Re: [Qemu-devel] [PATCH v4 2/4] block: support passing 'backing': '' to 'blockdev-add'

[Fam Zheng <famz@redhat.com>]

On Mon, 09/14 19:01, Alberto Garcia wrote:
> Passing an empty string allows opening an image but not its backing
> file. This was already described in the API documentation, only the
> implementation was missing.
> 
> This is useful for creating snapshots using images opened with
> blockdev-add, since they are not supposed to have a backing image
> before the operation.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/block.c b/block.c
> index 22d3b0e..ad1792d 100644
> --- a/block.c
> +++ b/block.c
> @@ -1402,6 +1402,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, const char *filename,
>      BlockDriverState *file = NULL, *bs;
>      BlockDriver *drv = NULL;
>      const char *drvname;
> +    const char *backing;
>      Error *local_err = NULL;
>      int snapshot_flags = 0;
>  
> @@ -1469,6 +1470,12 @@ static int bdrv_open_inherit(BlockDriverState **pbs, const char *filename,
>  
>      assert(drvname || !(flags & BDRV_O_PROTOCOL));
>  
> +    backing = qdict_get_try_str(options, "backing");
> +    if (backing && *backing == '\0') {
> +        flags |= BDRV_O_NO_BACKING;
> +    }
> +    qdict_del(options, "backing");
> +
>      bs->open_flags = flags;
>      bs->options = options;
>      options = qdict_clone_shallow(options);
> -- 
> 2.5.1
> 
> 

Specifying a non-empty "backing" will be a slient nop now, but it used to be an
error before.  Should we return an error?

Fam