From 7c6f5ddca62905e67025aa0657e8a011cbdffa11 Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Thu, 17 Jan 2019 13:36:57 -0600 Subject: [PATCH] nbd/client: Work around 3.0 bug for listing meta contexts 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 Reviewed-by: Vladimir Sementsov-Ogievskiy Message-Id: <20190117193658.16413-21-eblake@redhat.com> --- nbd/client.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/nbd/client.c b/nbd/client.c index 798b82f205..8a083c2f42 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_meta_query(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 + * qemu:, then run a more specific query to make sure. + */ + seen_qemu = true; + if (nbd_send_meta_query(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; }