,因此发送命令时,必要情况下,需要提供一个回调函数,以便在收到命令回复时调用该函数。

             异步API涉及到的函数分别是:

    redisAsyncContext *redisAsyncConnect(const char *ip, int port);
    int redisAsyncCommand(redisAsyncContext *ac, redisCallbackFn *fn, void *privdata, const char *format, ...);
    int redisAsyncCommandArgv(redisAsyncContext *ac, redisCallbackFn *fn, void *privdata, int argc, const char **argv, const size_t *argvlen);
    void redisAsyncDisconnect(redisAsyncContext *ac);

             以上函数分别对应TCP建链、发送命令和TCP断链。

     

    一:异步上下文

             类似于同步操作API,异步操作API中也有一个上下文结构redisAsyncContext,用于维护异步链接中的各种状态。

             redisAsyncContext结构在同步上下文结构redisContext的基础上,增加了一些异步属性,它的定义如下:

    typedef struct redisAsyncContext {
        /* Hold the regular context, so it can be realloc'ed. */
        redisContext c;
    
        /* Setup error flags so they can be used directly. */
        int err;
        char *errstr;
    
        /* Not used by hiredis */
        void *data;
    
        /* Event library data and hooks */
        struct {
            void *data;
    
            /* Hooks that are called when the library expects to start
             * reading/writing. These functions should be idempotent. */
            void (*addRead)(void *privdata);
            void (*delRead)(void *privdata);
            void (*addWrite)(void *privdata);
            void (*delWrite)(void *privdata);
            void (*cleanup)(void *privdata);
        } ev;
    
        /* Called when either the connection is terminated due to an error or per
         * user request. The status is set accordingly (REDIS_OK, REDIS_ERR). */
        redisDisconnectCallback *onDisconnect;
    
        /* Called when the first write event was received. */
        redisConnectCallback *onConnect;
    
        /* Regular command callbacks */
        redisCallbackList replies;
    
        /* Subscription callbacks */
        struct {
            redisCallbackList invalid;
            struct dict *channels;
            struct dict *patterns;
        } sub;
    } redisAsyncContext;

             该结构的第一个属性就是同步上下文结构redisContext c,剩下的就是一些异步属性:

             结构体ev中包含了,当Hiredis异步API与事件库(libev,libevent, Redis ev)一起工作时,用于注册和删除读写事件的函数;

             回调函数onDisconnect,表示断链时会调用的函数,该属性可以通过redisAsyncSetDisconnectCallback函数设置;

             回调函数onConnect,表示TCP建链成功或失败之后会调用的函数,该属性可以通过redisAsyncSetConnectCallback函数设置;

             replies属性是一个redisCallbackList结构,也就是由回调结构redisCallback组成的单链表。当发送普通命令时,会依次将该命令对应的回调结构追加到链表中,当Redis服务器回复普通命令时,会依次调用该链表中的每个redisCallback结构中的回调函数;

             结构体sub用于处理订阅模式,其中的字典channels,以频道名为key,以回调结构redisCallback为value。当客户端使用Hiredis异步API发送”subscribe”命令后,服务器产生回复时,就会根据回复信息中的频道名查询字典channels,找到对应的回调结构,调用其中的回调函数。字典patterns与channels类似,只不过它用于”psubscirbe”命令,其中的key是频道名模式;回调链表invalid,当客户端处于订阅模式下,服务器发来了意想不到的回复时,会依次调用该链表中,每个回调结构中的回调函数。

     

    二:建链

             函数redisAsyncConnect执行异步操作中的TCP建链。

    redisAsyncContext *redisAsyncConnect(const char *ip, int port) {
        redisContext *c;
        redisAsyncContext *ac;
    
        c = redisConnectNonBlock(ip,port);
        if (c == NULL)
            return NULL;
    
        ac = redisAsyncInitialize(c);
        if (ac == NULL) {
            redisFree(c);
            return NULL;
        }
    
        __redisAsyncCopyError(ac);
        return ac;
    }

             该函数首先根据ip和port,调用redisConnectNonBlock函数向Redis服务器发起非阻塞的建链操作,然后调用redisAsyncInitialize函数创建异步上下文结构redisAsyncContext。

     

             redisAsyncSetConnectCallback函数用于设置异步上下文中的建链回调函数。其代码如下:

    int redisAsyncSetConnectCallback(redisAsyncContext *ac, redisConnectCallback *fn) {
        if (ac->onConnect == NULL) {
            ac->onConnect = fn;
    
