Print useful error on rootwrap daemon failure

If the rootwrap daemon fails to execute a command, it
generates a cryptic message:

Unserializable message: ('#ERROR', ValueError('I/O operation on closed file',))

We should at least log the command that we were trying
to run, which will help users figure out why it failed.

Change-Id: I2c94e5a226630432028351f8287868f4fe5d2fa1
Closes-bug: #1677742

neutron/agent/linux/utils.py

@@ -107,7 +107,11 @@ def execute_rootwrap_daemon(cmd, process_input, addl_env):
     # just logging the execution error.
     LOG.debug("Running command (rootwrap daemon): %s", cmd)
     client = RootwrapDaemonHelper.get_client()
-    return client.execute(cmd, process_input)
+    try:
+        return client.execute(cmd, process_input)
+    except Exception:
+        with excutils.save_and_reraise_exception():
+            LOG.error(_LE("Rootwrap error running command: %s"), cmd)
 
 
 def execute(cmd, process_input=None, addl_env=None,

neutron/tests/unit/agent/linux/test_utils.py

@@ -58,6 +58,17 @@ class AgentUtilsExecuteTest(base.BaseTestCase):
         result = utils.execute(["ls", self.test_file], run_as_root=True)
         self.assertEqual(result, expected)
 
+    @mock.patch.object(utils.RootwrapDaemonHelper, 'get_client')
+    def test_with_helper_exception(self, get_client):
+        client_inst = mock.Mock()
+        client_inst.execute.side_effect = RuntimeError
+        get_client.return_value = client_inst
+        self.config(group='AGENT', root_helper_daemon='echo')
+        with mock.patch.object(utils, 'LOG') as log:
+            self.assertRaises(RuntimeError, utils.execute,
+                              ['ls'], run_as_root=True)
+            self.assertTrue(log.error.called)
+
     def test_stderr_true(self):
         expected = "%s\n" % self.test_file
         self.mock_popen.return_value = [expected, ""]