ubuntu-advantage: Add new config module to support ubuntu-advantage-tools

ubuntu-advantage-tools is a package for enabling and disabling extended
support services such as Extended Security Maintenance (ESM), Canonical
Livepatch and FIPS certified PPAs. Simplify Ubuntu Advantage setup on
machines by allowing users to provide a list of ubuntu-advantage commands
in cloud-config.

6 files changed, 563 insertions, 95 deletions

diff --git a/cloudinit/config/cc_snap.py b/cloudinit/config/cc_snap.py
index db965291..34a53fd4 100644
--- a/cloudinit/config/cc_snap.py
+++ b/cloudinit/config/cc_snap.py
@@ -11,6 +11,7 @@ from cloudinit import log as logging
 from cloudinit.config.schema import (
     get_schema_doc, validate_cloudconfig_schema)
 from cloudinit.settings import PER_INSTANCE
+from cloudinit.subp import prepend_base_command
 from cloudinit import util
 
 
@@ -160,50 +161,6 @@ def add_assertions(assertions):
     util.subp(snap_cmd + [ASSERTIONS_FILE], capture=True)
 
 
-def prepend_snap_commands(commands):
-    """Ensure user-provided commands start with SNAP_CMD, warn otherwise.
-
-    Each command is either a list or string. Perform the following:
-       - When the command is a list, pop the first element if it is None
-       - When the command is a list, insert SNAP_CMD as the first element if
-         not present.
-       - When the command is a string containing a non-snap command, warn.
-
-    Support cut-n-paste snap command sets from public snappy documentation.
-    Allow flexibility to provide non-snap environment/config setup if needed.
-
-    @commands: List of commands. Each command element is a list or string.
-
-    @return: List of 'fixed up' snap commands.
-    @raise: TypeError on invalid config item type.
-    """
-    warnings = []
-    errors = []
-    fixed_commands = []
-    for command in commands:
-        if isinstance(command, list):
-            if command[0] is None:  # Avoid warnings by specifying None
-                command = command[1:]
-            elif command[0] != SNAP_CMD:  # Automatically prepend SNAP_CMD
-                command.insert(0, SNAP_CMD)
-        elif isinstance(command, str):
-            if not command.startswith('%s ' % SNAP_CMD):
-                warnings.append(command)
-        else:
-            errors.append(str(command))
-            continue
-        fixed_commands.append(command)
-
-    if warnings:
-        LOG.warning(
-            'Non-snap commands in snap config:\n%s', '\n'.join(warnings))
-    if errors:
-        raise TypeError(
-            'Invalid snap config.'
-            ' These commands are not a string or list:\n' + '\n'.join(errors))
-    return fixed_commands
-
-
 def run_commands(commands):
     """Run the provided commands provided in snap:commands configuration.
 
@@ -224,7 +181,7 @@ def run_commands(commands):
             'commands parameter was not a list or dict: {commands}'.format(
                 commands=commands))
 
-    fixed_snap_commands = prepend_snap_commands(commands)
+    fixed_snap_commands = prepend_base_command('snap', commands)
 
     cmd_failures = []
     for command in fixed_snap_commands:
diff --git a/cloudinit/config/cc_ubuntu_advantage.py b/cloudinit/config/cc_ubuntu_advantage.py
new file mode 100644
index 00000000..16b1868b
--- /dev/null
+++ b/cloudinit/config/cc_ubuntu_advantage.py
@@ -0,0 +1,173 @@
+# Copyright (C) 2018 Canonical Ltd.
+#
+# This file is part of cloud-init. See LICENSE file for license information.
+
+"""Ubuntu advantage: manage ubuntu-advantage offerings from Canonical."""
+
+import sys
+from textwrap import dedent
+
+from cloudinit import log as logging
+from cloudinit.config.schema import (
+    get_schema_doc, validate_cloudconfig_schema)
+from cloudinit.settings import PER_INSTANCE
+from cloudinit.subp import prepend_base_command
+from cloudinit import util
+
+
+distros = ['ubuntu']
+frequency = PER_INSTANCE
+
+LOG = logging.getLogger(__name__)
+
+schema = {
+    'id': 'cc_ubuntu_advantage',
+    'name': 'Ubuntu Advantage',
+    'title': 'Install, configure and manage ubuntu-advantage offerings',
+    'description': dedent("""\
+        This module provides configuration options to setup ubuntu-advantage
+        subscriptions.
