Re: [PATCH RFC v2 5/5] block: Deprecate transaction type drive-backup

[Eric Blake]

On Sat, Oct 09, 2021 at 02:09:44PM +0200, Markus Armbruster wrote:
> Several moons ago, Vladimir posted
> 
>     Subject: [PATCH v2 3/3] qapi: deprecate drive-backup
>     Date: Wed,  5 May 2021 16:58:03 +0300
>     Message-Id: <20210505135803.67896-4-vsementsov@virtuozzo.com>
>     https://lists.gnu.org/archive/html/qemu-devel/2021-05/msg01394.html
> 
> with this
> 
>     TODO: We also need to deprecate drive-backup transaction action..
>     But union members in QAPI doesn't support 'deprecated' feature. I tried
>     to dig a bit, but failed :/ Markus, could you please help with it? At
>     least by advice?
> 
> This is one way to resolve it.  Sorry it took so long.
> 
> John explored another way, namely adding feature flags to union
> branches.  Could also be useful, say to add different features to
> branches in multiple unions sharing the same tag enum.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  qapi/transaction.json | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/qapi/transaction.json b/qapi/transaction.json
> index d175b5f863..0564a893b3 100644
> --- a/qapi/transaction.json
> +++ b/qapi/transaction.json
> @@ -54,6 +54,9 @@
>  # @blockdev-snapshot-sync: since 1.1
>  # @drive-backup: Since 1.6
>  #
> +# Features:
> +# @deprecated: Member @drive-backup is deprecated.  Use FIXME instead.

Obviously, we'd need to flesh this out ("'blockdev-backup' with proper
node names"? something else?) before dropping RFC on this patch.

And we'd want to edit docs/about/deprecated.rst to match.

> +#
>  # Since: 1.1
>  ##
>  { 'enum': 'TransactionActionKind',
> @@ -62,7 +65,7 @@
>              'block-dirty-bitmap-disable', 'block-dirty-bitmap-merge',
>              'blockdev-backup', 'blockdev-snapshot',
>              'blockdev-snapshot-internal-sync', 'blockdev-snapshot-sync',
> -            'drive-backup' ] }
> +            { 'name': 'drive-backup', 'features': [ 'deprecated' ] } ] }
>  
>  ##
>  # @AbortWrapper:
> -- 
> 2.31.1
>

But the idea is reasonable, and I'm not sure if we're any closer to
John's idea of feature flags on union branches.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org