            /* The common way to detect an established connection is to wait for
             * the first write event to be fired. This assumes the related event
             * library functions are already set. */
            _EL_ADD_WRITE(ac);
            return REDIS_OK;
        }
        return REDIS_ERR;
    }

             该函数中,如果之前已经设置过建链回调函数了,则直接返回REDIS_ERR。

             该函数除了设置异步上下文中的建链回调函数之外,还会调用_EL_ADD_WRITE,注册可写事件。对于使用Redis的ae事件库的客户端来说,该宏定义实际上就是调用redisAeAddWrite函数:

    static void redisAeAddWrite(void *privdata) {
        redisAeEvents *e = (redisAeEvents*)privdata;
        aeEventLoop *loop = e->loop;
        if (!e->writing) {
            e->writing = 1;
            aeCreateFileEvent(loop,e->fd,AE_WRITABLE,redisAeWriteEvent,e);
        }
    }

             可写事件的回调函数是redisAeWriteEvent,该函数调用redisAsyncHandleWrite实现。redisAsyncHandleWrite中,处理建链的代码如下:

    void redisAsyncHandleWrite(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
        int done = 0;
    
        if (!(c->flags & REDIS_CONNECTED)) {
            /* Abort connect was not successful. */
            if (__redisAsyncHandleConnect(ac) != REDIS_OK)
                return;
            /* Try again later when the context is still not connected. */
            if (!(c->flags & REDIS_CONNECTED))
                return;
        }
        . . .
    }

             在该函数中,如果上下文标志位中还没有设置REDIS_CONNECTED标记,说明目前还没有检测是否建链成功,因此调用__redisAsyncHandleConnect,判断建链是否成功,如果建链成功,则会在异步上下文的标志位中增加REDIS_CONNECTED标记,如果还没有建链成功,则直接返回。

     

             __redisAsyncHandleConnect的代码如下:

    static int __redisAsyncHandleConnect(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
    
        if (redisCheckSocketError(c) == REDIS_ERR) {
            /* Try again later when connect(2) is still in progress. */
            if (errno == EINPROGRESS)
                return REDIS_OK;
    
            if (ac->onConnect) ac->onConnect(ac,REDIS_ERR);
            __redisAsyncDisconnect(ac);
            return REDIS_ERR;
        }
    
        /* Mark context as connected. */
        c->flags |= REDIS_CONNECTED;
        if (ac->onConnect) ac->onConnect(ac,REDIS_OK);
        return REDIS_OK;
    }

             该函数中,首先调用redisCheckSocketError判断当前TCP是否建链成功,如果该函数返回REDIS_ERR,在errno为EINPROGRESS的情况下,说明TCP尚在建链中,这种情况直接返回REDIS_OK,等待下次处理;其他情况说明建链失败,以REDIS_ERR为参数,调用异步上下文中的建链回调函数,然后调用__redisAsyncDisconnect做清理工作,最后返回REDIS_ERR;

             如果redisCheckSocketError函数返回REDIS_OK,则将REDIS_CONNECTED标记增加到上下文标志位中,并以REDIS_OK为参数调用异步上下文中的建链回调函数;最后返回REDIS_OK;

     

    二:发送命令,接收回复

             redisAsyncCommand函数,是异步API中用于向Redis发送命令的函数。该函数与同步API中发送命令的函数redisCommand类似,同样支持printf式的可变参数。该函数的原型如下:

    int redisAsyncCommand(redisAsyncContext *ac, redisCallbackFn *fn, void *privdata, const char *format, ...);

             这里的fn和privdata分别表示收到命令回复后要调用的回调函数及其参数。因为Redis是单线程处理命令,因此当客户端使用异步API与事件库的结合之后,命令就自动的管道化了。也就是客户端在单线程模式下,发送命令的顺序和接收回复的顺序是一致的。因此,当发送命令时,就会将回调函数fn和参数privdata封装成回调结构redisCallback,并将该结构记录到单链表或者字典中。当收到回复后,就会依次得到链表或者字典中的redisCallback结构,调用其中的回调函数。

     

             redisAsyncCommand函数主要是调用redisvAsyncCommand实现,而redisvAsyncCommand函数又是通过调用redisvFormatCommand和__redisAsyncCommand函数实现的。

             redisvFormatCommand,解析用户输入的命令,转换成统一请求协议格式的字符串cmd,然后调用__redisAsyncCommand函数,将cmd发送给Redis,并且记录相应的回调函数。