+
+        .. note::
+            Both ``commands`` value can be either a dictionary or a list. If
+            the configuration provided is a dictionary, the keys are only used
+            to order the execution of the commands and the dictionary is
+            merged with any vendor-data ubuntu-advantage configuration
+            provided. If a ``commands`` is provided as a list, any vendor-data
+            ubuntu-advantage ``commands`` are ignored.
+
+        Ubuntu-advantage ``commands`` is a dictionary or list of
+        ubuntu-advantage commands to run on the deployed machine.
+        These commands can be used to enable or disable subscriptions to
+        various ubuntu-advantage products. See 'man ubuntu-advantage' for more
+        information on supported subcommands.
+
+        .. note::
+           Each command item can be a string or list. If the item is a list,
+           'ubuntu-advantage' can be omitted and it will automatically be
+           inserted as part of the command.
+        """),
+    'distros': distros,
+    'examples': [dedent("""\
+        # Enable Extended Security Maintenance using your service auth token
+        ubuntu-advantage:
+            commands:
+              00: ubuntu-advantage enable-esm <token>
+    """), dedent("""\
+        # Enable livepatch by providing your livepatch token
+        ubuntu-advantage:
+            commands:
+                00: ubuntu-advantage enable-livepatch <livepatch-token>
+
+    """), dedent("""\
+        # Convenience: the ubuntu-advantage command can be omitted when
+        # specifying commands as a list and 'ubuntu-advantage' will
+        # automatically be prepended.
+        # The following commands are equivalent
+        ubuntu-advantage:
+            commands:
+                00: ['enable-livepatch', 'my-token']
+                01: ['ubuntu-advantage', 'enable-livepatch', 'my-token']
+                02: ubuntu-advantage enable-livepatch my-token
+                03: 'ubuntu-advantage enable-livepatch my-token'
+    """)],
+    'frequency': PER_INSTANCE,
+    'type': 'object',
+    'properties': {
+        'ubuntu-advantage': {
+            'type': 'object',
+            'properties': {
+                'commands': {
+                    'type': ['object', 'array'],  # Array of strings or dict
+                    'items': {
+                        'oneOf': [
+                            {'type': 'array', 'items': {'type': 'string'}},
+                            {'type': 'string'}]
+                    },
+                    'additionalItems': False,  # Reject non-string & non-list
+                    'minItems': 1,
+                    'minProperties': 1,
+                    'uniqueItems': True
+                }
+            },
+            'additionalProperties': False,  # Reject keys not in schema
+            'required': ['commands']
+        }
+    }
+}
+
+# TODO schema for 'assertions' and 'commands' are too permissive at the moment.
+# Once python-jsonschema supports schema draft 6 add support for arbitrary
+# object keys with 'patternProperties' constraint to validate string values.
+
+__doc__ = get_schema_doc(schema)  # Supplement python help()
+
+UA_CMD = "ubuntu-advantage"
+
+
+def run_commands(commands):
+    """Run the commands provided in ubuntu-advantage:commands config.
+
+     Commands are run individually. Any errors are collected and reported
+     after attempting all commands.
+
+     @param commands: A list or dict containing commands to run. Keys of a
+         dict will be used to order the commands provided as dict values.
+     """
+    if not commands:
+        return
+    LOG.debug('Running user-provided ubuntu-advantage commands')
+    if isinstance(commands, dict):
+        # Sort commands based on dictionary key
+        commands = [v for _, v in sorted(commands.items())]
+    elif not isinstance(commands, list):
+        raise TypeError(
+            'commands parameter was not a list or dict: {commands}'.format(
+                commands=commands))
+
+    fixed_ua_commands = prepend_base_command('ubuntu-advantage', commands)
+
+    cmd_failures = []
+    for command in fixed_ua_commands:
+        shell = isinstance(command, str)
+        try:
+            util.subp(command, shell=shell, status_cb=sys.stderr.write)
+        except util.ProcessExecutionError as e:
+            cmd_failures.append(str(e))
+    if cmd_failures:
+        msg = (
+            'Failures running ubuntu-advantage commands:\n'
+            '{cmd_failures}'.format(
+                cmd_failures=cmd_failures))
+        util.logexc(LOG, msg)
+        raise RuntimeError(msg)
+
+
+def maybe_install_ua_tools(cloud):
+    """Install ubuntu-advantage-tools if not present."""
