Redis（六）：list/lpush/lrange/lpop 命令源码解析

    {"rpush",rpushCommand,-3,"wmF",0,NULL,1,1,1,0,0},    {"lpush",lpushCommand,-3,"wmF",0,NULL,1,1,1,0,0},    {"rpushx",rpushxCommand,3,"wmF",0,NULL,1,1,1,0,0},    {"lpushx",lpushxCommand,3,"wmF",0,NULL,1,1,1,0,0},    {"linsert",linsertCommand,5,"wm",0,NULL,1,1,1,0,0},    {"rpop",rpopCommand,2,"wF",0,NULL,1,1,1,0,0},    {"lpop",lpopCommand,2,"wF",0,NULL,1,1,1,0,0},    {"brpop",brpopCommand,-3,"ws",0,NULL,1,1,1,0,0},    {"brpoplpush",brpoplpushCommand,4,"wms",0,NULL,1,2,1,0,0},    {"blpop",blpopCommand,-3,"ws",0,NULL,1,-2,1,0,0},    {"llen",llenCommand,2,"rF",0,NULL,1,1,1,0,0},    {"lindex",lindexCommand,3,"r",0,NULL,1,1,1,0,0},    {"lset",lsetCommand,4,"wm",0,NULL,1,1,1,0,0},    {"lrange",lrangeCommand,4,"r",0,NULL,1,1,1,0,0},    {"ltrim",ltrimCommand,4,"w",0,NULL,1,1,1,0,0},    {"lrem",lremCommand,4,"w",0,NULL,1,1,1,0,0},    {"rpoplpush",rpoplpushCommand,3,"wm",0,NULL,1,2,1,0,0},

// quicklist 是其实数据容器，由head,tail 进行迭代，所以算是一个双向链表/* quicklist is a 32 byte struct (on 64-bit systems) describing a quicklist. * 'count' is the number of total entries. * 'len' is the number of quicklist nodes. * 'compress' is: -1 if compression disabled, otherwise it's the number *                of quicklistNodes to leave uncompressed at ends of quicklist. * 'fill' is the user-requested (or default) fill factor. */typedef struct quicklist {    // 头节点    quicklistNode *head;    // 尾节点    quicklistNode *tail;    // 现有元素个数    unsigned long count;        /* total count of all entries in all ziplists */    // 现有的 quicklistNode 个数，一个 node 可能包含n个元素    unsigned int len;           /* number of quicklistNodes */    // 填充因子    int fill : 16;              /* fill factor for individual nodes */    // 多深的链表无需压缩    unsigned int compress : 16; /* depth of end nodes not to compress;0=off */} quicklist;// 链表中的每个节点typedef struct quicklistEntry {    const quicklist *quicklist;    quicklistNode *node;    // 当前迭代元素的ziplist的偏移位置指针    unsigned char *zi;    // 纯粹的 value, 值来源 zi    unsigned char *value;    // 占用空间大小    unsigned int sz;    long long longval;    // 当前节点偏移    int offset;} quicklistEntry;// 链表元素节点使用 quicklistNode /* quicklistNode is a 32 byte struct describing a ziplist for a quicklist. * We use bit fields keep the quicklistNode at 32 bytes. * count: 16 bits, max 65536 (max zl bytes is 65k, so max count actually < 32k). * encoding: 2 bits, RAW=1, LZF=2. * container: 2 bits, NONE=1, ZIPLIST=2. * recompress: 1 bit, bool, true if node is temporarry decompressed for usage. * attempted_compress: 1 bit, boolean, used for verifying during testing. * extra: 12 bits, free for future use; pads out the remainder of 32 bits */typedef struct quicklistNode {    struct quicklistNode *prev;    struct quicklistNode *next;    // zl 为ziplist链表，保存count个元素值    unsigned char *zl;    unsigned int sz;             /* ziplist size in bytes */    unsigned int count : 16;     /* count of items in ziplist */    unsigned int encoding : 2;   /* RAW==1 or LZF==2 */    unsigned int container : 2;  /* NONE==1 or ZIPLIST==2 */    unsigned int recompress : 1; /* was this node previous compressed? */    unsigned int attempted_compress : 1; /* node can't compress; too small */    unsigned int extra : 10; /* more bits to steal for future usage */} quicklistNode;// list迭代器typedef struct quicklistIter {    const quicklist *quicklist;    quicklistNode *current;    unsigned char *zi;    long offset; /* offset in current ziplist */    int direction;} quicklistIter;// ziplist 数据结构 typedef struct zlentry {    unsigned int prevrawlensize, prevrawlen;    unsigned int lensize, len;    unsigned int headersize;    unsigned char encoding;    unsigned char *p;} zlentry;

// t_list.c, lpushvoid lpushCommand(client *c) {    // 使用 LIST_HEAD|LIST_TAIL 作为插入位置标识    pushGenericCommand(c,LIST_HEAD);}void rpushCommand(client *c) {    pushGenericCommand(c,LIST_TAIL);}// t_list.c, 实际的push操作void pushGenericCommand(client *c, int where) {    int j, waiting = 0, pushed = 0;    // 在db中查找对应的key实例，查到或者查不到    robj *lobj = lookupKeyWrite(c->db,c->argv[1]);    // 查到的情况下，需要验证数据类型    if (lobj && lobj->type != OBJ_LIST) {        addReply(c,shared.wrongtypeerr);        return;    }
    for (j = 2; j < c->argc; j++) {        c->argv[j] = tryObjectEncoding(c->argv[j]);        if (!lobj) {            // 1. 在没有key实例的情况下，先创建key实例到db中            lobj = createQuicklistObject();            // 2. 设置 fill和depth 参数            // fill 默认: -2            // depth 默认: 0            quicklistSetOptions(lobj->ptr, server.list_max_ziplist_size,                                server.list_compress_depth);            dbAdd(c->db,c->argv[1],lobj);        }        // 3. 一个个元素添加进去        listTypePush(lobj,c->argv[j],where);        pushed++;    }    // 返回list长度    addReplyLongLong(c, waiting + (lobj ? listTypeLength(lobj) : 0));    if (pushed) {        // 命令传播        char *event = (where == LIST_HEAD) ? "lpush" : "rpush";
        signalModifiedKey(c->db,c->argv[1]);        notifyKeyspaceEvent(NOTIFY_LIST,event,c->argv[1],c->db->id);    }    server.dirty += pushed;}// 1. 创建初始list// object.c, 创建初始listrobj *createQuicklistObject(void) {    quicklist *l = quicklistCreate();    robj *o = createObject(OBJ_LIST,l);    o->encoding = OBJ_ENCODING_QUICKLIST;    return o;}// quicklist.c, 创建一个新的list容器，初始化默认值/* Create a new quicklist. * Free with quicklistRelease(). */quicklist *quicklistCreate(void) {    struct quicklist *quicklist;
    quicklist = zmalloc(sizeof(*quicklist));    quicklist->head = quicklist->tail = NULL;    quicklist->len = 0;    quicklist->count = 0;    quicklist->compress = 0;    quicklist->fill = -2;    return quicklist;}
// 2. 设置quicklist 的fill和depth 值// quicklist.cvoid quicklistSetOptions(quicklist *quicklist, int fill, int depth) {    quicklistSetFill(quicklist, fill);    quicklistSetCompressDepth(quicklist, depth);}// quicklist.c, 设置 fill 参数void quicklistSetFill(quicklist *quicklist, int fill) {    if (fill > FILL_MAX) {        fill = FILL_MAX;    } else if (fill < -5) {        fill = -5;    }    quicklist->fill = fill;}// quicklist.c, 设置 depth 参数void quicklistSetCompressDepth(quicklist *quicklist, int compress) {    if (compress > COMPRESS_MAX) {        compress = COMPRESS_MAX;    } else if (compress < 0) {        compress = 0;    }    quicklist->compress = compress;}
