Commit 7c6f5ddc authored Jan 17, 2019 by Eric Blake

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 <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190117193658.16413-21-eblake@redhat.com>

parent 68b96f15

nbd/client.c

+19 −0

Original line number	Diff line number	Diff line
		@@ -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;
		}