redis源码阅读之rio

redis当中的rio的话就是在后端执行读写操作，bio相当于它的一个补充操作，它使用一个大结构体将操作全部封装起来，有点像linux当中驱动的写法，方便统一接口，具体结构体如下：

struct _rio {
    /* Backend functions.
     * Since this functions do not tolerate short writes or reads the return
     * value is simplified to: zero on error, non zero on complete success. */
    size_t (*read)(struct _rio *, void *buf, size_t len);
    size_t (*write)(struct _rio *, const void *buf, size_t len);
    off_t (*tell)(struct _rio *);
    int (*flush)(struct _rio *);
    /* The update_cksum method if not NULL is used to compute the checksum of
     * all the data that was read or written so far. The method should be
     * designed so that can be called with the current checksum, and the buf
     * and len fields pointing to the new block of data to add to the checksum
     * computation. */
    void (*update_cksum)(struct _rio *, const void *buf, size_t len);

    /* The current checksum */
    uint64_t cksum;

    /* number of bytes read or written */
    size_t processed_bytes;

    /* maximum single read or write chunk size */
    size_t max_processing_chunk;

    /* Backend-specific vars. */
    union {
        /* In-memory buffer target. */
        struct {
            sds ptr;
            off_t pos;
        } buffer;
        /* Stdio file pointer target. */
        struct {
            FILE *fp;
            off_t buffered; /* Bytes written since last fsync. */
            off_t autosync; /* fsync after 'autosync' bytes written. */
        } file;
        /* Multiple FDs target (used to write to N sockets). */
        struct {
            int *fds; /* File descriptors. */
            int *state; /* Error state of each fd. 0 (if ok) or errno. */
            int numfds;
            off_t pos;
            sds buf;
        } fdset;
    } io;
};

typedef struct _rio rio;

由结构可知，rio会处理三种不同的read和write，分别为
1. 内存buffer，内部由sds表示
2. 文件指针，内部由FILE*表示
3. 文件描述符数组，内部由int *表示
rio结构留给外部调用的接口有read，write，tell，flush和update_cksum，而整个rio留给外部调用的接口是将结构体的接口进一步封装了：

static inline size_t rioWrite(rio *r, const void *buf, size_t len) {
    while (len) {
        size_t bytes_to_write = (r->max_processing_chunk && r->max_processing_chunk < len) ? r->max_processing_chunk : len;
        if (r->update_cksum) r->update_cksum(r,buf,bytes_to_write);
        if (r->write(r,buf,bytes_to_write) == 0) /*0表示失败；非0表示成功*/
            return 0;
        buf = (char*)buf + bytes_to_write;
        len -= bytes_to_write;
        r->processed_bytes += bytes_to_write;
    }
    return 1;
}

static inline size_t rioRead(rio *r, void *buf, size_t len) {
    while (len) {
        size_t bytes_to_read = (r->max_processing_chunk && r->max_processing_chunk < len) ? r->max_processing_chunk : len;
        if (r->read(r,buf,bytes_to_read) == 0)/*0表示失败；非0表示成功*/
            return 0;
        if (r->update_cksum) r->update_cksum(r,buf,bytes_to_read);
        buf = (char*)buf + bytes_to_read;
        len -= bytes_to_read;
        r->processed_bytes += bytes_to_read;
    }
    return 1;
}

static inline off_t rioTell(rio *r) {
    return r->tell(r);
}

static inline int rioFlush(rio *r) {
    return r->flush(r);
}

接下来聊聊内部实现，还是按照之前说过的顺序内存buffer-> 文件指针->文件描述符数组来进行，其总体思路就是自定义对应类别的io函数，然后实例化结构体，将对应io函数填入结构体
1. 内存buffer主要是针对sds的封装

/* Returns 1 or 0 for success/failure. */
static size_t rioBufferWrite(rio *r, const void *buf, size_t len) {
    r->io.buffer.ptr = sdscatlen(r->io.buffer.ptr,(char*)buf,len);
    r->io.buffer.pos += len;
    return 1;
}

/* Returns 1 or 0 for success/failure. */
static size_t rioBufferRead(rio *r, void *buf, size_t len) {
    if (sdslen(r->io.buffer.ptr)-r->io.buffer.pos < len)
        return 0; /* not enough buffer to return len bytes. */
    memcpy(buf,r->io.buffer.ptr+r->io.buffer.pos,len);
    r->io.buffer.pos += len;
    return 1;
}

/* Returns read/write position in buffer. */
static off_t rioBufferTell(rio *r) {
    return r->io.buffer.pos;
}