// 3. 将元素添加进list中// t_list.c, /* The function pushes an element to the specified list object 'subject', * at head or tail position as specified by 'where'. * * There is no need for the caller to increment the refcount of 'value' as * the function takes care of it if needed. */void listTypePush(robj *subject, robj *value, int where) {    if (subject->encoding == OBJ_ENCODING_QUICKLIST) {        int pos = (where == LIST_HEAD) ? QUICKLIST_HEAD : QUICKLIST_TAIL;        // 解码value        value = getDecodedObject(value);        size_t len = sdslen(value->ptr);        // 将value添加到链表中        quicklistPush(subject->ptr, value->ptr, len, pos);        // 减小value的引用，如果是被解编码后的对象，此时会将内存释放        decrRefCount(value);    } else {        serverPanic("Unknown list encoding");    }}// object.c/* Get a decoded version of an encoded object (returned as a new object). * If the object is already raw-encoded just increment the ref count. */robj *getDecodedObject(robj *o) {    robj *dec;    // OBJ_ENCODING_RAW,OBJ_ENCODING_EMBSTR 编码直接返回,引用计数+1(原因是: 原始robj一个引用，转换后的robj一个引用)    if (sdsEncodedObject(o)) {        incrRefCount(o);        return o;    }    if (o->type == OBJ_STRING && o->encoding == OBJ_ENCODING_INT) {        char buf[32];        // 整型转换为字符型，返回string型的robj        ll2string(buf,32,(long)o->ptr);        dec = createStringObject(buf,strlen(buf));        return dec;    } else {        serverPanic("Unknown encoding type");    }}
// quicklist.c, 添加value到链表中/* Wrapper to allow argument-based switching between HEAD/TAIL pop */void quicklistPush(quicklist *quicklist, void *value, const size_t sz,                   int where) {    // 根据where决定添加到表头还表尾    if (where == QUICKLIST_HEAD) {        quicklistPushHead(quicklist, value, sz);    } else if (where == QUICKLIST_TAIL) {        quicklistPushTail(quicklist, value, sz);    }}// quicklist.c, 添加表头数据/* Add new entry to head node of quicklist. * * Returns 0 if used existing head. * Returns 1 if new head created. */int quicklistPushHead(quicklist *quicklist, void *value, size_t sz) {    quicklistNode *orig_head = quicklist->head;    // likely 对不同平台处理 __builtin_expect(!!(x), 1),     // 判断是否允许插入元素,实际上是判断 head 的ziplist空间是否已占满, 没有则直接往里面插入即可    // fill 默认: -2    // depth 默认: 0    if (likely(            _quicklistNodeAllowInsert(quicklist->head, quicklist->fill, sz))) {        // 3.1. 添加head节点的zl链表中, zl 为ziplist 链表节点        quicklist->head->zl =            ziplistPush(quicklist->head->zl, value, sz, ZIPLIST_HEAD);        // 3.2. 更新头节点size大小        quicklistNodeUpdateSz(quicklist->head);    } else {        // 如果head已占满，则创建一个新的 quicklistNode 节点进行插入        quicklistNode *node = quicklistCreateNode();        node->zl = ziplistPush(ziplistNew(), value, sz, ZIPLIST_HEAD);
        quicklistNodeUpdateSz(node);        // 3.3. 插入新节点到head之前        _quicklistInsertNodeBefore(quicklist, quicklist->head, node);    }    // 将链表计数+1, 避免获取总数时迭代计算    quicklist->count++;    quicklist->head->count++;    return (orig_head != quicklist->head);}// quicklist.c, 判断是否允许插入元素REDIS_STATIC int _quicklistNodeAllowInsert(const quicklistNode *node,                                           const int fill, const size_t sz) {    if (unlikely(!node))        return 0;
    int ziplist_overhead;    /* size of previous offset */    if (sz < 254)        ziplist_overhead = 1;    else        ziplist_overhead = 5;
    /* size of forward offset */    if (sz < 64)        ziplist_overhead += 1;    else if (likely(sz < 16384))        ziplist_overhead += 2;    else        ziplist_overhead += 5;
    /* new_sz overestimates if 'sz' encodes to an integer type */    // 加上需要添加的新元素的长度后，进行阀值判定，如果在阀值内，则返回1，否则返回0    unsigned int new_sz = node->sz + sz + ziplist_overhead;    // 使用fill参数判定    if (likely(_quicklistNodeSizeMeetsOptimizationRequirement(new_sz, fill)))        return 1;    else if (!sizeMeetsSafetyLimit(new_sz))        return 0;    else if ((int)node->count < fill)        return 1;    else        return 0;}// quicklist.c REDIS_STATIC int_quicklistNodeSizeMeetsOptimizationRequirement(const size_t sz,                                               const int fill) {    if (fill >= 0)        return 0;
    size_t offset = (-fill) - 1;    // /* Optimization levels for size-based filling */    // static const size_t optimization_level[] = {4096, 8192, 16384, 32768, 65536};    // offset < 5, offset 默认将等于 1, sz <= 8292    if (offset < (sizeof(optimization_level) / sizeof(*optimization_level))) {        if (sz <= optimization_level[offset]) {            return 1;        } else {            return 0;        }    } else {        return 0;    }}// SIZE_SAFETY_LIMIT 8192#define sizeMeetsSafetyLimit(sz) ((sz) <= SIZE_SAFETY_LIMIT)
// 3.1. 向每个链表节点中添加value, 实际是向 ziplist push 数据// ziplist.c, push *s 数据到 zl 中unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where) {    unsigned char *p;    p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);    // 具体添加元素方法，有点复杂。简单点说就是 判断容量、扩容、按照ziplist协议添加元素    return __ziplistInsert(zl,p,s,slen);}// ziplist.c, 在hash的数据介绍时已详细介绍/* Insert item at "p". */static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {    size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen;    unsigned int prevlensize, prevlen = 0;    size_t offset;    int nextdiff = 0;    unsigned char encoding = 0;    long long value = 123456789; /* initialized to avoid warning. Using a value                                    that is easy to see if for some reason                                    we use it uninitialized. */    zlentry tail;
    /* Find out prevlen for the entry that is inserted. */    if (p[0] != ZIP_END) {        ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);    } else {        unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);        if (ptail[0] != ZIP_END) {            prevlen = zipRawEntryLength(ptail);        }    }
    /* See if the entry can be encoded */    if (zipTryEncoding(s,slen,&value,&encoding)) {        /* 'encoding' is set to the appropriate integer encoding */        reqlen = zipIntSize(encoding);    } else {        /* 'encoding' is untouched, however zipEncodeLength will use the         * string length to figure out how to encode it. */        reqlen = slen;    }    /* We need space for both the length of the previous entry and     * the length of the payload. */    reqlen += zipPrevEncodeLength(NULL,prevlen);    reqlen += zipEncodeLength(NULL,encoding,slen);
