Redis C客户端Hiredis代码分析

初始化

redisContext - Redis连接的上下文

/* Context for a connection to Redis */
typedef struct redisContext {
    int err; /* Error flags, 0 when there is no error */
    char errstr[128]; /* String representation of error when applicable */
    int fd;
    int flags;
    char *obuf; /* Write buffer */
    redisReader *reader; /* Protocol reader */

    enum redisConnectionType connection_type;
    struct timeval *timeout;

    struct {
        char *host;
        char *source_addr;
        int port;
    } tcp;

    struct {
        char *path;
    } unix_sock;

} redisContext;

每一次连接成功, 都会创建一个redisContext数据, 用于后续的操作

redisReader - 结果读取

typedef struct redisReader {
    int err; /* Error flags, 0 when there is no error */
    char errstr[128]; /* String representation of error when applicable */

    char *buf; /* Read buffer */
    size_t pos; /* Buffer cursor */
    size_t len; /* Buffer length */
    size_t maxbuf; /* Max length of unused buffer */

    redisReadTask rstack[9];
    int ridx; /* Index of current read task */
    void *reply; /* Temporary reply pointer */

    redisReplyObjectFunctions *fn;
    void *privdata;
} redisReader;

redisReader在redisContext中使用redisReplyObjectFunctions参数进行初始化, 用于读取执行结果, 其中 redisReplyObjectFunctions 是一个函数指针列表, 用于读取不同类型的返回数据, 

typedef struct redisReplyObjectFunctions {
    void *(*createString)(const redisReadTask*, char*, size_t);
    void *(*createArray)(const redisReadTask*, int);
    void *(*createInteger)(const redisReadTask*, long long);
    void *(*createNil)(const redisReadTask*);
    void (*freeObject)(void*);
} redisReplyObjectFunctions;

一般使用的是默认的defaultFunctions进行初始化

/* Default set of functions to build the reply. Keep in mind that such a
 * function returning NULL is interpreted as OOM. */
static redisReplyObjectFunctions defaultFunctions = {
    createStringObject,
    createArrayObject,
    createIntegerObject,
    createNilObject,
    freeReplyObject
};

redisReader中的另一个结构体是 redisReadTask数组, 需要配合ridx一起使用, 用于给redisReplyObjectFunctions中的函数提供输入参数

typedef struct redisReadTask {
    int type;
    int elements; /* number of elements in multibulk container */
    int idx; /* index in parent (array) object */
    void *obj; /* holds user-generated value for a read task */
    struct redisReadTask *parent; /* parent task */
    void *privdata; /* user-settable arbitrary field */
} redisReadTask;

在defaultFunctions下面的函数是 createStringObject, createArrayObject, createIntegerObject, createNilObject, freeReplyObject, 这些函数的输入都是rediskReadTask, 但是根据类型不同会有不同的附带参数, 例如array就会有elements元素数量, 而string就会有char *str和size_t len, 都会在处理中产生redisReply对象, 并将这个对象挂载到上一级redisReply对象的element变量上(如果有上一级的话).

结果解析

执行命令时, 实际调用的处理方法是redisvCommand, 

void *redisvCommand(redisContext *c, const char *format, va_list ap) {
    if (redisvAppendCommand(c,format,ap) != REDIS_OK)
        return NULL;
    return __redisBlockForReply(c);
}

在这里会调用__redisBlockForReply来处理存在redisContext *c中的结果

/* Helper function for the redisCommand* family of functions.
 *
 * Write a formatted command to the output buffer. If the given context is
 * blocking, immediately read the reply into the "reply" pointer. When the
 * context is non-blocking, the "reply" pointer will not be used and the
 * command is simply appended to the write buffer.
 *
 * Returns the reply when a reply was successfully retrieved. Returns NULL
 * otherwise. When NULL is returned in a blocking context, the error field
 * in the context will be set.
 */
static void *__redisBlockForReply(redisContext *c) {
    void *reply;

    if (c->flags & REDIS_BLOCK) {
        if (redisGetReply(c,&reply) != REDIS_OK)
            return NULL;
        return reply;
    }
    return NULL;
}

这里会将错误情况排除, 实际的处理方法是 redisGetReply, 这里的处理方式是阻塞的, 如果aux已经读到error就返回, 否则一直在等直到有值

int redisGetReply(redisContext *c, void **reply) {
    int wdone = 0;
    void *aux = NULL;

    /* Try to read pending replies */
    if (redisGetReplyFromReader(c,&aux) == REDIS_ERR)
        return REDIS_ERR;

    /* For the blocking context, flush output buffer and read reply */
    if (aux == NULL && c->flags & REDIS_BLOCK) {
        /* Write until done */
        do {
            if (redisBufferWrite(c,&wdone) == REDIS_ERR)
                return REDIS_ERR;
        } while (!wdone);

        /* Read until there is a reply */
        do {
            if (redisBufferRead(c) == REDIS_ERR)
                return REDIS_ERR;
            if (redisGetReplyFromReader(c,&aux) == REDIS_ERR)
                return REDIS_ERR;
        } while (aux == NULL);
    }

    /* Set reply object */
    if (reply != NULL) *reply = aux;
    return REDIS_OK;
}

在阻塞读取的过程中, 会先通过redisBufferRead尝试读取, 成功后再用redisGetReplyFromReader尝试解析

/* Use this function to handle a read event on the descriptor. It will try
 * and read some bytes from the socket and feed them to the reply parser.
 *
 * After this function is called, you may use redisContextReadReply to
 * see if there is a reply available. */
int redisBufferRead(redisContext *c) {
    char buf[1024*16];
    int nread;

    /* Return early when the context has seen an error. */
    if (c->err)
        return REDIS_ERR;

    nread = read(c->fd,buf,sizeof(buf));
    if (nread == -1) {
        if ((errno == EAGAIN && !(c->flags & REDIS_BLOCK)) || (errno == EINTR)) {
            /* Try again later */
        } else {
            __redisSetError(c,REDIS_ERR_IO,NULL);
            return REDIS_ERR;
        }
    } else if (nread == 0) {
        __redisSetError(c,REDIS_ERR_EOF,"Server closed the connection");
        return REDIS_ERR;
    } else {
        if (redisReaderFeed(c->reader,buf,nread) != REDIS_OK) {
            __redisSetError(c,c->reader->err,c->reader->errstr);
            return REDIS_ERR;
        }
    }
    return REDIS_OK;
}

在redisGetReplyFromReader中调用redisReaderGetReply解析, 再往内部, 就是循环调用processItem来处理结果了

/* Internal helper function to try and get a reply from the reader,
 * or set an error in the context otherwise. */
int redisGetReplyFromReader(redisContext *c, void **reply) {
    if (redisReaderGetReply(c->reader,reply) == REDIS_ERR) {
        __redisSetError(c,c->reader->err,c->reader->errstr);
        return REDIS_ERR;
    }
    return REDIS_OK;
}

int redisReaderGetReply(redisReader *r, void **reply) {
    /* Default target pointer to NULL. */
    if (reply != NULL)
        *reply = NULL;

    /* Return early when this reader is in an erroneous state. */
    if (r->err)
        return REDIS_ERR;

    /* When the buffer is empty, there will never be a reply. */
    if (r->len == 0)
        return REDIS_OK;

    /* Set first item to process when the stack is empty. */
    if (r->ridx == -1) {
        r->rstack[0].type = -1;
        r->rstack[0].elements = -1;
        r->rstack[0].idx = -1;
        r->rstack[0].obj = NULL;
        r->rstack[0].parent = NULL;
        r->rstack[0].privdata = r->privdata;
        r->ridx = 0;
    }

    /* Process items in reply. */
    while (r->ridx >= 0)
        if (processItem(r) != REDIS_OK)
            break;

    /* Return ASAP when an error occurred. */
    if (r->err)
        return REDIS_ERR;

    /* Discard part of the buffer when we've consumed at least 1k, to avoid
     * doing unnecessary calls to memmove() in sds.c. */
    if (r->pos >= 1024) {
        sdsrange(r->buf,r->pos,-1);
        r->pos = 0;
        r->len = sdslen(r->buf);
    }

    /* Emit a reply when there is one. */
    if (r->ridx == -1) {
        if (reply != NULL)
            *reply = r->reply;
        r->reply = NULL;
    }
    return REDIS_OK;
}

在processItem中, 会根据第一个字节确定当前redisReadTask的类型, 然后对类型为integer, string和array的数据进行处理, 其他的直接返回error

static int processItem(redisReader *r) {
    redisReadTask *cur = &(r->rstack[r->ridx]);
    char *p;

    /* check if we need to read type */
    if (cur->type < 0) {
        if ((p = readBytes(r,1)) != NULL) {
            switch (p[0]) {
            case '-':
                cur->type = REDIS_REPLY_ERROR;
                break;
            case '+':
                cur->type = REDIS_REPLY_STATUS;
                break;
            case ':':
                cur->type = REDIS_REPLY_INTEGER;
                break;
            case '$':
                cur->type = REDIS_REPLY_STRING;
                break;
            case '*':
                cur->type = REDIS_REPLY_ARRAY;
                break;
            default:
                __redisReaderSetErrorProtocolByte(r,*p);
                return REDIS_ERR;
            }
        } else {
            /* could not consume 1 byte */
            return REDIS_ERR;
        }
    }