/* Flushes any buffer to target device if applicable. Returns 1 on success
 * and 0 on failures. */
static int rioBufferFlush(rio *r) {
    UNUSED(r);
    return 1; /* Nothing to do, our write just appends to the buffer. */
}

static const rio rioBufferIO = {
    rioBufferRead,
    rioBufferWrite,
    rioBufferTell,
    rioBufferFlush,
    NULL, /* update_checksum */
    0, /* current checksum */
    0, /* bytes read or written */
    0, /* read/write chunk size */
    { { NULL, 0 } } /* union for io-specific vars */
};

void rioInitWithBuffer(rio *r, sds s) {
    *r = rioBufferIO;
    r->io.buffer.ptr = s;
    r->io.buffer.pos = 0;
}

2. 文件指针操作的封装：

/* Returns 1 or 0 for success/failure. */
static size_t rioFileWrite(rio *r, const void *buf, size_t len) {
    size_t retval;

    retval = fwrite(buf,len,1,r->io.file.fp);
    r->io.file.buffered += len;

    if (r->io.file.autosync &&
        r->io.file.buffered >= r->io.file.autosync)
    {
        fflush(r->io.file.fp);
        aof_fsync(fileno(r->io.file.fp));
        r->io.file.buffered = 0;
    }
    return retval;
}

/* Returns 1 or 0 for success/failure. */
static size_t rioFileRead(rio *r, void *buf, size_t len) {
    return fread(buf,len,1,r->io.file.fp);
}

/* Returns read/write position in file. */
static off_t rioFileTell(rio *r) {
    return ftello(r->io.file.fp);
}

/* Flushes any buffer to target device if applicable. Returns 1 on success
 * and 0 on failures. */
static int rioFileFlush(rio *r) {
    return (fflush(r->io.file.fp) == 0) ? 1 : 0;
}

static const rio rioFileIO = {
    rioFileRead,
    rioFileWrite,
    rioFileTell,
    rioFileFlush,
    NULL, /* update_checksum */
    0, /* current checksum */
    0, /* bytes read or written */
    0, /* read/write chunk size */
    { { NULL, 0 } } /* union for io-specific vars */
};

void rioInitWithFile(rio *r, FILE *fp) {
    *r = rioFileIO;
    r->io.file.fp = fp;
    r->io.file.buffered = 0;
    r->io.file.autosync = 0;
}

3. 文件描述符数组，这里要注意的是写操作，如果要写的buffer为空，会写入文件描述符数组自身的buffer，并且每个文件描述符最多写1024个字节，可以并行执行，代码如下：

/* Returns 1 or 0 for success/failure.
 * The function returns success as long as we are able to correctly write
 * to at least one file descriptor.
 *
 * When buf is NULL and len is 0, the function performs a flush operation
 * if there is some pending buffer, so this function is also used in order
 * to implement rioFdsetFlush(). */
static size_t rioFdsetWrite(rio *r, const void *buf, size_t len) {
    ssize_t retval;
    int j;
    unsigned char *p = (unsigned char*) buf;
    int doflush = (buf == NULL && len == 0);

    /* To start we always append to our buffer. If it gets larger than
     * a given size, we actually write to the sockets. */
    if (len) {
        r->io.fdset.buf = sdscatlen(r->io.fdset.buf,buf,len);
        len = 0; /* Prevent entering the while below if we don't flush. */
        if (sdslen(r->io.fdset.buf) > PROTO_IOBUF_LEN) doflush = 1;
    }

    if (doflush) {
        p = (unsigned char*) r->io.fdset.buf;
        len = sdslen(r->io.fdset.buf);
    }

    /* Write in little chunchs so that when there are big writes we
     * parallelize while the kernel is sending data in background to
     * the TCP socket. */
    while(len) {
        size_t count = len < 1024 ? len : 1024;
        int broken = 0;
        for (j = 0; j < r->io.fdset.numfds; j++) {
            if (r->io.fdset.state[j] != 0) {
                /* Skip FDs alraedy in error. */
                broken++;
                continue;
            }

            /* Make sure to write 'count' bytes to the socket regardless
             * of short writes. */
            size_t nwritten = 0;
            while(nwritten != count) {
                retval = write(r->io.fdset.fds[j],p+nwritten,count-nwritten);
                if (retval <= 0) {
                    /* With blocking sockets, which is the sole user of this
                     * rio target, EWOULDBLOCK is returned only because of
                     * the SO_SNDTIMEO socket option, so we translate the error
                     * into one more recognizable by the user. */
                    if (retval == -1 && errno == EWOULDBLOCK) errno = ETIMEDOUT;
                    break;
                }
                nwritten += retval;
            }

            if (nwritten != count) {
                /* Mark this FD as broken. */
                r->io.fdset.state[j] = errno;
                if (r->io.fdset.state[j] == 0) r->io.fdset.state[j] = EIO;
            }
        }
        if (broken == r->io.fdset.numfds) return 0; /* All the FDs in error. */
        p += count;