+    if util.which('ubuntu-advantage'):
+        return
+    try:
+        cloud.distro.update_package_sources()
+    except Exception as e:
+        util.logexc(LOG, "Package update failed")
+        raise
+    try:
+        cloud.distro.install_packages(['ubuntu-advantage-tools'])
+    except Exception as e:
+        util.logexc(LOG, "Failed to install ubuntu-advantage-tools")
+        raise
+
+
+def handle(name, cfg, cloud, log, args):
+    cfgin = cfg.get('ubuntu-advantage')
+    if cfgin is None:
+        LOG.debug(("Skipping module named %s,"
+                   " no 'ubuntu-advantage' key in configuration"), name)
+        return
+
+    validate_cloudconfig_schema(cfg, schema)
+    maybe_install_ua_tools(cloud)
+    run_commands(cfgin.get('commands', []))
+
+# vi: ts=4 expandtab
diff --git a/cloudinit/config/tests/test_snap.py b/cloudinit/config/tests/test_snap.py
index cb1205e9..988e7f7c 100644
--- a/cloudinit/config/tests/test_snap.py
+++ b/cloudinit/config/tests/test_snap.py
@@ -4,8 +4,8 @@ import re
 from six import StringIO
 
 from cloudinit.config.cc_snap import (
-    ASSERTIONS_FILE, add_assertions, handle, prepend_snap_commands,
-    maybe_install_squashfuse, run_commands, schema)
+    ASSERTIONS_FILE, add_assertions, handle, maybe_install_squashfuse,
+    run_commands, schema)
 from cloudinit.config.schema import validate_cloudconfig_schema
 from cloudinit import util
 from cloudinit.tests.helpers import CiTestCase, mock, wrap_and_call
@@ -158,54 +158,6 @@ class TestAddAssertions(CiTestCase):
             util.load_file(compare_file), util.load_file(assert_file))
 
 
-class TestPrepentSnapCommands(CiTestCase):
-
-    with_logs = True
-
-    def test_prepend_snap_commands_errors_on_neither_string_nor_list(self):
-        """Raise an error for each command which is not a string or list."""
-        orig_commands = ['ls', 1, {'not': 'gonna work'}, ['snap', 'list']]
-        with self.assertRaises(TypeError) as context_manager:
-            prepend_snap_commands(orig_commands)
-        self.assertEqual(
-            "Invalid snap config. These commands are not a string or list:\n"
-            "1\n{'not': 'gonna work'}",
-            str(context_manager.exception))
-
-    def test_prepend_snap_commands_warns_on_non_snap_string_commands(self):
-        """Warn on each non-snap for commands of type string."""
-        orig_commands = ['ls', 'snap list', 'touch /blah', 'snap install x']
-        fixed_commands = prepend_snap_commands(orig_commands)
-        self.assertEqual(
-            'WARNING: Non-snap commands in snap config:\n'
-            'ls\ntouch /blah\n',
-            self.logs.getvalue())
-        self.assertEqual(orig_commands, fixed_commands)
-
-    def test_prepend_snap_commands_prepends_on_non_snap_list_commands(self):
-        """Prepend 'snap' for each non-snap command of type list."""
-        orig_commands = [['ls'], ['snap', 'list'], ['snapa', '/blah'],
-                         ['snap', 'install', 'x']]
-        expected = [['snap', 'ls'], ['snap', 'list'],
-                    ['snap', 'snapa', '/blah'],
-                    ['snap', 'install', 'x']]
-        fixed_commands = prepend_snap_commands(orig_commands)
-        self.assertEqual('', self.logs.getvalue())
-        self.assertEqual(expected, fixed_commands)
-
-    def test_prepend_snap_commands_removes_first_item_when_none(self):
-        """Remove the first element of a non-snap command when it is None."""
