Re: [Qemu-devel] [PATCH v2 2/3] block/rbd: Attempt to parse legacy filenames

[Eric Blake <eblake@redhat.com>]

On 9/11/18 3:43 PM, 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.
> 
> We do not support mixed modern usage alongside legacy keyvalue pair
> usage.
> 
> A deprecation warning has been added, although care should be taken
> when actually deprecating since the impact is not limited to
> commandline or qapi usage, but also opening existing images.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>   block/rbd.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 51 insertions(+), 2 deletions(-)

Where's the patch to qemu/qemu-deprecated.texi to mention the new 
message and our advice on upgrading images to avoid triggering it?

> 
> diff --git a/block/rbd.c b/block/rbd.c
> index b199450f9f..5090e4f662 100644
> --- a/block/rbd.c
> +++ b/block/rbd.c
> @@ -678,6 +678,33 @@ static int qemu_rbd_convert_options(QDict *options, BlockdevOptionsRbd **opts,
>       return 0;
>   }
>   
> +static int qemu_rbd_attempt_legacy_options(QDict *options,
> +                                           BlockdevOptionsRbd **opts,
> +                                           char **keypairs)
> +{
> +    char *filename;
> +    int r;
> +
> +    filename = g_strdup(qdict_get_try_str(options, "filename"));
> +    if (!filename) {
> +        return -EINVAL;
> +    }
> +    qdict_del(options, "filename");
> +
> +    qemu_rbd_parse_filename(filename, options, NULL);

Intentionally ignoring errors here,

> +
> +    /* keypairs freed by caller */
> +    *keypairs = g_strdup(qdict_get_try_str(options, "=keyvalue-pairs"));
> +    if (*keypairs) {
> +        qdict_del(options, "=keyvalue-pairs");
> +    }
> +
> +    r = qemu_rbd_convert_options(options, opts, NULL);

and here. I guess we'll see how the caller is expecting things to behave.

> +
> +    g_free(filename);
> +    return r;
> +}
> +
>   static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
>                            Error **errp)
>   {
> @@ -700,8 +727,30 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
>   
>       r = qemu_rbd_convert_options(options, &opts, &local_err);
>       if (local_err) {
> -        error_propagate(errp, local_err);
> -        goto out;
> +        /* If keypairs are present, that means some options are present in
> +         * the modern option format.  Don't attempt to parse legacy option
> +         * formats, as we won't support mixed usage. */
> +        if (keypairs) {
> +            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. */
> +        r = qemu_rbd_attempt_legacy_options(options, &opts, &keypairs);
> +        if (r < 0) {
> +            error_propagate(errp, local_err);
> +            goto out;
> +        }
> +        /* Take care whenever deciding to actually deprecate; once this ability
> +         * is removed, we will not be able to open any images with legacy-styled
> +         * backing image strings. */
> +        error_report("RBD options encoded in the filename as keyvalue pairs "
> +                     "is deprecated.  Future versions may cease to parse "
> +                     "these options in the future.");

Okay, so you're making a best-effort fallback to scrape out any 
remaining keyvalue pairs. If there was no filename (and hence no 
keyvalue pairs populated), we know up front that we lack enough 
information, so attempt_legacy_options() returns failure right away; if 
there was a filename, we don't know if we got all the required options 
(so ignoring errors was okay) - that will be up to later code to 
decipher, after we emit our warning that we already relied on legacy 
options (if the later code has all it needed, then only the warning is 
emitted; if later code fails, the user now has the warning in addition 
to the later failure message to help them resolve their images).

A bit confusing how it all fits together, but it seems to work out in 
the end.

Reviewed-by: Eric Blake <eblake@redhat.com>

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