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 <ebl...@redhat.com> > Message-Id: <20181215135324.152629-22-ebl...@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsement...@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