Don't depend on translated strings for error check

Currently, execute() may raise an exception that contains a *translated*
string that starts with 'Exit code: %(returncode)d...' if the returncode
of a process was not 0. find_child_pids() will then check if the
raised exception contains 'Exit code: 1' (to check if the returncode is
1), but in non-English locales this will fail as the 2 strings are not
encoded the same.

This patch adds a new ProcessExecutionError (which inherits from
RuntimeError, so as to not change all the code that currently depends on
execute() returning RuntimeError) which now accepts a returncode. This
can be changed explicitly without depending on the error message.

Later patches can move ProcessExecutionError to neutron-lib, if this is
needed - this patch intends to write the smallest piece of code that can
be backported.

Closes-Bug: #1638273
Change-Id: I85d3bec13e852918eb13e73c1367c70e1f4d34b1

neutron/agent/linux/utils.py

@@ -45,6 +45,12 @@ from neutron import wsgi
 LOG = logging.getLogger(__name__)
 
 
+class ProcessExecutionError(RuntimeError):
+    def __init__(self, message, returncode):
+        super(ProcessExecutionError, self).__init__(message)
+        self.returncode = returncode
+
+
 class RootwrapDaemonHelper(object):
     __client = None
     __lock = threading.Lock()
@@ -136,7 +142,7 @@ def execute(cmd, process_input=None, addl_env=None,
             if log_fail_as_error:
                 LOG.error(msg)
             if check_exit_code:
-                raise RuntimeError(msg)
+                raise ProcessExecutionError(msg, returncode=returncode)
         else:
             LOG.debug("Exit code: %d", returncode)
 
@@ -169,11 +175,11 @@ def find_child_pids(pid):
     try:
         raw_pids = execute(['ps', '--ppid', pid, '-o', 'pid='],
                            log_fail_as_error=False)
-    except RuntimeError as e:
+    except ProcessExecutionError as e:
         # Unexpected errors are the responsibility of the caller
         with excutils.save_and_reraise_exception() as ctxt:
             # Exception has already been logged by execute
-            no_children_found = 'Exit code: 1' in str(e)
+            no_children_found = e.returncode == 1
             if no_children_found:
                 ctxt.reraise = False
                 return []

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

@@ -122,7 +122,7 @@ class AgentUtilsExecuteTest(base.BaseTestCase):
         self.mock_popen.return_value = ('', '')
         self.process.return_value.returncode = 1
         with mock.patch.object(utils, 'LOG') as log:
-            self.assertRaises(RuntimeError, utils.execute,
+            self.assertRaises(utils.ProcessExecutionError, utils.execute,
                               ['ls'], log_fail_as_error=False)
             self.assertFalse(log.error.called)
 
@@ -185,7 +185,8 @@ class TestFindChildPids(base.BaseTestCase):
 
     def test_returns_empty_list_for_exit_code_1(self):
         with mock.patch.object(utils, 'execute',
-                               side_effect=RuntimeError('Exit code: 1')):
+                               side_effect=utils.ProcessExecutionError(
+                                   '', returncode=1)):
             self.assertEqual([], utils.find_child_pids(-1))
 
     def test_returns_empty_list_for_no_output(self):