-        orig_commands = [[None, 'ls'], ['snap', 'list'],
-                         [None, 'touch', '/blah'],
-                         ['snap', 'install', 'x']]
-        expected = [['ls'], ['snap', 'list'],
-                    ['touch', '/blah'],
-                    ['snap', 'install', 'x']]
-        fixed_commands = prepend_snap_commands(orig_commands)
-        self.assertEqual('', self.logs.getvalue())
-        self.assertEqual(expected, fixed_commands)
-
-
 class TestRunCommands(CiTestCase):
 
     with_logs = True
diff --git a/cloudinit/config/tests/test_ubuntu_advantage.py b/cloudinit/config/tests/test_ubuntu_advantage.py
new file mode 100644
index 00000000..0eeadd43
--- /dev/null
+++ b/cloudinit/config/tests/test_ubuntu_advantage.py
@@ -0,0 +1,268 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+
+import re
+from six import StringIO
+
+from cloudinit.config.cc_ubuntu_advantage import (
+    handle, maybe_install_ua_tools, run_commands, schema)
+from cloudinit.config.schema import validate_cloudconfig_schema
+from cloudinit import util
+from cloudinit.tests.helpers import CiTestCase, mock
+
+
+# Module path used in mocks
+MPATH = 'cloudinit.config.cc_ubuntu_advantage'
+
+
+class FakeCloud(object):
+    def __init__(self, distro):
+        self.distro = distro
+
+
+class TestRunCommands(CiTestCase):
+
+    with_logs = True
+
+    def setUp(self):
+        super(TestRunCommands, self).setUp()
+        self.tmp = self.tmp_dir()
+
+    @mock.patch('%s.util.subp' % MPATH)
+    def test_run_commands_on_empty_list(self, m_subp):
+        """When provided with an empty list, run_commands does nothing."""
+        run_commands([])
+        self.assertEqual('', self.logs.getvalue())
+        m_subp.assert_not_called()
+
+    def test_run_commands_on_non_list_or_dict(self):
+        """When provided an invalid type, run_commands raises an error."""
+        with self.assertRaises(TypeError) as context_manager:
+            run_commands(commands="I'm Not Valid")
+        self.assertEqual(
+            "commands parameter was not a list or dict: I'm Not Valid",
+            str(context_manager.exception))
+
+    def test_run_command_logs_commands_and_exit_codes_to_stderr(self):
+        """All exit codes are logged to stderr."""
+        outfile = self.tmp_path('output.log', dir=self.tmp)
+
+        cmd1 = 'echo "HI" >> %s' % outfile
+        cmd2 = 'bogus command'
+        cmd3 = 'echo "MOM" >> %s' % outfile
+        commands = [cmd1, cmd2, cmd3]
+
+        mock_path = '%s.sys.stderr' % MPATH
+        with mock.patch(mock_path, new_callable=StringIO) as m_stderr:
+            with self.assertRaises(RuntimeError) as context_manager:
+                run_commands(commands=commands)
+
+        self.assertIsNotNone(
+            re.search(r'bogus: (command )?not found',
+                      str(context_manager.exception)),
+            msg='Expected bogus command not found')
+        expected_stderr_log = '\n'.join([
+            'Begin run command: {cmd}'.format(cmd=cmd1),
+            'End run command: exit(0)',
+            'Begin run command: {cmd}'.format(cmd=cmd2),
+            'ERROR: End run command: exit(127)',
+            'Begin run command: {cmd}'.format(cmd=cmd3),
+            'End run command: exit(0)\n'])
+        self.assertEqual(expected_stderr_log, m_stderr.getvalue())
+
+    def test_run_command_as_lists(self):
+        """When commands are specified as a list, run them in order."""
+        outfile = self.tmp_path('output.log', dir=self.tmp)
+
+        cmd1 = 'echo "HI" >> %s' % outfile
+        cmd2 = 'echo "MOM" >> %s' % outfile
+        commands = [cmd1, cmd2]
+        with mock.patch('%s.sys.stderr' % MPATH, new_callable=StringIO):
+            run_commands(commands=commands)
+
+        self.assertIn(
+            'DEBUG: Running user-provided ubuntu-advantage commands',
+            self.logs.getvalue())
+        self.assertEqual('HI\nMOM\n', util.load_file(outfile))
+        self.assertIn(
+            'WARNING: Non-ubuntu-advantage commands in ubuntu-advantage'
+            ' config:',
+            self.logs.getvalue())
+
+    def test_run_command_dict_sorted_as_command_script(self):
+        """When commands are a dict, sort them and run."""
+        outfile = self.tmp_path('output.log', dir=self.tmp)
+        cmd1 = 'echo "HI" >> %s' % outfile
+        cmd2 = 'echo "MOM" >> %s' % outfile
+        commands = {'02': cmd1, '01': cmd2}
+        with mock.patch('%s.sys.stderr' % MPATH, new_callable=StringIO):
+            run_commands(commands=commands)
+
+        expected_messages = [
+            'DEBUG: Running user-provided ubuntu-advantage commands']
+        for message in expected_messages:
+            self.assertIn(message, self.logs.getvalue())
+        self.assertEqual('MOM\nHI\n', util.load_file(outfile))
+
+
+class TestSchema(CiTestCase):
+
+    with_logs = True
+
+    def test_schema_warns_on_ubuntu_advantage_not_as_dict(self):
+        """If ubuntu-advantage configuration is not a dict, emit a warning."""
+        validate_cloudconfig_schema({'ubuntu-advantage': 'wrong type'}, schema)
+        self.assertEqual(
+            "WARNING: Invalid config:\nubuntu-advantage: 'wrong type' is not"
+            " of type 'object'\n",
+            self.logs.getvalue())
+
+    @mock.patch('%s.run_commands' % MPATH)
+    def test_schema_disallows_unknown_keys(self, _):
+        """Unknown keys in ubuntu-advantage configuration emit warnings."""
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {'commands': ['ls'], 'invalid-key': ''}},
+            schema)
+        self.assertIn(
+            'WARNING: Invalid config:\nubuntu-advantage: Additional properties'
+            " are not allowed ('invalid-key' was unexpected)",
+            self.logs.getvalue())
+
+    def test_warn_schema_requires_commands(self):
+        """Warn when ubuntu-advantage configuration lacks commands."""
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {}}, schema)
+        self.assertEqual(
+            "WARNING: Invalid config:\nubuntu-advantage: 'commands' is a"
+            " required property\n",
+            self.logs.getvalue())
+
+    @mock.patch('%s.run_commands' % MPATH)
+    def test_warn_schema_commands_is_not_list_or_dict(self, _):
+        """Warn when ubuntu-advantage:commands config is not a list or dict."""
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {'commands': 'broken'}}, schema)
+        self.assertEqual(
+            "WARNING: Invalid config:\nubuntu-advantage.commands: 'broken' is"
+            " not of type 'object', 'array'\n",
+            self.logs.getvalue())
+
+    @mock.patch('%s.run_commands' % MPATH)
+    def test_warn_schema_when_commands_is_empty(self, _):
+        """Emit warnings when ubuntu-advantage:commands is empty."""
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {'commands': []}}, schema)
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {'commands': {}}}, schema)
+        self.assertEqual(
+            "WARNING: Invalid config:\nubuntu-advantage.commands: [] is too"
+            " short\nWARNING: Invalid config:\nubuntu-advantage.commands: {}"
+            " does not have enough properties\n",
+            self.logs.getvalue())
+
+    @mock.patch('%s.run_commands' % MPATH)
+    def test_schema_when_commands_are_list_or_dict(self, _):
+        """No warnings when ubuntu-advantage:commands are a list or dict."""
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {'commands': ['valid']}}, schema)
+        validate_cloudconfig_schema(
+            {'ubuntu-advantage': {'commands': {'01': 'also valid'}}}, schema)
+        self.assertEqual('', self.logs.getvalue())
+
+
+class TestHandle(CiTestCase):
+
+    with_logs = True
+
+    def setUp(self):
+        super(TestHandle, self).setUp()
+        self.tmp = self.tmp_dir()
+
+    @mock.patch('%s.run_commands' % MPATH)
+    @mock.patch('%s.validate_cloudconfig_schema' % MPATH)
+    def test_handle_no_config(self, m_schema, m_run):
+        """When no ua-related configuration is provided, nothing happens."""
+        cfg = {}
+        handle('ua-test', cfg=cfg, cloud=None, log=self.logger, args=None)
+        self.assertIn(
+            "DEBUG: Skipping module named ua-test, no 'ubuntu-advantage' key"
+            " in config",
+            self.logs.getvalue())
+        m_schema.assert_not_called()
+        m_run.assert_not_called()
+
+    @mock.patch('%s.maybe_install_ua_tools' % MPATH)
+    def test_handle_tries_to_install_ubuntu_advantage_tools(self, m_install):
+        """If ubuntu_advantage is provided, try installing ua-tools package."""