    /* When the insert position is not equal to the tail, we need to     * make sure that the next entry can hold this entry's length in     * its prevlen field. */    nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0;
    /* Store offset because a realloc may change the address of zl. */    offset = p-zl;    zl = ziplistResize(zl,curlen+reqlen+nextdiff);    p = zl+offset;
    /* Apply memory move when necessary and update tail offset. */    if (p[0] != ZIP_END) {        /* Subtract one because of the ZIP_END bytes */        memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);
        /* Encode this entry's raw length in the next entry. */        zipPrevEncodeLength(p+reqlen,reqlen);
        /* Update offset for tail */        ZIPLIST_TAIL_OFFSET(zl) =            intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+reqlen);
        /* When the tail contains more than one entry, we need to take         * "nextdiff" in account as well. Otherwise, a change in the         * size of prevlen doesn't have an effect on the *tail* offset. */        zipEntry(p+reqlen, &tail);        if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {            ZIPLIST_TAIL_OFFSET(zl) =                intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);        }    } else {        /* This element will be the new tail. */        ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);    }
    /* When nextdiff != 0, the raw length of the next entry has changed, so     * we need to cascade the update throughout the ziplist */    if (nextdiff != 0) {        offset = p-zl;        zl = __ziplistCascadeUpdate(zl,p+reqlen);        p = zl+offset;    }
    /* Write the entry */    p += zipPrevEncodeLength(p,prevlen);    p += zipEncodeLength(p,encoding,slen);    if (ZIP_IS_STR(encoding)) {        memcpy(p,s,slen);    } else {        zipSaveInteger(p,value,encoding);    }    ZIPLIST_INCR_LENGTH(zl,1);    return zl;}// 3.2. 更新node的size （实际占用内存空间大小）// quicklist.c, 更新node的size, 其实就是重新统计node的ziplist长度#define quicklistNodeUpdateSz(node)                                            \    do {                                                                       \        (node)->sz = ziplistBlobLen((node)->zl);                               \    } while (0)
// 3.3. 添加新链表节点到head之前// quicklist.c, /* Wrappers for node inserting around existing node. */REDIS_STATIC void _quicklistInsertNodeBefore(quicklist *quicklist,                                             quicklistNode *old_node,                                             quicklistNode *new_node) {    __quicklistInsertNode(quicklist, old_node, new_node, 0);}/* Insert 'new_node' after 'old_node' if 'after' is 1. * Insert 'new_node' before 'old_node' if 'after' is 0. * Note: 'new_node' is *always* uncompressed, so if we assign it to *       head or tail, we do not need to uncompress it. */REDIS_STATIC void __quicklistInsertNode(quicklist *quicklist,                                        quicklistNode *old_node,                                        quicklistNode *new_node, int after) {    if (after) {        new_node->prev = old_node;        if (old_node) {            new_node->next = old_node->next;            if (old_node->next)                old_node->next->prev = new_node;            old_node->next = new_node;        }        if (quicklist->tail == old_node)            quicklist->tail = new_node;    } else {        // 插入new_node到old_node之前        new_node->next = old_node;        if (old_node) {            new_node->prev = old_node->prev;            if (old_node->prev)                old_node->prev->next = new_node;            old_node->prev = new_node;        }        // 替换头节点位置        if (quicklist->head == old_node)            quicklist->head = new_node;    }    /* If this insert creates the only element so far, initialize head/tail. */    // 第一个元素    if (quicklist->len == 0) {        quicklist->head = quicklist->tail = new_node;    }    // 压缩list    if (old_node)        quicklistCompress(quicklist, old_node);
    quicklist->len++;}// quicklist.c, 压缩list#define quicklistCompress(_ql, _node)                                          \    do {                                                                       \        if ((_node)->recompress)                                               \            // recompress            quicklistCompressNode((_node));                                    \        else                                                                   \            //             __quicklistCompress((_ql), (_node));                               \    } while (0)// recompress    /* Compress only uncompressed nodes. */#define quicklistCompressNode(_node)                                           \    do {                                                                       \        if ((_node) && (_node)->encoding == QUICKLIST_NODE_ENCODING_RAW) {     \            __quicklistCompressNode((_node));                                  \        }                                                                      \    } while (0)/* Compress the ziplist in 'node' and update encoding details. * Returns 1 if ziplist compressed successfully. * Returns 0 if compression failed or if ziplist too small to compress. */REDIS_STATIC int __quicklistCompressNode(quicklistNode *node) {#ifdef REDIS_TEST    node->attempted_compress = 1;#endif
    /* Don't bother compressing small values */    if (node->sz < MIN_COMPRESS_BYTES)        return 0;
    quicklistLZF *lzf = zmalloc(sizeof(*lzf) + node->sz);
    /* Cancel if compression fails or doesn't compress small enough */    // lzf 压缩算法，有点复杂咯    if (((lzf->sz = lzf_compress(node->zl, node->sz, lzf->compressed,                                 node->sz)) == 0) ||        lzf->sz + MIN_COMPRESS_IMPROVE >= node->sz) {        /* lzf_compress aborts/rejects compression if value not compressable. */        zfree(lzf);        return 0;    }    lzf = zrealloc(lzf, sizeof(*lzf) + lzf->sz);    zfree(node->zl);    node->zl = (unsigned char *)lzf;    node->encoding = QUICKLIST_NODE_ENCODING_LZF;    node->recompress = 0;    return 1;}
