[L3][HA] Retry when setting HA router GW status.

When a HA router instance changes the state (active, backup), the
GW interface is set to up or down. As reported in the bug, while
keepalived is configuring the interface, the interface disappears
and appears again from the kernel namespace, as seen in the udev
messages.

This patch is a workaround until the real issue is addressed (if
possible), retrying the interface configuration for a small period
of time.

Related-Bug: #1916024

Change-Id: I8ced69f4f8e7d7c73da130a57e89e9d66590390b
(cherry picked from commit 662f483120)

neutron/agent/l3/ha_router.py

@@ -547,8 +547,11 @@ class HaRouter(router.RouterInfo):
         if ex_gw_port_id:
             interface_name = self.get_external_device_name(ex_gw_port_id)
             ns_name = self.get_gw_ns_name()
-            self.driver.set_link_status(interface_name, ns_name,
-                                        link_up=link_up)
+            if (not self.driver.set_link_status(
+                    interface_name, namespace=ns_name, link_up=link_up) and
+                    link_up):
+                LOG.error('Gateway interface for router %s was not set up; '
+                          'router will not work properly', self.router_id)
             if link_up and set_gw:
                 preserve_ips = self.get_router_preserve_ips()
                 self._external_gateway_settings(ex_gw_port, interface_name,

neutron/agent/linux/interface.py

@@ -323,14 +323,20 @@ class LinuxInterfaceDriver(object):
 
     def set_link_status(self, device_name, namespace=None, link_up=True):
         ns_dev = ip_lib.IPWrapper(namespace=namespace).device(device_name)
-        if not ns_dev.exists():
-            LOG.debug("Device %s may concurrently be deleted.", device_name)
-            return
+        try:
+            utils.wait_until_true(ns_dev.exists, timeout=3)
+        except utils.WaitTimeout:
+            LOG.debug('Device %s may have been deleted concurrently',
+                      device_name)
+            return False
+
         if link_up:
             ns_dev.link.set_up()
         else:
             ns_dev.link.set_down()
 
+        return True
+
 
 class NullDriver(LinuxInterfaceDriver):
     def plug_new(self, network_id, port_id, device_name, mac_address,