+        cfg = {'ubuntu-advantage': {}}
+        mycloud = FakeCloud(None)
+        handle('nomatter', cfg=cfg, cloud=mycloud, log=self.logger, args=None)
+        m_install.assert_called_once_with(mycloud)
+
+    @mock.patch('%s.maybe_install_ua_tools' % MPATH)
+    def test_handle_runs_commands_provided(self, m_install):
+        """When commands are specified as a list, run them."""
+        outfile = self.tmp_path('output.log', dir=self.tmp)
+
+        cfg = {
+            'ubuntu-advantage': {'commands': ['echo "HI" >> %s' % outfile,
+                                              'echo "MOM" >> %s' % outfile]}}
+        mock_path = '%s.sys.stderr' % MPATH
+        with mock.patch(mock_path, new_callable=StringIO):
+            handle('nomatter', cfg=cfg, cloud=None, log=self.logger, args=None)
+        self.assertEqual('HI\nMOM\n', util.load_file(outfile))
+
+
+class TestMaybeInstallUATools(CiTestCase):
+
+    with_logs = True
+
+    def setUp(self):
+        super(TestMaybeInstallUATools, self).setUp()
+        self.tmp = self.tmp_dir()
+
+    @mock.patch('%s.util.which' % MPATH)
+    def test_maybe_install_ua_tools_noop_when_ua_tools_present(self, m_which):
+        """Do nothing if ubuntu-advantage-tools already exists."""
+        m_which.return_value = '/usr/bin/ubuntu-advantage'  # already installed
+        distro = mock.MagicMock()
+        distro.update_package_sources.side_effect = RuntimeError(
+            'Some apt error')
+        maybe_install_ua_tools(cloud=FakeCloud(distro))  # No RuntimeError
+
+    @mock.patch('%s.util.which' % MPATH)
+    def test_maybe_install_ua_tools_raises_update_errors(self, m_which):
+        """maybe_install_ua_tools logs and raises apt update errors."""
+        m_which.return_value = None
+        distro = mock.MagicMock()
+        distro.update_package_sources.side_effect = RuntimeError(
+            'Some apt error')
+        with self.assertRaises(RuntimeError) as context_manager:
+            maybe_install_ua_tools(cloud=FakeCloud(distro))
+        self.assertEqual('Some apt error', str(context_manager.exception))
+        self.assertIn('Package update failed\nTraceback', self.logs.getvalue())
+
+    @mock.patch('%s.util.which' % MPATH)
+    def test_maybe_install_ua_raises_install_errors(self, m_which):
+        """maybe_install_ua_tools logs and raises package install errors."""
+        m_which.return_value = None
+        distro = mock.MagicMock()
+        distro.update_package_sources.return_value = None
+        distro.install_packages.side_effect = RuntimeError(
+            'Some install error')
+        with self.assertRaises(RuntimeError) as context_manager:
+            maybe_install_ua_tools(cloud=FakeCloud(distro))
+        self.assertEqual('Some install error', str(context_manager.exception))
+        self.assertIn(
+            'Failed to install ubuntu-advantage-tools\n', self.logs.getvalue())
+
+    @mock.patch('%s.util.which' % MPATH)
+    def test_maybe_install_ua_tools_happy_path(self, m_which):
+        """maybe_install_ua_tools installs ubuntu-advantage-tools."""
+        m_which.return_value = None
+        distro = mock.MagicMock()  # No errors raised
+        maybe_install_ua_tools(cloud=FakeCloud(distro))
+        distro.update_package_sources.assert_called_once_with()
+        distro.install_packages.assert_called_once_with(
+            ['ubuntu-advantage-tools'])
+
+# vi: ts=4 expandtab
diff --git a/cloudinit/subp.py b/cloudinit/subp.py
new file mode 100644
index 00000000..0ad09306
--- /dev/null
+++ b/cloudinit/subp.py
@@ -0,0 +1,57 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+"""Common utility functions for interacting with subprocess."""
+
+# TODO move subp shellify and runparts related functions out of util.py
+
+import logging
+
+LOG = logging.getLogger(__name__)
+
+
+def prepend_base_command(base_command, commands):
+    """Ensure user-provided commands start with base_command; warn otherwise.
+
+    Each command is either a list or string. Perform the following:
+       - If the command is a list, pop the first element if it is None
+       - If the command is a list, insert base_command as the first element if
+         not present.
