Add bus for auto-loading charm modules.

charms_openstack.bus.discover() can now be used to load the openstack
charm modules placed under lib/charm/openstack/. This is particularly
useful when using the generic provide_charm_instance() code e.g.

```
charms_openstack.bus.discover()

with charms_openstack.charm.provide_charm_instance() as ci:
    ci.some_charm_instance_method()
```

Change-Id: Ic1dfb58a37b3f283bbc5b31b5ea192527fc8e57d

charms_openstack/bus.py

@@ -0,0 +1,80 @@
+# Copyright 2014-2018 Canonical Limited.
+#
+# This file is part of charms.reactive.
+#
+# charms.reactive is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# charm-helpers is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with charm-helpers.  If not, see <http://www.gnu.org/licenses/>.
+
+import importlib
+import os
+import charmhelpers.core.hookenv as hookenv
+
+# Code below is based on charms.reactive.bus
+
+
+def discover():
+    """Discover Openstack handlers based on convention.
+
+    Handlers will be loaded from the following directory and its
+    subdirectories:
+
+      * ``$CHARM_DIR/lib/charm/openstack``
+
+    The Python files will be imported and decorated functions registered.
+    """
+    search_path = os.path.join(
+        hookenv.charm_dir(), 'lib', 'charm', 'openstack')
+    base_path = os.path.join(hookenv.charm_dir(), 'lib', 'charm')
+    for dirpath, dirnames, filenames in os.walk(search_path):
+        for filename in filenames:
+            filepath = os.path.join(dirpath, filename)
+            _register_handlers_from_file(base_path, filepath)
+
+
+def _load_module(root, filepath):
+    """Import the supplied module.
+
+    :param root: Module root directory eg directory that the import is
+                 relative to.
+    :type root: str
+    :param filepath: Module file.
+    :type filepath: str
+    """
+    assert filepath.startswith(root + os.sep)
+    assert filepath.endswith('.py')
+    package = os.path.basename(root)
+    module = filepath[len(root):-3].replace(os.sep, '.')
+    if module.endswith('.__init__'):
+        module = module[:-9]
+
+    # Standard import.
+    importlib.import_module(package + module)
+
+
+def _register_handlers_from_file(root, filepath):
+    """Import the supplied module if its a good candidate.
+
+    :param root: Module root directory eg directory that the import is
+                 relative to.
+    :type root: str
+    :param filepath: Module file.
+    :type filepath: str
+    """
+    no_exec_blacklist = (
+        '.md', '.yaml', '.txt', '.ini',
+        'makefile', '.gitignore',
+        'copyright', 'license')
+    if filepath.lower().endswith(no_exec_blacklist):
+        # Don't load handlers with one of the blacklisted extensions
+        return
+    if filepath.endswith('.py'):
+        _load_module(root, filepath)

unit_tests/test_charms_openstack_bus.py

@@ -0,0 +1,58 @@
+# Copyright 2014-2018 Canonical Limited.
+#
+# This file is part of charms.reactive.
+#
+# charms.reactive is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# charm-helpers is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with charm-helpers.  If not, see <http://www.gnu.org/licenses/>.
+
+import os
+import unittest
+
+import mock
+
+import charms_openstack.bus as bus
+
+
+class TestBus(unittest.TestCase):
+
+    @mock.patch.object(bus.os, 'walk')
+    @mock.patch.object(bus, '_register_handlers_from_file')
+    @mock.patch('charmhelpers.core.hookenv.charm_dir')
+    def test_discover(self, charm_dir, _register_handlers_from_file, walk):
+        os.walk.return_value = [(
+            '/x/unit-aodh-1/charm/lib/charm/openstack',
+            ['__pycache__'],
+            ['__init__.py', 'aodh.py'])]
+
+        charm_dir.return_value = '/x/unit-aodh-1/charm'
+        bus.discover()
+        expect_calls = [
+            mock.call(
+                '/x/unit-aodh-1/charm/lib/charm',
+                '/x/unit-aodh-1/charm/lib/charm/openstack/__init__.py'),
+            mock.call(
+                '/x/unit-aodh-1/charm/lib/charm',
+                '/x/unit-aodh-1/charm/lib/charm/openstack/aodh.py')]
+        _register_handlers_from_file.assert_has_calls(expect_calls)
+
+    @mock.patch.object(bus.importlib, 'import_module')
+    def test_load_module(self, import_module):
+        import_module.side_effect = lambda x: x
+        bus._load_module(
+            '/x/charm/lib/charm',
+            '/x/charm/lib/charm/openstack/aodh.py'),
+        import_module.assert_called_once_with('charm.openstack.aodh')
+
+    @mock.patch.object(bus, '_load_module')
+    def test_register_handlers_from_file(self, _load_module):
+        bus._register_handlers_from_file('reactive', 'reactive/foo.py')
+        _load_module.assert_called_once_with('reactive', 'reactive/foo.py')