net: fix qemu_flush_queued_packets() in presence of a hub

When frontend and backend are connected through a hub as below
(showing only one direction), and the frontend (or in general, all
output ports of the hub) cannot accept more traffic, the backend
queues packets in queue-A.

When the frontend (or in general, one output port) becomes ready again,
quemu tries to flush packets from queue-B, which is unfortunately empty.

  e1000.0 <--[queue B]-- hub0port0(hub)hub0port1 <--[queue A]-- tap.0

To fix this i propose to introduce a new function net_hub_flush()
which is called when trying to flush a queue connected to a hub.

Signed-off-by: Luigi Rizzo <rizzo@iet.unipi.it>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

net/hub.c

@@ -338,3 +338,17 @@ void net_hub_check_clients(void)
         }
     }
 }
+
+bool net_hub_flush(NetClientState *nc)
+{
+    NetHubPort *port;
+    NetHubPort *source_port = DO_UPCAST(NetHubPort, nc, nc);
+    int ret = 0;
+
+    QLIST_FOREACH(port, &source_port->hub->ports, next) {
+        if (port != source_port) {
+            ret += qemu_net_queue_flush(port->nc.send_queue);
+        }
+    }
+    return ret ? true : false;
+}

net/hub.h

@@ -21,5 +21,6 @@ NetClientState *net_hub_add_port(int hub_id, const char *name);
 NetClientState *net_hub_find_client_by_name(int hub_id, const char *name);
 void net_hub_info(Monitor *mon);
 void net_hub_check_clients(void);
+bool net_hub_flush(NetClientState *nc);
 
 #endif /* NET_HUB_H */

net/net.c

@@ -441,6 +441,12 @@ void qemu_flush_queued_packets(NetClientState *nc)
 {
     nc->receive_disabled = 0;
 
+    if (nc->peer && nc->peer->info->type == NET_CLIENT_OPTIONS_KIND_HUBPORT) {
+        if (net_hub_flush(nc->peer)) {
+            qemu_notify_event();
+        }
+        return;
+    }
     if (qemu_net_queue_flush(nc->send_queue)) {
         /* We emptied the queue successfully, signal to the IO thread to repoll
          * the file descriptor (for tap, for example).