+       - When the command is a string not starting with 'base-command', warn.
+
+    Allow flexibility to provide non-base-command environment/config setup if
+    needed.
+
+    @commands: List of commands. Each command element is a list or string.
+
+    @return: List of 'fixed up' commands.
+    @raise: TypeError on invalid config item type.
+    """
+    warnings = []
+    errors = []
+    fixed_commands = []
+    for command in commands:
+        if isinstance(command, list):
+            if command[0] is None:  # Avoid warnings by specifying None
+                command = command[1:]
+            elif command[0] != base_command:  # Automatically prepend
+                command.insert(0, base_command)
+        elif isinstance(command, str):
+            if not command.startswith('%s ' % base_command):
+                warnings.append(command)
+        else:
+            errors.append(str(command))
+            continue
+        fixed_commands.append(command)
+
+    if warnings:
+        LOG.warning(
+            'Non-%s commands in %s config:\n%s',
+            base_command, base_command, '\n'.join(warnings))
+    if errors:
+        raise TypeError(
+            'Invalid {name} config.'
+            ' These commands are not a string or list:\n{errors}'.format(
+                name=base_command, errors='\n'.join(errors)))
+    return fixed_commands
+
+
+# vi: ts=4 expandtab
diff --git a/cloudinit/tests/test_subp.py b/cloudinit/tests/test_subp.py
new file mode 100644
index 00000000..448097d3
--- /dev/null
+++ b/cloudinit/tests/test_subp.py
@@ -0,0 +1,61 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+
+"""Tests for cloudinit.subp utility functions"""
+
+from cloudinit import subp
+from cloudinit.tests.helpers import CiTestCase
+
+
+class TestPrependBaseCommands(CiTestCase):
+
+    with_logs = True
+
+    def test_prepend_base_command_errors_on_neither_string_nor_list(self):
+        """Raise an error for each command which is not a string or list."""
+        orig_commands = ['ls', 1, {'not': 'gonna work'}, ['basecmd', 'list']]
+        with self.assertRaises(TypeError) as context_manager:
+            subp.prepend_base_command(
+                base_command='basecmd', commands=orig_commands)
+        self.assertEqual(
+            "Invalid basecmd config. These commands are not a string or"
+            " list:\n1\n{'not': 'gonna work'}",
+            str(context_manager.exception))
+
+    def test_prepend_base_command_warns_on_non_base_string_commands(self):
+        """Warn on each non-base for commands of type string."""
+        orig_commands = [
+            'ls', 'basecmd list', 'touch /blah', 'basecmd install x']
+        fixed_commands = subp.prepend_base_command(
+            base_command='basecmd', commands=orig_commands)
+        self.assertEqual(
+            'WARNING: Non-basecmd commands in basecmd config:\n'
+            'ls\ntouch /blah\n',
+            self.logs.getvalue())
+        self.assertEqual(orig_commands, fixed_commands)
+
+    def test_prepend_base_command_prepends_on_non_base_list_commands(self):
+        """Prepend 'basecmd' for each non-basecmd command of type list."""
+        orig_commands = [['ls'], ['basecmd', 'list'], ['basecmda', '/blah'],
+                         ['basecmd', 'install', 'x']]
+        expected = [['basecmd', 'ls'], ['basecmd', 'list'],
+                    ['basecmd', 'basecmda', '/blah'],
+                    ['basecmd', 'install', 'x']]
+        fixed_commands = subp.prepend_base_command(
+            base_command='basecmd', commands=orig_commands)
+        self.assertEqual('', self.logs.getvalue())
+        self.assertEqual(expected, fixed_commands)
+
+    def test_prepend_base_command_removes_first_item_when_none(self):
+        """Remove the first element of a non-basecmd when it is None."""
+        orig_commands = [[None, 'ls'], ['basecmd', 'list'],
+                         [None, 'touch', '/blah'],
+                         ['basecmd', 'install', 'x']]
+        expected = [['ls'], ['basecmd', 'list'],
+                    ['touch', '/blah'],
+                    ['basecmd', 'install', 'x']]
+        fixed_commands = subp.prepend_base_command(
+            base_command='basecmd', commands=orig_commands)
+        self.assertEqual('', self.logs.getvalue())
+        self.assertEqual(expected, fixed_commands)
+
+# vi: ts=4 expandtab