nbd/server: Allow MULTI_CONN for shared writable exports

According to the NBD spec, a server that advertises
NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will
not see any cache inconsistencies: when properly separated by a single
flush, actions performed by one client will be visible to another
client, regardless of which client did the flush.

We always satisfy these conditions in qemu - even when we support
multiple clients, ALL clients go through a single point of reference
into the block layer, with no local caching.  The effect of one client
is instantly visible to the next client.  Even if our backend were a
network device, we argue that any multi-path caching effects that
would cause inconsistencies in back-to-back actions not seeing the
effect of previous actions would be a bug in that backend, and not the
fault of caching in qemu.  As such, it is safe to unconditionally
advertise CAN_MULTI_CONN for any qemu NBD server situation that
supports parallel clients.

Note, however, that we don't want to advertise CAN_MULTI_CONN when we
know that a second client cannot connect (for historical reasons,
qemu-nbd defaults to a single connection while nbd-server-add and QMP
commands default to unlimited connections; but we already have
existing means to let either style of NBD server creation alter those
defaults).  This is visible by no longer advertising MULTI_CONN for
'qemu-nbd -r' without -e, as in the iotest nbd-qemu-allocation.

The harder part of this patch is setting up an iotest to demonstrate
behavior of multiple NBD clients to a single server.  It might be
possible with parallel qemu-io processes, but I found it easier to do
in python with the help of libnbd, and help from Nir and Vladimir in
writing the test.

Signed-off-by: Eric Blake <eblake@redhat.com>
Suggested-by: Nir Soffer <nsoffer@redhat.com>
Suggested-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru>
Message-Id: <20220512004924.417153-3-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/nbd/server.c b/nbd/server.c
index 4cdbc062c1..213e00e761 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2021 Red Hat, Inc.
+ *  Copyright (C) 2016-2022 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device Server Side
@@ -1642,7 +1642,6 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
     int64_t size;
     uint64_t perm, shared_perm;
     bool readonly = !exp_args->writable;
-    bool shared = !exp_args->writable;
     BlockDirtyBitmapOrStrList *bitmaps;
     size_t i;
     int ret;
@@ -1693,11 +1692,12 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
     exp->description = g_strdup(arg->description);
     exp->nbdflags = (NBD_FLAG_HAS_FLAGS | NBD_FLAG_SEND_FLUSH |
                      NBD_FLAG_SEND_FUA | NBD_FLAG_SEND_CACHE);
+
+    if (nbd_server_max_connections() != 1) {
+        exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
+    }
     if (readonly) {
         exp->nbdflags |= NBD_FLAG_READ_ONLY;
-        if (shared) {
-            exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
-        }
     } else {
         exp->nbdflags |= (NBD_FLAG_SEND_TRIM | NBD_FLAG_SEND_WRITE_ZEROES |
                           NBD_FLAG_SEND_FAST_ZERO);