network: reconnect tap devices during networkNotifyActualDevice

If a network is destroyed and restarted, or its bridge is changed, any
tap devices that had previously been connected to the bridge will no
longer be connected. As a first step in automating a reconnection of
all tap devices when this happens, this patch modifies
networkNotifyActualDevice() (which is called once for every
<interface> of every active domain whenever libvirtd is restarted) to
reconnect any tap devices that it finds disconnected.

With this patch in place, you will need to restart libvirtd to
reconnect all the taps to their proper bridges. A future patch will
add a callback that hypervisor drivers can register with the network
driver to that the network driver can trigger this behavior
automatically whenever a network is started.

src/network/bridge_driver.c

@@ -4612,7 +4612,8 @@ networkAllocateActualDevice(virDomainDefPtr dom,
  * Called to notify the network driver when libvirtd is restarted and
  * finds an already running domain. If appropriate it will force an
  * allocation of the actual->direct.linkdev to get everything back in
- * order.
+ * order, or re-attach the interface's tap device to the network's
+ * bridge.
  *
  * Returns 0 on success, -1 on failure.
  */
@@ -4627,6 +4628,7 @@ networkNotifyActualDevice(virDomainDefPtr dom,
     virNetworkForwardIfDefPtr dev = NULL;
     size_t i;
     int ret = -1;
+    char *master = NULL;
 
     if (iface->type != VIR_DOMAIN_NET_TYPE_NETWORK)
         return 0;
@@ -4651,6 +4653,31 @@ networkNotifyActualDevice(virDomainDefPtr dom,
                     netdef->bridge) < 0))
             goto error;
 
+    /* see if we're connected to the correct bridge */
+    if (netdef->bridge) {
+        if (virNetDevGetMaster(iface->ifname, &master) < 0)
+            goto error;
+
+        if (STRNEQ_NULLABLE(netdef->bridge, master)) {
+            /* disconnect from current (incorrect) bridge */
+            if (master)
+                ignore_value(virNetDevBridgeRemovePort(master, iface->ifname));
+
+            /* attach/reattach to correct bridge.
+             * NB: we can't notify the guest of any MTU change anyway,
+             * so there is no point in trying to learn the actualMTU
+             * (final arg to virNetDevTapAttachBridge())
+             */
+            if (virNetDevTapAttachBridge(iface->ifname, netdef->bridge,
+                                         &iface->mac, dom->uuid,
+                                         virDomainNetGetActualVirtPortProfile(iface),
+                                         virDomainNetGetActualVlan(iface),
+                                         iface->mtu, NULL) < 0) {
+                goto error;
+            }
+        }
+    }
+
     if (!iface->data.network.actual ||
         (actualType != VIR_DOMAIN_NET_TYPE_DIRECT &&
          actualType != VIR_DOMAIN_NET_TYPE_HOSTDEV)) {
@@ -4783,6 +4810,7 @@ networkNotifyActualDevice(virDomainDefPtr dom,
     ret = 0;
  cleanup:
     virNetworkObjEndAPI(&network);
+    VIR_FREE(master);
     return ret;
 
  error: