Re: [Qemu-devel] [FIX V2] block: Fix device snapshots broken by the block filter snapshots patchset.

[Fam Zheng <famz@redhat.com>]

On Mon, 02/10 22:49, Benoît Canet wrote:
> Take into account the fact that a block filter like quorum will be in bs->file
> while a regular block driver device is really on the top level.
> 
> Signed-off-by: Benoit Canet <benoit@irqsave.net>
> ---
>  block.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 07ac50a..d04f535 100644
> --- a/block.c
> +++ b/block.c
> @@ -5400,14 +5400,16 @@ bool bdrv_is_first_non_filter(BlockDriverState *candidate)
>  
>      /* walk down the bs forest recursively */
>      QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> -        bool perm;
> -
> -        if (!bs->file) {
> -            continue;
> +        bool perm = false;
> +
> +        if (bs->file &&
> +            bs->file->drv &&
> +            bs->file->drv->authorizations[BS_IS_A_FILTER]) {
> +            perm = bdrv_recurse_is_first_non_filter(bs->file, candidate);
> +        } else if (bs == candidate) {
> +            perm = true;
>          }
>  
> -        perm = bdrv_recurse_is_first_non_filter(bs->file, candidate);
> -
>          /* candidate is the first non filter */
>          if (perm) {
>              return true;

With this change, if the top level driver has ->file, its implementation of
.bdrv_recurse_is_first_non_filter() is skipped and bs->file is the start point.

So we have an implication that single child block drivers (that has ->file)
doesn't need to, and shouldn't implement this operation, as commentted above
bdrv_generic_is_first_non_filter.

Tested that this patch fixes the external snapshot problem, but didn't test
the "quorum as bs->file case".

Thanks,

Reviewed-by: Fam Zheng <famz@redhat.com>