Redis C客戶端Hiredis代碼分析

初始化

redisContext - Redis連接的上下文

/* 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;

每一次連接成功, 都會創建一個redisContext數據, 用於后續的操作

redisReader - 結果讀取

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參數進行初始化, 用於讀取執行結果, 其中 redisReplyObjectFunctions 是一個函數指針列表, 用於讀取不同類型的返回數據, 

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;

一般使用的是默認的defaultFunctions進行初始化

/* 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中的另一個結構體是 redisReadTask數組, 需要配合ridx一起使用, 用於給redisReplyObjectFunctions中的函數提供輸入參數

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下面的函數是 createStringObject, createArrayObject, createIntegerObject, createNilObject, freeReplyObject, 這些函數的輸入都是rediskReadTask, 但是根據類型不同會有不同的附帶參數, 例如array就會有elements元素數量, 而string就會有char *str和size_t len, 都會在處理中產生redisReply對象, 並將這個對象掛載到上一級redisReply對象的element變量上(如果有上一級的話).

結果解析

執行命令時, 實際調用的處理方法是redisvCommand, 

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

在這里會調用__redisBlockForReply來處理存在redisContext *c中的結果

/* 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;
}

這里會將錯誤情況排除, 實際的處理方法是 redisGetReply, 這里的處理方式是阻塞的, 如果aux已經讀到error就返回, 否則一直在等直到有值

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;
}

在阻塞讀取的過程中, 會先通過redisBufferRead嘗試讀取, 成功后再用redisGetReplyFromReader嘗試解析

/* 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中調用redisReaderGetReply解析, 再往內部, 就是循環調用processItem來處理結果了

/* 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, 會根據第一個字節確定當前redisReadTask的類型, 然后對類型為integer, string和array的數據進行處理, 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. */
    }
}

對redisReadTask隊列和ridx的操作只發生在類型為array的情況下, 是在processMultiBulkItem的處理中進行的, 這里會取出一整個這一層array里的數據, 如果遇到元素的elements大於0的, 說明這個元素也是一個數組, 那么就會ridx加一, 在rstack中增加一個redisReadTask, 這時候就不再繼續處理同層數據了, 而是等着外循環去進入下一層處理.

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;
}

這里要注意的是moveToNextTask這個函數, 這是去移動到當前層的下一個元素, 如果當前層已經空了就直接返回, 如果在當前層的編號已經到最后一個, 就返回上一層. 所以這個hiredis對返回數組的處理是一個深度遍歷, 並且規定了層數不能超過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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