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

[Eric Blake <eblake@redhat.com>]

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>
---
 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
+             * 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;
     }
-- 
2.20.1