Re: [Qemu-devel] [PATCH 2/2] block/rbd: Attempt to parse legacy filenames

[Jeff Cody <jcody@redhat.com>]

On Tue, Sep 11, 2018 at 02:22:31PM -0400, John Snow wrote:
> 
> 
> On 09/11/2018 01:15 AM, Jeff Cody wrote:
> > When we converted rbd to get rid of the older key/value-centric
> > encoding format, we broke compatibility with image files with backing
> > file strings encoded in the old format.
> > 
> > This leaves a bit of an ugly conundrum, and a hacky solution.
> > 
> > If the initial attempt to parse the "proper" options fails, it assumes
> > that we may have an older key/value encoded filename.  Fall back to
> > attempting to parse the filename, and extract the required options from
> > it.  If that fails, pass along the original error message.
> > 
> > This approach has a potential drawback: if for some reason there are
> > some options supplied the new way, and some the old way, we may not
> > catch all the old options if they are not required options (since it
> > won't cause the initial failure).
> > 
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > ---
> >  block/rbd.c | 30 +++++++++++++++++++++++++++---
> >  1 file changed, 27 insertions(+), 3 deletions(-)
> > 
> > diff --git a/block/rbd.c b/block/rbd.c
> > index a8e79d01d2..bce86b8bde 100644
> > --- a/block/rbd.c
> > +++ b/block/rbd.c
> > @@ -685,7 +685,7 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
> >      BlockdevOptionsRbd *opts = NULL;
> >      const QDictEntry *e;
> >      Error *local_err = NULL;
> > -    char *keypairs, *secretid;
> > +    char *keypairs, *secretid, *filename;
> >      int r;
> >  
> >      keypairs = g_strdup(qdict_get_try_str(options, "=keyvalue-pairs"));
> > @@ -700,8 +700,32 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
> >  
> >      r = qemu_rbd_convert_options(bs, options, &opts, &local_err);
> >      if (local_err) {
> > -        error_propagate(errp, local_err);
> > -        goto out;
> > +        /* If the initial attempt to convert and process the options failed,
> > +         * we may be attempting to open an image file that has the rbd options
> > +         * specified in the older format consisting of all key/value pairs
> > +         * encoded in the filename.  Go ahead and attempt to parse the
> > +         * filename, and see if we can pull out the required options */
> 
> Is it worth splitting out the legacy parsing routine here into its own
> function, given that we will generally depend on it not being invoked?
> i.e., for readability, it doesn't need to distract us.
> 

Yeah, that would probably be good.

> > +        Error *parse_err = NULL;
> > +
> > +        filename = g_strdup(qdict_get_try_str(options, "filename"));
> > +        qdict_del(options, "filename");
> > +
> > +        qemu_rbd_parse_filename(filename, options, NULL);
> > +
> > +        g_free(keypairs);
> 
> As Eric already noticed, better to just return with error if this is
> already set.
> 
> > +        keypairs = g_strdup(qdict_get_try_str(options, "=keyvalue-pairs"));
> > +        if (keypairs) {
> > +            qdict_del(options, "=keyvalue-pairs");
> > +        }
> > +
> > +        r = qemu_rbd_convert_options(bs, options, &opts, &parse_err);
> > +        if (parse_err) {
> > +            /* if the second attempt failed, pass along the original error
> > +             * message for the current format */
> > +            error_propagate(errp, local_err);
> > +            error_free(parse_err);
> > +            goto out;
> > +        }
> 
> If it does succeed, though, ought we emit a deprecated warning that the
> old specification syntax is not supported?
>

I don't know.  Without this support, we can't open some existing images.  At
what point would we actually remove that support?

> Once we load the image, will the header get rewritten into a compliant
> format?
> 

Hmm - I think in some code paths, but not all.  I don't think the answer is
'yes' universally, alas.