Re: [PATCH v12 13/14] stream: skip filters when writing backing file name to QCOW2 header

[Vladimir Sementsov-Ogievskiy]

27.10.2020 19:01, Andrey Shinkevich wrote:
> On 27.10.2020 18:09, Vladimir Sementsov-Ogievskiy wrote:
> > 22.10.2020 21:13, Andrey Shinkevich wrote:
> > > Avoid writing a filter JSON file name and a filter format name to QCOW2
> > > image when the backing file is changed after the block stream job.
> > > A user is still able to assign the 'backing-file' parameter for a
> > > block-stream job keeping in mind the possible issue mentioned above.
> > > If the user does not specify the 'backing-file' parameter, QEMU will
> > > assign it automatically.
> > >
> > > Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> > > ---
> > >   block/stream.c | 15 +++++++++++++--
> > >   blockdev.c     |  9 ++-------
> > >   2 files changed, 15 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/block/stream.c b/block/stream.c
> > > index e0540ee..1ba74ab 100644
> > > --- a/block/stream.c
> > > +++ b/block/stream.c
> > > @@ -65,6 +65,7 @@ static int stream_prepare(Job *job)
> > >       BlockDriverState *bs = blk_bs(bjob->blk);
> > >       BlockDriverState *unfiltered_bs = bdrv_skip_filters(bs);
> > >       BlockDriverState *base = bdrv_filter_or_cow_bs(s->above_base);
> > > +    BlockDriverState *base_unfiltered = NULL;
> > >       Error *local_err = NULL;
> > >       int ret = 0;
> > > @@ -75,8 +76,18 @@ static int stream_prepare(Job *job)
> > >           const char *base_id = NULL, *base_fmt = NULL;
> > >           if (base) {
> > >               base_id = s->backing_file_str;
> > > -            if (base->drv) {
> > > -                base_fmt = base->drv->format_name;
> > > +            if (base_id) {
> > > +                if (base->drv) {
> > > +                    base_fmt = base->drv->format_name;
> >
> > hmm. this doesn't make real sense: so, we assume that user specified 
> > backing_file_str, which may not relate to base, but we use 
> > base->drv->format_name? But it may be name of the filter driver, which would be 
> > wrong..
> >
> > Any ideas?
> >
> > 1. we can use base_fmt=NULL, to provoke probing on next open of the qcow2 file..
>
> I would choose this item #1 but have to check the probing code logic... 
> Particularly, I do not remember now if the probing is able to recognize a 
> protocol.
> The logic for the format_name in the QEMU existent code (I has kept it here in 
> the patch) is a slippery way for an imprudent user. That's why I staked on the 
> backing_file_str deprication in the previous version.
>
> > 2. we can do probing now
> > 3. we can at least check, if backing_file_str == 
>
> Not bad for the sanity check but we will search a node by the file name again - 
> not good ((

Not search, but only check one very likely option.

Additionally to 1. or 3. (or combined), or even keeping things as is (i.e. 
wrong, but it is preexisting), we can:

 - add backing-format argument to qapi as pair for backing-file
 - deprecate using backing-file without backing-format.

Then, after deprecation period we'll have correct code. This may be done in 
separate.

> > base_unfiltered->filename, in this case we can use 
> > base_unfiltered->drv->format_name
> >
> >
> > > +                }
> > > +            } else {
> > > +                base_unfiltered = bdrv_skip_filters(base);
> > > +                if (base_unfiltered) {
> > > +                    base_id = base_unfiltered->filename;
> > > +                    if (base_unfiltered->drv) {
> > > +                        base_fmt = base_unfiltered->drv->format_name;
> > > +                    }
> > > +                }
> > >               }
> > >           }
> > >           bdrv_set_backing_hd(unfiltered_bs, base, &local_err);
> > > diff --git a/blockdev.c b/blockdev.c
> > > index c917625..0e9c783 100644
> > > --- a/blockdev.c
> > > +++ b/blockdev.c
>
> [...]
>
> > > -    stream_start(has_job_id ? job_id : NULL, bs, base_bs, base_name,
> > > +    stream_start(has_job_id ? job_id : NULL, bs, base_bs,
> > > +                 has_backing_file ? backing_file : NULL,
> >
> > backing_file should be NULL if has_backing_file is false, so you can use just 
> > backing_file instead of ternary operator.
>
> Yes, if reliable. I has kept the conformation with the ternary operator at the 
> first parameter above.
>
> Andrey
>
> > >                    job_flags, has_speed ? speed : 0, on_error,
> > >                    filter_node_name, &local_err);
> > >       if (local_err) {


--
Best regards,
Vladimir