Re: [Qemu-devel] [PATCH v3 18/19] nbd/client: Work around 3.0 bug for listing meta contexts

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

12.01.2019 20:58, Eric Blake wrote:
> Commit 3d068aff forgot to advertise available qemu: contexts
> when the client requests a list with 0 queries. Furthermore,
> 3.0 shipped with a qemu-img hack of x-dirty-bitmap (commit
> 216ee365) that _silently_ acts as though the entire image is
> clean if a requested bitmap is not present.  Both bugs have
> been recently fixed, so that a modern qemu server gives full
> context output right away, and the client refuses a
> connection if a requested x-dirty-bitmap was not found.
> 
> Still, it is likely that there will be users that have to
> work with a mix of old and new qemu versions, depending on
> which features get backported where, at which point being
> able to rely on 'qemu-img --list' output to know for sure
> whether a given NBD export has the desired dirty bitmap is
> much nicer than blindly connecting and risking that the
> entire image may appear clean.  We can make our --list code
> smart enough to work around buggy servers by tracking
> whether we've seen any qemu: replies in the original 0-query
> list; if not, repeat with a single query on "qemu:" (which
> may still have no replies, but then we know for sure we
> didn't trip up on the server bug).
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> Message-Id: <20181215135324.152629-22-eblake@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> ---
>   nbd/client.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
> 
> diff --git a/nbd/client.c b/nbd/client.c
> index 2001e6e8160..64f3e45edd4 100644
> --- a/nbd/client.c
> +++ b/nbd/client.c
> @@ -21,6 +21,7 @@
>   #include "qapi/error.h"
>   #include "trace.h"
>   #include "nbd-internal.h"
> +#include "qemu/cutils.h"
> 
>   /* Definitions for opaque data types */
> 
> @@ -828,6 +829,8 @@ static int nbd_list_meta_contexts(QIOChannel *ioc,
>                                     Error **errp)
>   {
>       int ret;
> +    int seen_any = false;
> +    int seen_qemu = false;
> 
>       if (nbd_send_one_meta_context(ioc, NBD_OPT_LIST_META_CONTEXT,
>                                     info->name, NULL, errp) < 0) {
> @@ -839,9 +842,25 @@ static int nbd_list_meta_contexts(QIOChannel *ioc,
> 
>           ret = nbd_receive_one_meta_context(ioc, NBD_OPT_LIST_META_CONTEXT,
>                                              &context, NULL, errp);
> +        if (ret == 0 && seen_any && !seen_qemu) {
> +            /*
> +             * Work around qemu 3.0 bug: the server forgot to send
> +             * "qemu:" replies to 0 queries. If we saw at least one
> +             * reply (probably base:allocation), but none of them were

if we are saying about 3.0, it is base:allocation for sure, isn't it?

> +             * qemu:, then run a more specific query to make sure.
> +             */
> +            seen_qemu = true;
> +            if (nbd_send_one_meta_context(ioc, NBD_OPT_LIST_META_CONTEXT,
> +                                          info->name, "qemu:", errp) < 0) {
> +                return -1;
> +            }
> +            continue;
> +        }
>           if (ret <= 0) {
>               return ret;
>           }
> +        seen_any = true;
> +        seen_qemu |= strstart(context, "qemu:", NULL);
>           info->contexts = g_renew(char *, info->contexts, ++info->n_contexts);
>           info->contexts[info->n_contexts - 1] = context;
>       }
> 


-- 
Best regards,
Vladimir