nbd: Fully initialize client in case of failed negotiation

If a non-NBD client connects to qemu-nbd, we would end up with
a SIGSEGV in nbd_client_put() because we were trying to
unregister the client's association to the export, even though
we skipped inserting the client into that list.  Easy trigger
in two terminals:

$ qemu-nbd -p 30001 --format=raw file
$ nmap 127.0.0.1 -p 30001

nmap claims that it thinks it connected to a pago-services1
server (which probably means nmap could be updated to learn the
NBD protocol and give a more accurate diagnosis of the open
port - but that's not our problem), then terminates immediately,
so our call to nbd_negotiate() fails.  The fix is to reorder
nbd_co_client_start() to ensure that all initialization occurs
before we ever try talking to a client in nbd_negotiate(), so
that the teardown sequence on negotiation failure doesn't fault
while dereferencing a half-initialized object.

While debugging this, I also noticed that nbd_update_server_watch()
called by nbd_client_closed() was still adding a channel to accept
the next client, even when the state was no longer RUNNING.  That
is fixed by making nbd_can_accept() pay attention to the current
state.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1451614Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170527030421.28366-1-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

nbd/server.c

@@ -1358,16 +1358,14 @@ static coroutine_fn void nbd_co_client_start(void *opaque)
 
     if (exp) {
         nbd_export_get(exp);
+        QTAILQ_INSERT_TAIL(&exp->clients, client, next);
     }
+    qemu_co_mutex_init(&client->send_lock);
+
     if (nbd_negotiate(data)) {
         client_close(client);
         goto out;
     }
-    qemu_co_mutex_init(&client->send_lock);
-
-    if (exp) {
-        QTAILQ_INSERT_TAIL(&exp->clients, client, next);
-    }
 
     nbd_client_receive_next_request(client);
 

qemu-nbd.c

@@ -325,7 +325,7 @@ out:
 
 static int nbd_can_accept(void)
 {
-    return nb_fds < shared;
+    return state == RUNNING && nb_fds < shared;
 }
 
 static void nbd_export_closed(NBDExport *exp)