[Qemu-devel] non-file-based storage and -snapshot ?

[Michael Tokarev]

It is interesting that no one hit this bug before.

Apparenlty, no non-file-based storage works with
-snapshot or ,snapshot=on today.  Here's how:

$ qemu -drive file=nbd:localhost:12345 -snapshot
qemu: -drive file=nbd:localhost:12345: could not open disk image 
nbd:localhost:12345: No such file or directory

Here's the relevant code, block.c:bdrv_open(), with
irrelevant details removed:

    if (flags & BDRV_O_SNAPSHOT) {
        BlockDriverState *bs1;
        int is_protocol = 0;
        char backing_filename[PATH_MAX];

        /* if there is a backing file, use it */
        bs1 = bdrv_new("");
        ret = bdrv_open(bs1, filename, 0, drv);

        if (bs1->drv && bs1->drv->protocol_name)
            is_protocol = 1;

        /* Real path is meaningless for protocols */
        if (is_protocol)
            snprintf(backing_filename, sizeof(backing_filename),
                     "%s", filename);
        else if (!realpath(filename, backing_filename))   <=== it fails here
            return -errno;

        set_option_parameter(options, BLOCK_OPT_BACKING_FILE, backing_filename);
  }

Now, the "protocol" for this file is "raw", and
bs1->drv->protocol_name is NULL.

This thing is rather twisted, with lots of variables with
similar but slightly different meaning, so I'm not sure
how to fix this.

For now I changed the code to:

        set_option_parameter(options, BLOCK_OPT_BACKING_FILE, filename);

instead of

        set_option_parameter(options, BLOCK_OPT_BACKING_FILE, backing_filename);

and removed all the surrounding cruft.

I understand this will break for some things, but at least
it lets me to do my work now.

Any idea for a real fix?

Thanks!

/mjt