     

             __redisAsyncCommand函数的代码如下:

    static int __redisAsyncCommand(redisAsyncContext *ac, redisCallbackFn *fn, void *privdata, char *cmd, size_t len) {
        redisContext *c = &(ac->c);
        redisCallback cb;
        int pvariant, hasnext;
        char *cstr, *astr;
        size_t clen, alen;
        char *p;
        sds sname;
    
        /* Don't accept new commands when the connection is about to be closed. */
        if (c->flags & (REDIS_DISCONNECTING | REDIS_FREEING)) return REDIS_ERR;
    
        /* Setup callback */
        cb.fn = fn;
        cb.privdata = privdata;
    
        /* Find out which command will be appended. */
        p = nextArgument(cmd,&cstr,&clen);
        assert(p != NULL);
        hasnext = (p[0] == '$');
        pvariant = (tolower(cstr[0]) == 'p') ? 1 : 0;
        cstr += pvariant;
        clen -= pvariant;
    
        if (hasnext && strncasecmp(cstr,"subscribe
    ",11) == 0) {
            c->flags |= REDIS_SUBSCRIBED;
    
            /* Add every channel/pattern to the list of subscription callbacks. */
            while ((p = nextArgument(p,&astr,&alen)) != NULL) {
                sname = sdsnewlen(astr,alen);
                if (pvariant)
                    dictReplace(ac->sub.patterns,sname,&cb);
                else
                    dictReplace(ac->sub.channels,sname,&cb);
            }
        } else if (strncasecmp(cstr,"unsubscribe
    ",13) == 0) {
            /* It is only useful to call (P)UNSUBSCRIBE when the context is
             * subscribed to one or more channels or patterns. */
            if (!(c->flags & REDIS_SUBSCRIBED)) return REDIS_ERR;
    
            /* (P)UNSUBSCRIBE does not have its own response: every channel or
             * pattern that is unsubscribed will receive a message. This means we
             * should not append a callback function for this command. */
         } else if(strncasecmp(cstr,"monitor
    ",9) == 0) {
             /* Set monitor flag and push callback */
             c->flags |= REDIS_MONITORING;
             __redisPushCallback(&ac->replies,&cb);
        } else {
            if (c->flags & REDIS_SUBSCRIBED)
                /* This will likely result in an error reply, but it needs to be
                 * received and passed to the callback. */
                __redisPushCallback(&ac->sub.invalid,&cb);
            else
                __redisPushCallback(&ac->replies,&cb);
        }
    
        __redisAppendCommand(c,cmd,len);
    
        /* Always schedule a write when the write buffer is non-empty */
        _EL_ADD_WRITE(ac);
    
        return REDIS_OK;
    }

             在函数中,首先将回调函数fn,以及用户提供的该回调函数的私有参数privdata,封装到redisCallback回调结构的cb中。当然,用户如果没有提供回调函数和参数,则cb中相应的属性为NULL。

     

             然后解析用户输入命令,根据不同的命令,将回调函数追加到不同的链表或字典中:

             如果用户输入命令为"subscribe"或者"psubscribe",首先将REDIS_SUBSCRIBED标记增加到上下文标志中,表示当前客户端进入订阅模式;

             然后循环解析命令中的后续参数,这些参数表示订阅的频道名("subscribe"),或者订阅的频道名的匹配模式("psubscribe")。以这些频道名或匹配模式为key,以回调结构cb为value,插入到异步上下文的字典ac->sub.patterns或ac->sub.channels中。

     

             如果用户输入命令为"unsubscribe",这种情况无需记录回调函数。但是该命令只有客户端处于订阅模式下才有效,否则直接返回REDIS_ERR;

     

             如果用户输入命令为"monitor",则将REDIS_MONITORING标记增加到上下文标志位中,表示客户端进入monitor模式,然后调用__redisPushCallback,将回调结构cb追加到上下文的回调链表ac->replies中;

             如果用户输入的是其他命令,则若当前客户端处于订阅模式,因处于订阅模式中,客户端只能发送”subscribe/psubscribe/unsubscribe/punsubscribe”命令,走到这一步,说明客户端发送了其他命令,因此将回调结构cb追加到链表ac->sub.invalid中;

             其他情况,将回调结构cb追加到链表ac->replies中;

     

