Add snap package paths to default config dirs

With snap packaging (see snapcraft.io) the package is installed into a
read-only squashfs filesystem, which includes the default config. For
example, $SNAP/etc/nova/nova.conf. To override the defaults, a separate
writable directory is used, and this directory is also unique to the snap.
For example, either $SNAP_COMMON/etc/nova/nova.conf, or
$SNAP_COMMON/etc/nova/nova.conf.d/ can be used to override config.

This patch adds these snap directories to the default config paths where
oslo looks for config.

For more details on $SNAP and $SNAP_COMMON please refer to
https://snapcraft.io/docs/reference/env.

Change-Id: I83627e0f215382aedc7b32163e0303b39e8bccf8
Closes-Bug: 1696830

oslo_config/cfg.py

@@ -613,16 +613,27 @@ def _get_config_dirs(project=None):
       /etc/${project}/
       /etc/
 
-    Otherwise, these directories::
+    If a project is specified and installed from a snap package, following
+    directories are also returned:
+
+      ${SNAP}/etc/${project}
+      ${SNAP_COMMON}/etc/${project}
+
+    Otherwise, if project is not specified, these directories are returned:
 
       ~/
       /etc/
     """
+    snap = os.environ.get('SNAP')
+    snap_c = os.environ.get('SNAP_COMMON')
+
     cfg_dirs = [
         _fixpath(os.path.join('~', '.' + project)) if project else None,
         _fixpath('~'),
         os.path.join('/etc', project) if project else None,
-        '/etc'
+        '/etc',
+        os.path.join(snap, "etc", project) if snap and project else None,
+        os.path.join(snap_c, "etc", project) if snap_c and project else None,
     ]
     return [x for x in cfg_dirs if x]
 
@@ -673,6 +684,8 @@ def find_config_files(project=None, prog=None, extension='.conf'):
       ~/
       /etc/${project}/
       /etc/
+      ${SNAP}/etc/${project}
+      ${SNAP_COMMON}/etc/${project}
 
     We return an absolute path for (at most) one of each the default config
     files, for the topmost directory it exists in.
@@ -703,6 +716,8 @@ def find_config_dirs(project=None, prog=None, extension='.conf.d'):
       ~/
       /etc/${project}/
       /etc/
+      ${SNAP}/etc/${project}
+      ${SNAP_COMMON}/etc/${project}
 
     We return an absolute path for each of the two config dirs,
     in the first place we find it (iff we find it).

oslo_config/tests/test_cfg.py

@@ -223,6 +223,18 @@ class FindConfigFilesTestCase(BaseTestCase):
 
         self.assertEqual(cfg.find_config_files(project='blaa'), config_files)
 
+    def test_find_config_files_snap(self):
+        config_files = ['/snap/nova/current/etc/blaa/blaa.conf']
+        fake_env = {'SNAP': '/snap/nova/current/',
+                    'SNAP_COMMON': '/var/snap/nova/common/'}
+
+        self.useFixture(fixtures.MonkeyPatch('sys.argv', ['foo']))
+        self.useFixture(fixtures.MonkeyPatch('os.path.exists',
+                        lambda p: p in config_files))
+        self.useFixture(fixtures.MonkeyPatch('os.environ', fake_env))
+
+        self.assertEqual(cfg.find_config_files(project='blaa'), config_files)
+
     def test_find_config_files_with_extension(self):
         config_files = ['/etc/foo.json']
 
@@ -248,6 +260,18 @@ class FindConfigDirsTestCase(BaseTestCase):
 
         self.assertEqual(cfg.find_config_dirs(project='blaa'), config_dirs)
 
+    def test_find_config_dirs_snap(self):
+        config_dirs = ['/var/snap/nova/common/etc/blaa/blaa.conf.d']
+        fake_env = {'SNAP': '/snap/nova/current/',
+                    'SNAP_COMMON': '/var/snap/nova/common/'}
+
+        self.useFixture(fixtures.MonkeyPatch('sys.argv', ['foo']))
+        self.useFixture(fixtures.MonkeyPatch('os.path.exists',
+                                             lambda p: p in config_dirs))
+        self.useFixture(fixtures.MonkeyPatch('os.environ', fake_env))
+
+        self.assertEqual(cfg.find_config_dirs(project='blaa'), config_dirs)
+
     def test_find_config_dirs_non_exists(self):
         self.useFixture(fixtures.MonkeyPatch('sys.argv', ['foo']))
         self.assertEqual(cfg.find_config_dirs(project='blaa'), [])