/* Force 'quicklist' to meet compression guidelines set by compress depth. * The only way to guarantee interior nodes get compressed is to iterate * to our "interior" compress depth then compress the next node we find. * If compress depth is larger than the entire list, we return immediately. */REDIS_STATIC void __quicklistCompress(const quicklist *quicklist,                                      quicklistNode *node) {    /* If length is less than our compress depth (from both sides),     * we can't compress anything. */    if (!quicklistAllowsCompression(quicklist) ||        quicklist->len < (unsigned int)(quicklist->compress * 2))        return;
#if 0    /* Optimized cases for small depth counts */    if (quicklist->compress == 1) {        quicklistNode *h = quicklist->head, *t = quicklist->tail;        quicklistDecompressNode(h);        quicklistDecompressNode(t);        if (h != node && t != node)            quicklistCompressNode(node);        return;    } else if (quicklist->compress == 2) {        quicklistNode *h = quicklist->head, *hn = h->next, *hnn = hn->next;        quicklistNode *t = quicklist->tail, *tp = t->prev, *tpp = tp->prev;        quicklistDecompressNode(h);        quicklistDecompressNode(hn);        quicklistDecompressNode(t);        quicklistDecompressNode(tp);        if (h != node && hn != node && t != node && tp != node) {            quicklistCompressNode(node);        }        if (hnn != t) {            quicklistCompressNode(hnn);        }        if (tpp != h) {            quicklistCompressNode(tpp);        }        return;    }#endif
    /* Iterate until we reach compress depth for both sides of the list.a     * Note: because we do length checks at the *top* of this function,     *       we can skip explicit null checks below. Everything exists. */    quicklistNode *forward = quicklist->head;    quicklistNode *reverse = quicklist->tail;    int depth = 0;    int in_depth = 0;    while (depth++ < quicklist->compress) {        // 解压缩？？？        quicklistDecompressNode(forward);        quicklistDecompressNode(reverse);
        if (forward == node || reverse == node)            in_depth = 1;
        if (forward == reverse)            return;
        forward = forward->next;        reverse = reverse->prev;    }
    if (!in_depth)        quicklistCompressNode(node);
    if (depth > 2) {        /* At this point, forward and reverse are one node beyond depth */        // 压缩        quicklistCompressNode(forward);        quicklistCompressNode(reverse);    }}
/* Decompress only compressed nodes. */#define quicklistDecompressNode(_node)                                         \    do {                                                                       \        if ((_node) && (_node)->encoding == QUICKLIST_NODE_ENCODING_LZF) {     \            __quicklistDecompressNode((_node));                                \        }                                                                      \    } while (0)/* Uncompress the ziplist in 'node' and update encoding details. * Returns 1 on successful decode, 0 on failure to decode. */REDIS_STATIC int __quicklistDecompressNode(quicklistNode *node) {#ifdef REDIS_TEST    node->attempted_compress = 0;#endif
    void *decompressed = zmalloc(node->sz);    quicklistLZF *lzf = (quicklistLZF *)node->zl;    if (lzf_decompress(lzf->compressed, lzf->sz, decompressed, node->sz) == 0) {        /* Someone requested decompress, but we can't decompress.  Not good. */        zfree(decompressed);        return 0;    }    zfree(lzf);    node->zl = decompressed;    node->encoding = QUICKLIST_NODE_ENCODING_RAW;    return 1;}

// 1. 单个查询 lindex key index// t_list.c, 通过下标查找元素值void lindexCommand(client *c) {    robj *o = lookupKeyReadOrReply(c,c->argv[1],shared.nullbulk);    // 如果key本身就不存在，直接返回，空已响应    if (o == NULL || checkType(c,o,OBJ_LIST)) return;    long index;    robj *value = NULL;    // 解析index字段，赋给 index 变量    if ((getLongFromObjectOrReply(c, c->argv[2], &index, NULL) != C_OK))        return;
    if (o->encoding == OBJ_ENCODING_QUICKLIST) {        quicklistEntry entry;        // 根据index查询list数据        if (quicklistIndex(o->ptr, index, &entry)) {            // 使用两个字段来保存value            if (entry.value) {                value = createStringObject((char*)entry.value,entry.sz);            } else {                value = createStringObjectFromLongLong(entry.longval);            }            addReplyBulk(c,value);            decrRefCount(value);        } else {            addReply(c,shared.nullbulk);        }    } else {        serverPanic("Unknown list encoding");    }}// quicklist.c, 根据 index 查找元素/* Populate 'entry' with the element at the specified zero-based index * where 0 is the head, 1 is the element next to head * and so on. Negative integers are used in order to count * from the tail, -1 is the last element, -2 the penultimate * and so on. If the index is out of range 0 is returned. * * Returns 1 if element found * Returns 0 if element not found */int quicklistIndex(const quicklist *quicklist, const long long idx,                   quicklistEntry *entry) {    quicklistNode *n;    unsigned long long accum = 0;    unsigned long long index;    int forward = idx < 0 ? 0 : 1; /* < 0 -> reverse, 0+ -> forward */    // 初始化 quicklistEntry, 设置默认值    initEntry(entry);    entry->quicklist = quicklist;    // index为负数时，逆向搜索    if (!forward) {        index = (-idx) - 1;        n = quicklist->tail;    } else {        index = idx;        n = quicklist->head;    }
    if (index >= quicklist->count)        return 0;
    while (likely(n)) {        // n->count 代表每个list节点里的实际元素的个数（ziplist里可能包含n个元素）        // 此处代表只会迭代到 index 所在的list节点就停止了        if ((accum + n->count) > index) {            break;        } else {            D("Skipping over (%p) %u at accum %lld", (void *)n, n->count,              accum);            // 依次迭代            accum += n->count;            n = forward ? n->next : n->prev;        }    }    // 如果已经迭代完成，说明未找到index元素    if (!n)        return 0;
    D("Found node: %p at accum %llu, idx %llu, sub+ %llu, sub- %llu", (void *)n,      accum, index, index - accum, (-index) - 1 + accum);