             记录完回调函数之后,剩下的,就是调用__redisAppendCommand,将cmd追加到上下文的输出缓存中。

             然后调用_EL_ADD_WRITE,注册可写事件。对于使用Redis的ae事件库的客户端来说,该宏定义实际上就是调用redisAeAddWrite函数,可写事件的回调函数是redisAeWriteEvent,该函数调用redisAsyncHandleWrite实现。

             redisAsyncHandleWrite函数的全部代码如下:

    void redisAsyncHandleWrite(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
        int done = 0;
    
        if (!(c->flags & REDIS_CONNECTED)) {
            /* Abort connect was not successful. */
            if (__redisAsyncHandleConnect(ac) != REDIS_OK)
                return;
            /* Try again later when the context is still not connected. */
            if (!(c->flags & REDIS_CONNECTED))
                return;
        }
    
        if (redisBufferWrite(c,&done) == REDIS_ERR) {
            __redisAsyncDisconnect(ac);
        } else {
            /* Continue writing when not done, stop writing otherwise */
            if (!done)
                _EL_ADD_WRITE(ac);
            else
                _EL_DEL_WRITE(ac);
    
            /* Always schedule reads after writes */
            _EL_ADD_READ(ac);
        }
    }

             首先处理建链尚未成功的情况,之前已经讲过,不在赘述。

             建链成功之后,调用redisBufferWrite,将上下文中输出缓存的内容通过socket描述符发送出去。

             全部发送成功之后,调用_EL_DEL_WRITE,删除注册的可写事件。对于使用Redis的ae事件库的客户端来说,这里就是调用redisAeDelWrite函数,删除注册的可写事件。

             然后,调用_EL_ADD_READ,注册可读事件。对于使用Redis的ae事件库的客户端来说,这里就是调用redisAeAddRead函数,注册可读事件。事件回调函数为redisAeReadEvent。该回调函数主要是调用redisAsyncHandleRead实现。

     

             redisAsyncHandleRead函数的代码如下:

    void redisAsyncHandleRead(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
    
        if (!(c->flags & REDIS_CONNECTED)) {
            /* Abort connect was not successful. */
            if (__redisAsyncHandleConnect(ac) != REDIS_OK)
                return;
            /* Try again later when the context is still not connected. */
            if (!(c->flags & REDIS_CONNECTED))
                return;
        }
    
        if (redisBufferRead(c) == REDIS_ERR) {
            __redisAsyncDisconnect(ac);
        } else {
            /* Always re-schedule reads */
            _EL_ADD_READ(ac);
            redisProcessCallbacks(ac);
        }
    }

             该函数中,首先处理未建链的情况,与redisAsyncHandleWrite中的处理方式一致,不在赘述。

             建链成功之后,首先调用redisBufferRead,从socket中读取数据,并追加到解析器的输入缓存中,这在同步操作API中已讲过,不再赘述。

     

             读取成功之后,调用redisProcessCallbacks函数进行处理。该函数就是根据回复信息找到相应的回调结构,然后调用其中的回调函数。redisProcessCallbacks函数的代码如下:

    void redisProcessCallbacks(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
        redisCallback cb = {NULL, NULL, NULL};
        void *reply = NULL;
        int status;
    
        while((status = redisGetReply(c,&reply)) == REDIS_OK) {
            if (reply == NULL) {
                /* When the connection is being disconnected and there are
                 * no more replies, this is the cue to really disconnect. */
                if (c->flags & REDIS_DISCONNECTING && sdslen(c->obuf) == 0) {
                    __redisAsyncDisconnect(ac);
                    return;
                }
    
                /* If monitor mode, repush callback */
                if(c->flags & REDIS_MONITORING) {
                    __redisPushCallback(&ac->replies,&cb);
                }
    
                /* When the connection is not being disconnected, simply stop
                 * trying to get replies and wait for the next loop tick. */
                break;
            }
    
