Check for concurrent bridge creation in bridge add

With Linux bridge, either Neutron or Nova may create the bridge
associated with a network so this process must be robust to
concurrent creations of the same bridge.

This change just has the addbr call capture exceptions and avoid
reraising if the device already exists.

Closes-Bug: #1617447
Change-Id: Ib0266086e0caffecf3f9f2a8291369cfa155f386
(cherry picked from commit c25ce5a852)

vif_plug_linux_bridge/linux_net.py

@@ -24,6 +24,7 @@ import os
 from oslo_concurrency import lockutils
 from oslo_concurrency import processutils
 from oslo_log import log as logging
+from oslo_utils import excutils
 
 from vif_plug_linux_bridge import privsep
 
@@ -122,7 +123,11 @@ def _ensure_bridge_privileged(bridge, interface, net_attrs, gateway,
     """
     if not device_exists(bridge):
         LOG.debug('Starting Bridge %s', bridge)
-        processutils.execute('brctl', 'addbr', bridge)
+        try:
+            processutils.execute('brctl', 'addbr', bridge)
+        except Exception:
+            with excutils.save_and_reraise_exception() as ectx:
+                ectx.reraise = not device_exists(bridge)
         processutils.execute('brctl', 'setfd', bridge, 0)
         # processutils.execute('brctl setageing %s 10' % bridge)
         processutils.execute('brctl', 'stp', bridge, 'off')

vif_plug_linux_bridge/tests/test_linux_net.py

@@ -94,6 +94,26 @@ class LinuxNetTest(testtools.TestCase):
         self.assertEqual([], mock_exec.mock_calls)
         mock_dev_exists.assert_called_once_with("br0")
 
+    @mock.patch.object(processutils, "execute")
+    @mock.patch.object(linux_net, "device_exists", return_value=False)
+    def test_ensure_bridge_addbr_exception(self, mock_dev_exists, mock_exec):
+        mock_exec.side_effect = ValueError()
+        with testtools.ExpectedException(ValueError):
+            linux_net.ensure_bridge("br0", None, filtering=False)
+
+    @mock.patch.object(processutils, "execute")
+    @mock.patch.object(linux_net, "device_exists", side_effect=[False, True])
+    def test_ensure_bridge_concurrent_add(self, mock_dev_exists, mock_exec):
+        mock_exec.side_effect = [ValueError(), 0, 0, 0]
+        linux_net.ensure_bridge("br0", None, filtering=False)
+
+        calls = [mock.call('brctl', 'addbr', 'br0'),
+                 mock.call('brctl', 'setfd', 'br0', 0),
+                 mock.call('brctl', 'stp', 'br0', "off"),
+                 mock.call('ip', 'link', 'set', 'br0', "up")]
+        self.assertEqual(calls, mock_exec.mock_calls)
+        mock_dev_exists.assert_has_calls([mock.call("br0"), mock.call("br0")])
+
     @mock.patch.object(os.path, "exists", return_value=False)
     @mock.patch.object(processutils, "execute")
     @mock.patch.object(linux_net, "device_exists", return_value=False)