static int sd_open(BlockDriverState *bs, const char *filename, int flags)

{

    int ret, fd;

    uint32_t vid = 0;

    BDRVSheepdogState *s = bs->opaque;

    char vdi[SD_MAX_VDI_LEN], tag[SD_MAX_VDI_TAG_LEN];

    uint32_t snapid;

    char *buf = NULL;



    strstart(filename, "sheepdog:", (const char **)&filename);



    QLIST_INIT(&s->inflight_aio_head);

    QLIST_INIT(&s->pending_aio_head);

    s->fd = -1;



    memset(vdi, 0, sizeof(vdi));

    memset(tag, 0, sizeof(tag));

    if (parse_vdiname(s, filename, vdi, &snapid, tag) < 0) {

        ret = -EINVAL;

        goto out;

    }

    s->fd = get_sheep_fd(s);

    if (s->fd < 0) {

        ret = s->fd;

        goto out;

    }



    ret = find_vdi_name(s, vdi, snapid, tag, &vid, 0);

    if (ret) {

        goto out;

    }



    /*

     * QEMU block layer emulates writethrough cache as 'writeback + flush', so

     * we always set SD_FLAG_CMD_CACHE (writeback cache) as default.

     */

    s->cache_flags = SD_FLAG_CMD_CACHE;

    if (flags & BDRV_O_NOCACHE) {

        s->cache_flags = SD_FLAG_CMD_DIRECT;

    }



    if (s->cache_flags == SD_FLAG_CMD_CACHE) {

        s->flush_fd = connect_to_sdog(s->addr, s->port);

        if (s->flush_fd < 0) {

            error_report("failed to connect");

            ret = s->flush_fd;

            goto out;

        }

    }



    if (snapid || tag[0] != '\0') {

        dprintf("%" PRIx32 " snapshot inode was open.\n", vid);

        s->is_snapshot = true;

    }



    fd = connect_to_sdog(s->addr, s->port);

    if (fd < 0) {

        error_report("failed to connect");

        ret = fd;

        goto out;

    }



    buf = g_malloc(SD_INODE_SIZE);

    ret = read_object(fd, buf, vid_to_vdi_oid(vid), 0, SD_INODE_SIZE, 0,

                      s->cache_flags);



    closesocket(fd);



    if (ret) {

        goto out;

    }



    memcpy(&s->inode, buf, sizeof(s->inode));

    s->min_dirty_data_idx = UINT32_MAX;

    s->max_dirty_data_idx = 0;



    bs->total_sectors = s->inode.vdi_size / SECTOR_SIZE;

    pstrcpy(s->name, sizeof(s->name), vdi);

    qemu_co_mutex_init(&s->lock);

    g_free(buf);

    return 0;

out:

    qemu_aio_set_fd_handler(s->fd, NULL, NULL, NULL, NULL);

    if (s->fd >= 0) {

        closesocket(s->fd);

    }

    g_free(buf);

    return ret;

}