            /* Even if the context is subscribed, pending regular callbacks will
             * get a reply before pub/sub messages arrive. */
            if (__redisShiftCallback(&ac->replies,&cb) != REDIS_OK) {
                /*
                 * A spontaneous reply in a not-subscribed context can be the error
                 * reply that is sent when a new connection exceeds the maximum
                 * number of allowed connections on the server side.
                 *
                 * This is seen as an error instead of a regular reply because the
                 * server closes the connection after sending it.
                 *
                 * To prevent the error from being overwritten by an EOF error the
                 * connection is closed here. See issue #43.
                 *
                 * Another possibility is that the server is loading its dataset.
                 * In this case we also want to close the connection, and have the
                 * user wait until the server is ready to take our request.
                 */
                if (((redisReply*)reply)->type == REDIS_REPLY_ERROR) {
                    c->err = REDIS_ERR_OTHER;
                    snprintf(c->errstr,sizeof(c->errstr),"%s",((redisReply*)reply)->str);
                    c->reader->fn->freeObject(reply);
                    __redisAsyncDisconnect(ac);
                    return;
                }
                /* No more regular callbacks and no errors, the context *must* be subscribed or monitoring. */
                assert((c->flags & REDIS_SUBSCRIBED || c->flags & REDIS_MONITORING));
                if(c->flags & REDIS_SUBSCRIBED)
                    __redisGetSubscribeCallback(ac,reply,&cb);
            }
    
            if (cb.fn != NULL) {
                __redisRunCallback(ac,&cb,reply);
                c->reader->fn->freeObject(reply);
    
                /* Proceed with free'ing when redisAsyncFree() was called. */
                if (c->flags & REDIS_FREEING) {
                    __redisAsyncFree(ac);
                    return;
                }
            } else {
                /* No callback for this reply. This can either be a NULL callback,
                 * or there were no callbacks to begin with. Either way, don't
                 * abort with an error, but simply ignore it because the client
                 * doesn't know what the server will spit out over the wire. */
                c->reader->fn->freeObject(reply);
            }
        }
    
        /* Disconnect when there was an error reading the reply */
        if (status != REDIS_OK)
            __redisAsyncDisconnect(ac);
    }

             该函数循环调用redisGetReply,将解析器中输入缓存中的内容,组织成redisReply结构树,树的根节点通过参数reply返回。

             在循环中,如果取得的reply为NULL,说明输入缓存已空,这种情况下,如果当前上下文标志位中设置了REDIS_DISCONNECTING,说明之前某个命令的回调函数中,调用了redisAsyncDisconnect函数设置了该标记,因此在输出缓存为空,并且输入缓存也为空(reply为NULL)的条件下,调用__redisAsyncDisconnect开始执行断链操作,释放清理内存,最后返回。

             如果取得的reply为NULL,并且当前处于监控模式下,则将上次取出的回调结构cb,重新插入到链表ac->replies中。最后退出循环。

     

             如果取得的reply非空,则首先调用__redisShiftCallback,尝试从链表ac->replies中取出第一个回调结构cb。

             如果链表ac->replies已空,这种情况下,客户端要么是处于订阅模式下,要么就是服务器主动向客户端发送了某个错误信息,比如该客户端向服务器建链,服务器中已经超过了最大的客户端数,或者是服务器正在加载转储数据,而向客户端返回一个错误信息。

             如果回复类型为REDIS_REPLY_ERROR,则调用__redisAsyncDisconnect断链;如果回复类型不是REDIS_REPLY_ERROR,则当前客户端只能处于订阅模式或者监控模式,如果当前处于订阅模式下,则调用__redisGetSubscribeCallback,根据reply,从相应的字典中取出回调结构cb;

             取得回调结构cb之后,只要其中的回调函数不为空,就调用__redisRunCallback函数,调用其中的回调函数;对于回调函数为空的回调结构,直接释放reply即可。

            

             __redisGetSubscribeCallback函数根据回复信息,在字典结构中找到对应的回调结构并返回该结构。它的代码如下:

    static int __redisGetSubscribeCallback(redisAsyncContext *ac, redisReply *reply, redisCallback *dstcb) {
        redisContext *c = &(ac->c);
        dict *callbacks;
        dictEntry *de;
        int pvariant;
        char *stype;
        sds sname;
    
        /* Custom reply functions are not supported for pub/sub. This will fail
         * very hard when they are used... */
        if (reply->type == REDIS_REPLY_ARRAY) {
            assert(reply->elements >= 2);
            assert(reply->element[0]->type == REDIS_REPLY_STRING);
            stype = reply->element[0]->str;
            pvariant = (tolower(stype[0]) == 'p') ? 1 : 0;
    
            if (pvariant)
                callbacks = ac->sub.patterns;
            else
                callbacks = ac->sub.channels;
    
            /* Locate the right callback */
            assert(reply->element[1]->type == REDIS_REPLY_STRING);
            sname = sdsnewlen(reply->element[1]->str,reply->element[1]->len);
            de = dictFind(callbacks,sname);
            if (de != NULL) {
                memcpy(dstcb,dictGetEntryVal(de),sizeof(*dstcb));
    