    entry->node = n;    if (forward) {        /* forward = normal head-to-tail offset. */        // index-accum 代表index节点在 当前n节点中的偏移        entry->offset = index - accum;    } else {        /* reverse = need negative offset for tail-to-head, so undo         * the result of the original if (index < 0) above. */        // 逆向搜索定位 如-1=1-1+0,-2=2-1+0        entry->offset = (-index) - 1 + accum;    }    // 解压缩node数据    quicklistDecompressNodeForUse(entry->node);    // 根据offset，查找ziplist中的sds value    entry->zi = ziplistIndex(entry->node->zl, entry->offset);    // 从zi中获取value,sz,longval 返回 (ziplist 协议)    ziplistGet(entry->zi, &entry->value, &entry->sz, &entry->longval);    /* The caller will use our result, so we don't re-compress here.     * The caller can recompress or delete the node as needed. */    return 1;}// quicklist.c/* Simple way to give quicklistEntry structs default values with one call. */#define initEntry(e)                                                           \    do {                                                                       \        (e)->zi = (e)->value = NULL;                                           \        (e)->longval = -123456789;                                             \        (e)->quicklist = NULL;                                                 \        (e)->node = NULL;                                                      \        (e)->offset = 123456789;                                               \        (e)->sz = 0;                                                           \    } while (0)// 解压缩node数据/* Force node to not be immediately re-compresable */#define quicklistDecompressNodeForUse(_node)                                   \    do {                                                                       \        if ((_node) && (_node)->encoding == QUICKLIST_NODE_ENCODING_LZF) {     \            __quicklistDecompressNode((_node));                                \            (_node)->recompress = 1;                                           \        }                                                                      \    } while (0)/* Uncompress the ziplist in 'node' and update encoding details. * Returns 1 on successful decode, 0 on failure to decode. */REDIS_STATIC int __quicklistDecompressNode(quicklistNode *node) {#ifdef REDIS_TEST    node->attempted_compress = 0;#endif
    void *decompressed = zmalloc(node->sz);    quicklistLZF *lzf = (quicklistLZF *)node->zl;    if (lzf_decompress(lzf->compressed, lzf->sz, decompressed, node->sz) == 0) {        /* Someone requested decompress, but we can't decompress.  Not good. */        zfree(decompressed);        return 0;    }    zfree(lzf);    node->zl = decompressed;    node->encoding = QUICKLIST_NODE_ENCODING_RAW;    return 1;}// ziplist.c/* Returns an offset to use for iterating with ziplistNext. When the given * index is negative, the list is traversed back to front. When the list * doesn't contain an element at the provided index, NULL is returned. */unsigned char *ziplistIndex(unsigned char *zl, int index) {    unsigned char *p;    unsigned int prevlensize, prevlen = 0;    if (index < 0) {        index = (-index)-1;        p = ZIPLIST_ENTRY_TAIL(zl);        if (p[0] != ZIP_END) {            ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);            while (prevlen > 0 && index--) {                p -= prevlen;                ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);            }        }    } else {        p = ZIPLIST_ENTRY_HEAD(zl);        while (p[0] != ZIP_END && index--) {            p += zipRawEntryLength(p);        }    }    return (p[0] == ZIP_END || index > 0) ? NULL : p;}

// 2. lrange 范围查询// t_list.cvoid lrangeCommand(client *c) {    robj *o;    long start, end, llen, rangelen;    // 解析 start,end 参数    if ((getLongFromObjectOrReply(c, c->argv[2], &start, NULL) != C_OK) ||        (getLongFromObjectOrReply(c, c->argv[3], &end, NULL) != C_OK)) return;
    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.emptymultibulk)) == NULL         || checkType(c,o,OBJ_LIST)) return;    // list 长度获取, 有个计数器在    llen = listTypeLength(o);
    /* convert negative indexes */    if (start < 0) start = llen+start;    if (end < 0) end = llen+end;    // 将-xx的下标转换为正数查询，如果负数过大，则以0计算    if (start < 0) start = 0;
    /* Invariant: start >= 0, so this test will be true when end < 0.     * The range is empty when start > end or start >= length. */    if (start > end || start >= llen) {        addReply(c,shared.emptymultibulk);        return;    }    // end 过大，则限制    // end 不可能小于0，因为上一个 start > end 已限制    if (end >= llen) end = llen-1;    rangelen = (end-start)+1;
    /* Return the result in form of a multi-bulk reply */    addReplyMultiBulkLen(c,rangelen);    if (o->encoding == OBJ_ENCODING_QUICKLIST) {        // 返回列表迭代器, start-TAIL, LIST_TAIL 代表正向迭代        listTypeIterator *iter = listTypeInitIterator(o, start, LIST_TAIL);        // 迭代到 rangelen=0 为止，依次向输出缓冲输出        while(rangelen--) {            listTypeEntry entry;            // 获取下一个元素            listTypeNext(iter, &entry);            quicklistEntry *qe = &entry.entry;            if (qe->value) {                addReplyBulkCBuffer(c,qe->value,qe->sz);            } else {                addReplyBulkLongLong(c,qe->longval);            }        }        listTypeReleaseIterator(iter);    } else {        serverPanic("List encoding is not QUICKLIST!");    }}// t_list.c, 统计list长度unsigned long listTypeLength(robj *subject) {    if (subject->encoding == OBJ_ENCODING_QUICKLIST) {        return quicklistCount(subject->ptr);    } else {        serverPanic("Unknown list encoding");    }}/* Return cached quicklist count */unsigned int quicklistCount(quicklist *ql) { return ql->count; }// 初始化 list 迭代器/* Initialize an iterator at the specified index. */listTypeIterator *listTypeInitIterator(robj *subject, long index,                                       unsigned char direction) {    listTypeIterator *li = zmalloc(sizeof(listTypeIterator));    li->subject = subject;    li->encoding = subject->encoding;    li->direction = direction;    li->iter = NULL;    /* LIST_HEAD means start at TAIL and move *towards* head.     * LIST_TAIL means start at HEAD and move *towards tail. */    int iter_direction =        direction == LIST_HEAD ? AL_START_TAIL : AL_START_HEAD;    if (li->encoding == OBJ_ENCODING_QUICKLIST) {        li->iter = quicklistGetIteratorAtIdx(li->subject->ptr,                                             iter_direction, index);    } else {        serverPanic("Unknown list encoding");    }    return li;}
