[Libguestfs] [PATCH v2] appliance: extract UUID from QCOW2 disk image

[Vladimir Sementsov-Ogievskiy]

12.08.2020 18:36, Andrey Shinkevich wrote:
> For the appliance of the QCOW2 format, get the UUID of the disk by
> reading the first 256k bytes with 'qemu-img dd' command. Then pass the
> read block to the 'file' command. In case of failure, run the 'file'
> command again directly.
> 
> Suggested-by: Denis V. Lunev <den at openvz.org>
> Signed-off-by: Andrey Shinkevich <andrey.shinkevich at virtuozzo.com>
> ---
> v2:
>    01: The order of the function calls to direct <file> command on the appliance
>        and the indirect one (through the temporary file) has been swapped.
> 
>   lib/appliance-kcmdline.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/appliance-kcmdline.c b/lib/appliance-kcmdline.c
> index fbeb4f4..c78524b 100644
> --- a/lib/appliance-kcmdline.c
> +++ b/lib/appliance-kcmdline.c
> @@ -71,7 +71,7 @@ read_uuid (guestfs_h *g, void *retv, const char *line, size_t len)
>    * The L<file(1)> command does the hard work.
>    */
>   static char *
> -get_root_uuid (guestfs_h *g, const char *appliance)
> +do_get_root_uuid (guestfs_h *g, const char *appliance)
>   {
>     CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g);
>     char *ret = NULL;
> @@ -96,6 +96,74 @@ get_root_uuid (guestfs_h *g, const char *appliance)
>   }
>   
>   /**
> + * Read the first 256k bytes of the in_file with L<qemu-img(1)> command
> + * and write them into the out_file. That may be useful to get UUID of
> + * the QCOW2 disk image with further L<file(1)> command.
> + * The function returns zero if successful, otherwise -1.
> + */
> +static int
> +run_qemu_img_dd (guestfs_h *g, const char *in_file, char *out_file)
> +{
> +  CLEANUP_CMD_CLOSE struct command *cmd = guestfs_int_new_command (g);
> +  int r;
> +
> +  guestfs_int_cmd_add_arg (cmd, "qemu-img");
> +  guestfs_int_cmd_add_arg (cmd, "dd");
> +  guestfs_int_cmd_add_arg_format (cmd, "if=%s", in_file);
> +  guestfs_int_cmd_add_arg_format (cmd, "of=%s", out_file);
> +  guestfs_int_cmd_add_arg (cmd, "bs=256k");
> +  guestfs_int_cmd_add_arg (cmd, "count=1");
> +
> +  r = guestfs_int_cmd_run (cmd);
> +  if (r == -1) {
> +    error (g, "Failed to run qemu-img");
> +    return -1;
> +  }
> +  if (!WIFEXITED (r) || WEXITSTATUS (r) != 0) {
> +    guestfs_int_external_command_failed (g, r, "qemu-img dd", NULL);
> +    return -1;
> +  }
> +
> +  return 0;
> +}
> +
> +/**
> + * Get the UUID from the appliance disk image.
> + */
> +static char *
> +get_root_uuid (guestfs_h *g, const char *appliance)
> +{
> +  char *uuid = NULL;
> +  int ret;
> +  char tmp_file[] = "/tmp/libguestfsXXXXXX";
> +
> +  uuid = do_get_root_uuid (g, appliance);
> +  if (uuid) {
> +      return uuid;
> +  }
> +
> +  if (!mktemp (tmp_file)) {
> +    error (g, "get_root_uuid: mktemp failed");
> +    return NULL;
> +  }
> +
> +  ret = run_qemu_img_dd (g, appliance, tmp_file);
> +  if (ret == 0) {
> +    uuid = do_get_root_uuid (g, tmp_file);
> +    if (uuid) {
> +      goto out;
> +    }
> +  }
> +
> +  /* We get here in case of failure to extract UUID from the temporary file. */

Or if qemu-img dd failed (which is already reported).

A bit more native here seems to check error instead for checking success:

   ret = run_qemu_img_dd (g, appliance, tmp_file);
   if (ret < 0) {
     goto out;
   }

   uuid = do_get_root_uuid (g, tmp_file);
   if (!uuid) {
     error (g, "Failed to get the appliance UUID");
   }

out:
   unlink (tmp_file);
   return uuid;
}


> +  error (g, "Failed to get the appliance UUID");
> +
> +out:
> +  unlink (tmp_file);
> +  return uuid;
> +}
> +
> +/**
>    * Construct the Linux command line passed to the appliance.  This is
>    * used by the C<direct> and C<libvirt> backends, and is simply
>    * located in this file because it's a convenient place for this
> 

Still, good enough for me, and I was going to put Reviewed-by here, but noted new reply from Pino, so, you'll have to resend anyway.


-- 
Best regards,
Vladimir