                /* If this is an unsubscribe message, remove it. */
                if (strcasecmp(stype+pvariant,"unsubscribe") == 0) {
                    dictDelete(callbacks,sname);
    
                    /* If this was the last unsubscribe message, revert to
                     * non-subscribe mode. */
                    assert(reply->element[2]->type == REDIS_REPLY_INTEGER);
                    if (reply->element[2]->integer == 0)
                        c->flags &= ~REDIS_SUBSCRIBED;
                }
            }
            sdsfree(sname);
        } else {
            /* Shift callback for invalid commands. */
            __redisShiftCallback(&ac->sub.invalid,dstcb);
        }
        return REDIS_OK;
    }

             正常情况下,处于订阅模式下的客户端,接收到的消息类型应该是REDIS_REPLY_ARRAY类型,比如:

    1) "message"  
    2) "channel1"  
    3) "hi  
    
    1) "pmessage"  
    2) "channel.?*"  
    3) "channel.1"  
    4) "this is channel.1"  

             根据回复信息第一行的首字节是否为”p”,找到不同的字典结构callbacks。然后根据reply->element[1]的内容,也就是频道名或者频道名模式,从字典中找到相应的回调结构。

             如果Redis回复的信息是"unsubscribe",则从字典中删除相应的回调结构,此时reply->element[2]中的信息应该是个整数,表示当前客户端目前还订阅了多少频道,如果该值为0,表示客户端已经从最后一个频道中退订了,因此将REDIS_SUBSCRIBED标记从标志位c->flags中删除,表示该客户端退出订阅模式;

     

             如果Redis的回复信息不是REDIS_REPLY_ARRAY类型,说明发生了异常,此时从链表ac->sub.invalid中取出下一个回调结构即可。

     

    三:断链

             客户端可以通过调用redisAsyncDisconnect函数主动断链。该函数的代码如下:

    void redisAsyncDisconnect(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
        c->flags |= REDIS_DISCONNECTING;
        if (!(c->flags & REDIS_IN_CALLBACK) && ac->replies.head == NULL)
            __redisAsyncDisconnect(ac);
    }

             一般情况下,该函数是在某个命令回调函数中被调用。当调用该函数时,并不一定会立即进行断链操作,该函数将REDIS_DISCONNECTING标记增加到上下文的标志位中。只有当输出缓存中所有命令都发送完毕,并且收到他们的回复,调用了回调函数之后,才会真正的执行断链操作,这是在函数redisProcessCallbacks中处理的。

             设置了REDIS_DISCONNECTING标记后,在__redisAsyncCommand函数中,会直接返回REDIS_ERR,表示不再发送新的命令。

             真正的断链操作由函数__redisAsyncDisconnect实现。

     

             当客户与服务器之间的交互过程中发生了错误,或者是服务器主动断链时,就会调用__redisAsyncDisconnect进入断链流程。该函数代码如下:

    static void __redisAsyncDisconnect(redisAsyncContext *ac) {
        redisContext *c = &(ac->c);
    
        /* Make sure error is accessible if there is any */
        __redisAsyncCopyError(ac);
    
        if (ac->err == 0) {
            /* For clean disconnects, there should be no pending callbacks. */
            assert(__redisShiftCallback(&ac->replies,NULL) == REDIS_ERR);
        } else {
            /* Disconnection is caused by an error, make sure that pending
             * callbacks cannot call new commands. */
            c->flags |= REDIS_DISCONNECTING;
        }
    
        /* For non-clean disconnects, __redisAsyncFree() will execute pending
         * callbacks with a NULL-reply. */
        __redisAsyncFree(ac);
    }

             首先调用__redisAsyncCopyError,得到异步上下文中的err,如果err为0,则说明是客户端主动断链,这种情况下,链表ac->replies应该为空才对;否则,将上下文标志位中增加REDIS_DISCONNECTING标记,表明这是由于错误引起的断链,设置该标记后,不再发送新的命令给Redis。

             最终调用__redisAsyncFree函数,进行最后的清理。在__redisAsyncFree函数中,会议NULL为reply,调用所有异步上下文中尚存的回调函数。然后调用断链回调函数,最后调用redisFree关闭socket描述符,清理释放空间。

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  • 原文地址:https://www.cnblogs.com/gqtcgq/p/7247051.html
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