/* Initialize an iterator at a specific offset 'idx' and make the iterator * return nodes in 'direction' direction. */quicklistIter *quicklistGetIteratorAtIdx(const quicklist *quicklist,                                         const int direction,                                         const long long idx) {    quicklistEntry entry;    // 查找idx 元素先 （前面已介绍, 为 ziplist+quicklist 迭代获得）    if (quicklistIndex(quicklist, idx, &entry)) {        // 获取获取的是整个list的迭代器, 通过current和offset进行迭代        quicklistIter *base = quicklistGetIterator(quicklist, direction);        base->zi = NULL;        base->current = entry.node;        base->offset = entry.offset;        return base;    } else {        return NULL;    }}// quicklist, list迭代器初始化/* Returns a quicklist iterator 'iter'. After the initialization every * call to quicklistNext() will return the next element of the quicklist. */quicklistIter *quicklistGetIterator(const quicklist *quicklist, int direction) {    quicklistIter *iter;    // 迭代器只包含当前元素    iter = zmalloc(sizeof(*iter));
    if (direction == AL_START_HEAD) {        iter->current = quicklist->head;        iter->offset = 0;    } else if (direction == AL_START_TAIL) {        iter->current = quicklist->tail;        iter->offset = -1;    }
    iter->direction = direction;    iter->quicklist = quicklist;
    iter->zi = NULL;
    return iter;}// 迭代器携带整个list 引用，及当前节点，如何进行迭代，则是重点// t_list.c, 迭代list元素, 并将 当前节点赋给 entry/* Stores pointer to current the entry in the provided entry structure * and advances the position of the iterator. Returns 1 when the current * entry is in fact an entry, 0 otherwise. */int listTypeNext(listTypeIterator *li, listTypeEntry *entry) {    /* Protect from converting when iterating */    serverAssert(li->subject->encoding == li->encoding);
    entry->li = li;    if (li->encoding == OBJ_ENCODING_QUICKLIST) {        // 迭代iter(改变iter指向), 赋值给 entry->entry        return quicklistNext(li->iter, &entry->entry);    } else {        serverPanic("Unknown list encoding");    }    return 0;}// quicklist.c /* Get next element in iterator. * * Note: You must NOT insert into the list while iterating over it. * You *may* delete from the list while iterating using the * quicklistDelEntry() function. * If you insert into the quicklist while iterating, you should * re-create the iterator after your addition. * * iter = quicklistGetIterator(quicklist,<direction>); * quicklistEntry entry; * while (quicklistNext(iter, &entry)) { *     if (entry.value) *          [[ use entry.value with entry.sz ]] *     else *          [[ use entry.longval ]] * } * * Populates 'entry' with values for this iteration. * Returns 0 when iteration is complete or if iteration not possible. * If return value is 0, the contents of 'entry' are not valid. */int quicklistNext(quicklistIter *iter, quicklistEntry *entry) {    initEntry(entry);
    if (!iter) {        D("Returning because no iter!");        return 0;    }    // 保存当前node, 及quicklist引用    entry->quicklist = iter->quicklist;    entry->node = iter->current;
    if (!iter->current) {        D("Returning because current node is NULL")        return 0;    }
    unsigned char *(*nextFn)(unsigned char *, unsigned char *) = NULL;    int offset_update = 0;
    if (!iter->zi) {        /* If !zi, use current index. */        // 初始化时 zi 未赋值，所以直接使用当前元素，使用offset进行查找        quicklistDecompressNodeForUse(iter->current);        iter->zi = ziplistIndex(iter->current->zl, iter->offset);    } else {        /* else, use existing iterator offset and get prev/next as necessary. */        if (iter->direction == AL_START_HEAD) {            nextFn = ziplistNext;            offset_update = 1;        } else if (iter->direction == AL_START_TAIL) {            nextFn = ziplistPrev;            offset_update = -1;        }        // 向前或向后迭代元素        iter->zi = nextFn(iter->current->zl, iter->zi);        iter->offset += offset_update;    }
    entry->zi = iter->zi;    entry->offset = iter->offset;
    if (iter->zi) {        /* Populate value from existing ziplist position */        // 从 zi 中获取值返回 （按ziplist协议）        ziplistGet(entry->zi, &entry->value, &entry->sz, &entry->longval);        return 1;    } else {        /* We ran out of ziplist entries.         * Pick next node, update offset, then re-run retrieval. */        // 当前ziplist没有下一个元素了，递归查找下一个ziplist        quicklistCompress(iter->quicklist, iter->current);        if (iter->direction == AL_START_HEAD) {            /* Forward traversal */            D("Jumping to start of next node");            iter->current = iter->current->next;            iter->offset = 0;        } else if (iter->direction == AL_START_TAIL) {            /* Reverse traversal */            D("Jumping to end of previous node");            iter->current = iter->current->prev;            iter->offset = -1;        }        iter->zi = NULL;        return quicklistNext(iter, entry);    }}// ziplist.c/* Get entry pointed to by 'p' and store in either '*sstr' or 'sval' depending * on the encoding of the entry. '*sstr' is always set to NULL to be able * to find out whether the string pointer or the integer value was set. * Return 0 if 'p' points to the end of the ziplist, 1 otherwise. */unsigned int ziplistGet(unsigned char *p, unsigned char **sstr, unsigned int *slen, long long *sval) {    zlentry entry;    if (p == NULL || p[0] == ZIP_END) return 0;    if (sstr) *sstr = NULL;
    zipEntry(p, &entry);    if (ZIP_IS_STR(entry.encoding)) {        if (sstr) {            *slen = entry.len;            *sstr = p+entry.headersize;        }    } else {        if (sval) {            *sval = zipLoadInteger(p+entry.headersize,entry.encoding);        }    }    return 1;}

// LREM key count value, 只提供了范围删除的方式，单个数据删除可以通过此命令来完成// t_list.cvoid lremCommand(client *c) {    robj *subject, *obj;    obj = c->argv[3];    long toremove;    long removed = 0;
    if ((getLongFromObjectOrReply(c, c->argv[2], &toremove, NULL) != C_OK))        return;
    subject = lookupKeyWriteOrReply(c,c->argv[1],shared.czero);    if (subject == NULL || checkType(c,subject,OBJ_LIST)) return;    // 因是范围型删除，自然使用迭代删除是最好的选择了    listTypeIterator *li;    if (toremove < 0) {        toremove = -toremove;        li = listTypeInitIterator(subject,-1,LIST_HEAD);    } else {        li = listTypeInitIterator(subject,0,LIST_TAIL);    }