    /* process typed item */
    switch(cur->type) {
    case REDIS_REPLY_ERROR:
    case REDIS_REPLY_STATUS:
    case REDIS_REPLY_INTEGER:
        return processLineItem(r);
    case REDIS_REPLY_STRING:
        return processBulkItem(r);
    case REDIS_REPLY_ARRAY:
        return processMultiBulkItem(r);
    default:
        assert(NULL);
        return REDIS_ERR; /* Avoid warning. */
    }
}

对redisReadTask队列和ridx的操作只发生在类型为array的情况下, 是在processMultiBulkItem的处理中进行的, 这里会取出一整个这一层array里的数据, 如果遇到元素的elements大于0的, 说明这个元素也是一个数组, 那么就会ridx加一, 在rstack中增加一个redisReadTask, 这时候就不再继续处理同层数据了, 而是等着外循环去进入下一层处理.

static int processMultiBulkItem(redisReader *r) {
    redisReadTask *cur = &(r->rstack[r->ridx]);
    void *obj;
    char *p;
    long long elements;
    int root = 0, len;

    /* Set error for nested multi bulks with depth > 7 */
    if (r->ridx == 8) {
        __redisReaderSetError(r,REDIS_ERR_PROTOCOL,
            "No support for nested multi bulk replies with depth > 7");
        return REDIS_ERR;
    }

    if ((p = readLine(r,&len)) != NULL) {
        if (string2ll(p, len, &elements) == REDIS_ERR) {
            __redisReaderSetError(r,REDIS_ERR_PROTOCOL,
                    "Bad multi-bulk length");
            return REDIS_ERR;
        }

        root = (r->ridx == 0);

        if (elements < -1 || elements > INT_MAX) {
            __redisReaderSetError(r,REDIS_ERR_PROTOCOL,
                    "Multi-bulk length out of range");
            return REDIS_ERR;
        }

        if (elements == -1) {
            if (r->fn && r->fn->createNil)
                obj = r->fn->createNil(cur);
            else
                obj = (void*)REDIS_REPLY_NIL;

            if (obj == NULL) {
                __redisReaderSetErrorOOM(r);
                return REDIS_ERR;
            }

            moveToNextTask(r);
        } else {
            if (r->fn && r->fn->createArray)
                obj = r->fn->createArray(cur,elements);
            else
                obj = (void*)REDIS_REPLY_ARRAY;

            if (obj == NULL) {
                __redisReaderSetErrorOOM(r);
                return REDIS_ERR;
            }

            /* Modify task stack when there are more than 0 elements. */
            if (elements > 0) {
                cur->elements = elements;
                cur->obj = obj;
                r->ridx++;
                r->rstack[r->ridx].type = -1;
                r->rstack[r->ridx].elements = -1;
                r->rstack[r->ridx].idx = 0;
                r->rstack[r->ridx].obj = NULL;
                r->rstack[r->ridx].parent = cur;
                r->rstack[r->ridx].privdata = r->privdata;
            } else {
                moveToNextTask(r);
            }
        }

        /* Set reply if this is the root object. */
        if (root) r->reply = obj;
        return REDIS_OK;
    }

    return REDIS_ERR;
}

这里要注意的是moveToNextTask这个函数, 这是去移动到当前层的下一个元素, 如果当前层已经空了就直接返回, 如果在当前层的编号已经到最后一个, 就返回上一层. 所以这个hiredis对返回数组的处理是一个深度遍历, 并且规定了层数不能超过8

static void moveToNextTask(redisReader *r) {
    redisReadTask *cur, *prv;
    while (r->ridx >= 0) {
        /* Return a.s.a.p. when the stack is now empty. */
        if (r->ridx == 0) {
            r->ridx--;
            return;
        }

        cur = &(r->rstack[r->ridx]);
        prv = &(r->rstack[r->ridx-1]);
        assert(prv->type == REDIS_REPLY_ARRAY);
        if (cur->idx == prv->elements-1) {
            r->ridx--;
        } else {
            /* Reset the type because the next item can be anything */
            assert(cur->idx < prv->elements);
            cur->type = -1;
            cur->elements = -1;
            cur->idx++;
            return;
        }
    }
}

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