Make ProcessExecutionError picklable

Serialising/deserialising exceptions in python follows the protocol
defined by pickle.  It relies on the base Exception.__init__ setting
self.args, and later resurrects exceptions with class(*args).

This change rewrites processutils.ProcessExecutionError so it survives a
pickle.dumps/loads round-trip.

Change-Id: I9b8d104f60df868be7b808c72c932d08f1752777

oslo_concurrency/processutils.py

@@ -63,26 +63,33 @@ class UnknownArgumentError(Exception):
 class ProcessExecutionError(Exception):
     def __init__(self, stdout=None, stderr=None, exit_code=None, cmd=None,
                  description=None):
+        super(ProcessExecutionError, self).__init__(
+            stdout, stderr, exit_code, cmd, description)
         self.exit_code = exit_code
         self.stderr = stderr
         self.stdout = stdout
         self.cmd = cmd
         self.description = description
 
+    def __str__(self):
+        description = self.description
         if description is None:
             description = _("Unexpected error while running command.")
+
+        exit_code = self.exit_code
         if exit_code is None:
             exit_code = '-'
+
         message = _('%(description)s\n'
                     'Command: %(cmd)s\n'
                     'Exit code: %(exit_code)s\n'
                     'Stdout: %(stdout)r\n'
                     'Stderr: %(stderr)r') % {'description': description,
-                                             'cmd': cmd,
+                                             'cmd': self.cmd,
                                              'exit_code': exit_code,
-                                             'stdout': stdout,
+                                             'stdout': self.stdout,
-                                             'stderr': stderr}
+                                             'stderr': self.stderr}
-        super(ProcessExecutionError, self).__init__(message)
+        return message
 
 
 class NoRootWrapSpecified(Exception):

oslo_concurrency/tests/unit/test_processutils.py

@@ -19,6 +19,7 @@ import errno
 import logging
 import multiprocessing
 import os
+import pickle
 import resource
 import stat
 import subprocess
@@ -467,6 +468,21 @@ grep foo
     def test_binary_undecodable_bytes_error(self):
         self.check_undecodable_bytes_error(True)
 
+    def test_picklable(self):
+        exc = processutils.ProcessExecutionError(
+            stdout='my stdout', stderr='my stderr',
+            exit_code=42, cmd='my cmd',
+            description='my description')
+        exc_message = str(exc)
+
+        exc = pickle.loads(pickle.dumps(exc))
+        self.assertEqual('my stdout', exc.stdout)
+        self.assertEqual('my stderr', exc.stderr)
+        self.assertEqual(42, exc.exit_code)
+        self.assertEqual('my cmd', exc.cmd)
+        self.assertEqual('my description', exc.description)
+        self.assertEqual(exc_message, str(exc))
+
 
 class ProcessExecutionErrorLoggingTest(test_base.BaseTestCase):
     def setUp(self):