    listTypeEntry entry;    // 迭代方式我们在查找操作已详细说明    while (listTypeNext(li,&entry)) {        // 1. 比较元素是否是需要删除的对象，只有完全匹配才可以删除        if (listTypeEqual(&entry,obj)) {            // 2. 实际的删除动作            listTypeDelete(li, &entry);            server.dirty++;            removed++;            if (toremove && removed == toremove) break;        }    }    listTypeReleaseIterator(li);    // 如果没有任何元素后，将key从db中删除    if (listTypeLength(subject) == 0) {        dbDelete(c->db,c->argv[1]);    }
    addReplyLongLong(c,removed);    if (removed) signalModifiedKey(c->db,c->argv[1]);}// 1. 是否与指定robj相等// t_list.c, listTypeEntry 是否与指定robj相等/* Compare the given object with the entry at the current position. */int listTypeEqual(listTypeEntry *entry, robj *o) {    if (entry->li->encoding == OBJ_ENCODING_QUICKLIST) {        serverAssertWithInfo(NULL,o,sdsEncodedObject(o));        return quicklistCompare(entry->entry.zi,o->ptr,sdslen(o->ptr));    } else {        serverPanic("Unknown list encoding");    }}// t_list.cint quicklistCompare(unsigned char *p1, unsigned char *p2, int p2_len) {    // 元素本身是 ziplist 类型的，所以直接交由ziplist比对即可    return ziplistCompare(p1, p2, p2_len);}// ziplist.c/* Compare entry pointer to by 'p' with 'sstr' of length 'slen'. *//* Return 1 if equal. */unsigned int ziplistCompare(unsigned char *p, unsigned char *sstr, unsigned int slen) {    zlentry entry;    unsigned char sencoding;    long long zval, sval;    if (p[0] == ZIP_END) return 0;
    zipEntry(p, &entry);    if (ZIP_IS_STR(entry.encoding)) {        /* Raw compare */        if (entry.len == slen) {            return memcmp(p+entry.headersize,sstr,slen) == 0;        } else {            return 0;        }    } else {        /* Try to compare encoded values. Don't compare encoding because         * different implementations may encoded integers differently. */        if (zipTryEncoding(sstr,slen,&sval,&sencoding)) {          zval = zipLoadInteger(p+entry.headersize,entry.encoding);          return zval == sval;        }    }    return 0;}
/* Delete the element pointed to. */void listTypeDelete(listTypeIterator *iter, listTypeEntry *entry) {    if (entry->li->encoding == OBJ_ENCODING_QUICKLIST) {        quicklistDelEntry(iter->iter, &entry->entry);    } else {        serverPanic("Unknown list encoding");    }}
// 2. 执行删除操作// t_list.c /* Delete the element pointed to. */void listTypeDelete(listTypeIterator *iter, listTypeEntry *entry) {    if (entry->li->encoding == OBJ_ENCODING_QUICKLIST) {        quicklistDelEntry(iter->iter, &entry->entry);    } else {        serverPanic("Unknown list encoding");    }}// quicklist.c/* Delete one element represented by 'entry' * * 'entry' stores enough metadata to delete the proper position in * the correct ziplist in the correct quicklist node. */void quicklistDelEntry(quicklistIter *iter, quicklistEntry *entry) {    quicklistNode *prev = entry->node->prev;    quicklistNode *next = entry->node->next;    int deleted_node = quicklistDelIndex((quicklist *)entry->quicklist,                                         entry->node, &entry->zi);
    /* after delete, the zi is now invalid for any future usage. */    iter->zi = NULL;
    /* If current node is deleted, we must update iterator node and offset. */    if (deleted_node) {        // 如果node被删除，则移动quicklist指针        if (iter->direction == AL_START_HEAD) {            iter->current = next;            iter->offset = 0;        } else if (iter->direction == AL_START_TAIL) {            iter->current = prev;            iter->offset = -1;        }    }    /* else if (!deleted_node), no changes needed.     * we already reset iter->zi above, and the existing iter->offset     * doesn't move again because:     *   - [1, 2, 3] => delete offset 1 => [1, 3]: next element still offset 1     *   - [1, 2, 3] => delete offset 0 => [2, 3]: next element still offset 0     *  if we deleted the last element at offet N and now     *  length of this ziplist is N-1, the next call into     *  quicklistNext() will jump to the next node. */}// quicklist.c/* Delete one entry from list given the node for the entry and a pointer * to the entry in the node. * * Note: quicklistDelIndex() *requires* uncompressed nodes because you *       already had to get *p from an uncompressed node somewhere. * * Returns 1 if the entire node was deleted, 0 if node still exists. * Also updates in/out param 'p' with the next offset in the ziplist. */REDIS_STATIC int quicklistDelIndex(quicklist *quicklist, quicklistNode *node,                                   unsigned char **p) {    int gone = 0;    // 同样，到最后一级，依旧是调用ziplist的方法进行删除 (按照 ziplist 协议操作即可)    node->zl = ziplistDelete(node->zl, p);    node->count--;    // 如果node中没有元素了，就把当前node移除，否则更新 sz 大小    if (node->count == 0) {        gone = 1;        __quicklistDelNode(quicklist, node);    } else {        quicklistNodeUpdateSz(node);    }    quicklist->count--;    /* If we deleted the node, the original node is no longer valid */    return gone ? 1 : 0;}

// 用法: LPOP key// t_list.cvoid lpopCommand(client *c) {    popGenericCommand(c,LIST_HEAD);}void popGenericCommand(client *c, int where) {    robj *o = lookupKeyWriteOrReply(c,c->argv[1],shared.nullbulk);    if (o == NULL || checkType(c,o,OBJ_LIST)) return;    // 弹出元素，重点看一下这个方法    robj *value = listTypePop(o,where);    if (value == NULL) {        addReply(c,shared.nullbulk);    } else {        char *event = (where == LIST_HEAD) ? "lpop" : "rpop";
        addReplyBulk(c,value);        decrRefCount(value);        notifyKeyspaceEvent(NOTIFY_LIST,event,c->argv[1],c->db->id);        if (listTypeLength(o) == 0) {            notifyKeyspaceEvent(NOTIFY_GENERIC,"del",                                c->argv[1],c->db->id);            dbDelete(c->db,c->argv[1]);        }        signalModifiedKey(c->db,c->argv[1]);        server.dirty++;    }}// t_list.crobj *listTypePop(robj *subject, int where) {    long long vlong;    robj *value = NULL;
