stages: fix tracebacks if a module stage is undefined or empty

In /etc/cloud/cloud.cfg, users and imagees can configure which modules run
during a specific cloud-init stage by modifying one of the following
lists: cloud_init_modules, cloud_init_modules, cloud_init_final_modules.

If any of the configured module lists are absent or empty, cloud-init will
emit the same message it already does for existing lists that only contain
modules which are not unsupported on that platform:

No 'config' modules to run under section 'cloud_config_modules'

LP: #1770462

2 files changed, 33 insertions, 3 deletions

diff --git a/cloudinit/stages.py b/cloudinit/stages.py
index 3998cf68..286607bf 100644
--- a/cloudinit/stages.py
+++ b/cloudinit/stages.py
@@ -697,7 +697,9 @@ class Modules(object):
         module_list = []
         if name not in self.cfg:
             return module_list
-        cfg_mods = self.cfg[name]
+        cfg_mods = self.cfg.get(name)
+        if not cfg_mods:
+            return module_list
         # Create 'module_list', an array of hashes
         # Where hash['mod'] = module name
         #       hash['freq'] = frequency
diff --git a/tests/unittests/test_runs/test_simple_run.py b/tests/unittests/test_runs/test_simple_run.py
index 762974e9..d67c422c 100644
--- a/tests/unittests/test_runs/test_simple_run.py
+++ b/tests/unittests/test_runs/test_simple_run.py
@@ -1,5 +1,6 @@
 # This file is part of cloud-init. See LICENSE file for license information.
 
+import copy
 import os
 
 
@@ -127,8 +128,9 @@ class TestSimpleRun(helpers.FilesystemMockingTestCase):
         """run_section forced skipped modules by using unverified_modules."""
 
         # re-write cloud.cfg with unverified_modules override
-        self.cfg['unverified_modules'] = ['spacewalk']  # Would have skipped
-        cloud_cfg = util.yaml_dumps(self.cfg)
+        cfg = copy.deepcopy(self.cfg)
+        cfg['unverified_modules'] = ['spacewalk']  # Would have skipped
+        cloud_cfg = util.yaml_dumps(cfg)
         util.ensure_dir(os.path.join(self.new_root, 'etc', 'cloud'))
         util.write_file(os.path.join(self.new_root, 'etc',
                                      'cloud', 'cloud.cfg'), cloud_cfg)
@@ -150,4 +152,30 @@ class TestSimpleRun(helpers.FilesystemMockingTestCase):
             "running unverified_modules: 'spacewalk'",
             self.logs.getvalue())
 
+    def test_none_ds_run_with_no_config_modules(self):
+        """run_section will report no modules run when none are configured."""
+
+        # re-write cloud.cfg with unverified_modules override
+        cfg = copy.deepcopy(self.cfg)
+        # Represent empty configuration in /etc/cloud/cloud.cfg
+        cfg['cloud_init_modules'] = None
+        cloud_cfg = util.yaml_dumps(cfg)
+        util.ensure_dir(os.path.join(self.new_root, 'etc', 'cloud'))
+        util.write_file(os.path.join(self.new_root, 'etc',
+                                     'cloud', 'cloud.cfg'), cloud_cfg)
+
+        initer = stages.Init()
+        initer.read_cfg()
+        initer.initialize()
+        initer.fetch()
+        initer.instancify()
+        initer.update()
+        initer.cloudify().run('consume_data', initer.consume_data,
+                              args=[PER_INSTANCE], freq=PER_INSTANCE)
+
+        mods = stages.Modules(initer)
+        (which_ran, failures) = mods.run_section('cloud_init_modules')
+        self.assertTrue(len(failures) == 0)
+        self.assertEqual([], which_ran)
+
 # vi: ts=4 expandtab