    int ql_where = where == LIST_HEAD ? QUICKLIST_HEAD : QUICKLIST_TAIL;    if (subject->encoding == OBJ_ENCODING_QUICKLIST) {        if (quicklistPopCustom(subject->ptr, ql_where, (unsigned char **)&value,                               NULL, &vlong, listPopSaver)) {            if (!value)                value = createStringObjectFromLongLong(vlong);        }    } else {        serverPanic("Unknown list encoding");    }    return value;}// quicklist.c/* pop from quicklist and return result in 'data' ptr.  Value of 'data' * is the return value of 'saver' function pointer if the data is NOT a number. * * If the quicklist element is a long long, then the return value is returned in * 'sval'. * * Return value of 0 means no elements available. * Return value of 1 means check 'data' and 'sval' for values. * If 'data' is set, use 'data' and 'sz'.  Otherwise, use 'sval'. */int quicklistPopCustom(quicklist *quicklist, int where, unsigned char **data,                       unsigned int *sz, long long *sval,                       void *(*saver)(unsigned char *data, unsigned int sz)) {    unsigned char *p;    unsigned char *vstr;    unsigned int vlen;    long long vlong;    int pos = (where == QUICKLIST_HEAD) ? 0 : -1;
    if (quicklist->count == 0)        return 0;
    if (data)        *data = NULL;    if (sz)        *sz = 0;    if (sval)        *sval = -123456789;
    quicklistNode *node;    // 获取ziplist中的，第一个元素或者最后一个节点    if (where == QUICKLIST_HEAD && quicklist->head) {        node = quicklist->head;    } else if (where == QUICKLIST_TAIL && quicklist->tail) {        node = quicklist->tail;    } else {        return 0;    }    // 获取ziplist中的，第一个元素或者最后一个元素    p = ziplistIndex(node->zl, pos);    if (ziplistGet(p, &vstr, &vlen, &vlong)) {        if (vstr) {            if (data)                // 创建string 对象返回                *data = saver(vstr, vlen);            if (sz)                *sz = vlen;        } else {            if (data)                *data = NULL;            if (sval)                *sval = vlong;        }        // 删除获取数据后的元素        quicklistDelIndex(quicklist, node, &p);        return 1;    }    return 0;}

// 用法: BLPOP key1 [key2] timeout// t_list.c  同样 l/r 复用代码void blpopCommand(client *c) {    blockingPopGenericCommand(c,LIST_HEAD);}/* Blocking RPOP/LPOP */void blockingPopGenericCommand(client *c, int where) {    robj *o;    mstime_t timeout;    int j;
    if (getTimeoutFromObjectOrReply(c,c->argv[c->argc-1],&timeout,UNIT_SECONDS)        != C_OK) return;    // 循环查找多个key    for (j = 1; j < c->argc-1; j++) {        o = lookupKeyWrite(c->db,c->argv[j]);        if (o != NULL) {            if (o->type != OBJ_LIST) {                addReply(c,shared.wrongtypeerr);                return;            } else {                // 如果有值，则和非阻塞版本一样了，直接响应即可                if (listTypeLength(o) != 0) {                    /* Non empty list, this is like a non normal [LR]POP. */                    char *event = (where == LIST_HEAD) ? "lpop" : "rpop";                    robj *value = listTypePop(o,where);                    serverAssert(value != NULL);
                    addReplyMultiBulkLen(c,2);                    addReplyBulk(c,c->argv[j]);                    addReplyBulk(c,value);                    decrRefCount(value);                    notifyKeyspaceEvent(NOTIFY_LIST,event,                                        c->argv[j],c->db->id);                    if (listTypeLength(o) == 0) {                        dbDelete(c->db,c->argv[j]);                        notifyKeyspaceEvent(NOTIFY_GENERIC,"del",                                            c->argv[j],c->db->id);                    }                    signalModifiedKey(c->db,c->argv[j]);                    server.dirty++;
                    /* Replicate it as an [LR]POP instead of B[LR]POP. */                    rewriteClientCommandVector(c,2,                        (where == LIST_HEAD) ? shared.lpop : shared.rpop,                        c->argv[j]);                    // 获取到值后直接结束流程                    return;                }            }        }    }
    /* If we are inside a MULTI/EXEC and the list is empty the only thing     * we can do is treating it as a timeout (even with timeout 0). */    if (c->flags & CLIENT_MULTI) {        addReply(c,shared.nullmultibulk);        return;    }
    /* If the list is empty or the key does not exists we must block */    // 阻塞是在这里实现的    blockForKeys(c, c->argv + 1, c->argc - 2, timeout, NULL);}
/* This is how the current blocking POP works, we use BLPOP as example: * - If the user calls BLPOP and the key exists and contains a non empty list *   then LPOP is called instead. So BLPOP is semantically the same as LPOP *   if blocking is not required. * - If instead BLPOP is called and the key does not exists or the list is *   empty we need to block. In order to do so we remove the notification for *   new data to read in the client socket (so that we'll not serve new *   requests if the blocking request is not served). Also we put the client *   in a dictionary (db->blocking_keys) mapping keys to a list of clients *   blocking for this keys. * - If a PUSH operation against a key with blocked clients waiting is *   performed, we mark this key as "ready", and after the current command, *   MULTI/EXEC block, or script, is executed, we serve all the clients waiting *   for this list, from the one that blocked first, to the last, accordingly *   to the number of elements we have in the ready list. */
/* Set a client in blocking mode for the specified key, with the specified * timeout */void blockForKeys(client *c, robj **keys, int numkeys, mstime_t timeout, robj *target) {    dictEntry *de;    list *l;    int j;
    c->bpop.timeout = timeout;    c->bpop.target = target;
    if (target != NULL) incrRefCount(target);    // 阻塞入队判定    for (j = 0; j < numkeys; j++) {        /* If the key already exists in the dict ignore it. */        if (dictAdd(c->bpop.keys,keys[j],NULL) != DICT_OK) continue;        incrRefCount(keys[j]);
        /* And in the other "side", to map keys -> clients */        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();            // 将阻塞key放到 db 中，后台有线程去轮询            retval = dictAdd(c->db->blocking_keys,keys[j],l);            incrRefCount(keys[j]);            serverAssertWithInfo(c,keys[j],retval == DICT_OK);        } else {            l = dictGetVal(de);        }        // 将每个key 依次添加到 c->db->blocking_keys, 后续迭代将会重新检查取出        listAddNodeTail(l,c);    }    // 阻塞客户端，其实就是设置阻塞标识，然后等待key变更或超时，下一次扫描时将重新检测取出执行    blockClient(c,BLOCKED_LIST);}// block.c 设置阻塞标识/* Block a client for the specific operation type. Once the CLIENT_BLOCKED * flag is set client query buffer is not longer processed, but accumulated, * and will be processed when the client is unblocked. */void blockClient(client *c, int btype) {    c->flags |= CLIENT_BLOCKED;    c->btype = btype;    server.bpop_blocked_clients++;}