Allow placement to start without a config file

With oslo.config 6.7.0 configuration options may be found in environment
variables. Because of the small number of options that placement
requires to run it is straightforward to source all config from the
environment, useful in container-based environments.

However, prior to this change, placement-the-wsgi-app required that a
configuration file existed, placement.conf, either in /etc/placement or
in a custom directory.

This change makes it so that the placement.conf can be any of:

* in the custom directory
* in the default project locations (including /etc/placement)
* nowhere

The change maintains the behavior that if
[placement_database]/connection is not set the application will fail to
start.

Though this change is orthogonal to the oslo.config change, requirements
and lower-constraints are updated to ensure the desired behavior.

Closes-Bug: #1802925
Change-Id: Iefa8ad22dcb6a128293ea71ab77c377db56e8d70

lower-constraints.txt

@@ -77,7 +77,7 @@ os-xenapi==0.3.3
 osc-lib==1.10.0
 oslo.cache==1.26.0
 oslo.concurrency==3.26.0
-oslo.config==6.1.0
+oslo.config==6.7.0
 oslo.context==2.19.2
 oslo.db==4.40.0
 oslo.i18n==3.15.3

placement/wsgi.py

@@ -51,12 +51,20 @@ def setup_logging(config):
     py_logging.captureWarnings(True)
 
 
-def _get_config_file(env=None):
+def _get_config_files(env=None):
+    """Return a list of one file or None describing config location.
+
+    If None, that means oslo.config will look in the default locations
+    for a config file.
+    """
     if env is None:
         env = os.environ
 
-    dirname = env.get('OS_PLACEMENT_CONFIG_DIR', '/etc/placement').strip()
-    return os.path.join(dirname, CONFIG_FILE)
+    dirname = env.get('OS_PLACEMENT_CONFIG_DIR', '').strip()
+    if dirname:
+        return [os.path.join(dirname, CONFIG_FILE)]
+    else:
+        return None
 
 
 def _parse_args(argv, default_config_files):
@@ -100,19 +108,17 @@ def _set_middleware_defaults():
 
 def init_application():
     # initialize the config system
-    conffile = _get_config_file()
+    conffiles = _get_config_files()
 
     # NOTE(lyarwood): Call reset to ensure the ConfigOpts object doesn't
     # already contain registered options if the app is reloaded.
     conf.CONF.reset()
 
-    # This will raise cfg.ConfigFilesNotFoundError and cfg.RequiredOptError
-    # when either conffile is not there or some required option is not set
-    # (notably the database connection string). We want both of these to
-    # be a hard fail and prevent the application from starting so we hard
-    # fail here. The error will show up in the wsgi server's logs and the
-    # app will not start.
-    _parse_args([], default_config_files=[conffile])
+    # This will raise cfg.RequiredOptError when a required option is not set
+    # (notably the database connection string). We want this to be a hard fail
+    # that prevents the application from starting. The error will show up in
+    # the wsgi server's logs.
+    _parse_args([], default_config_files=conffiles)
     # initialize the logging system
     setup_logging(conf.CONF)
 

requirements.txt

@@ -12,7 +12,7 @@ requests>=2.14.2 # Apache-2.0
 six>=1.10.0 # MIT
 setuptools!=24.0.0,!=34.0.0,!=34.0.1,!=34.0.2,!=34.0.3,!=34.1.0,!=34.1.1,!=34.2.0,!=34.3.0,!=34.3.1,!=34.3.2,!=36.2.0,>=21.0.0 # PSF/ZPL
 oslo.concurrency>=3.26.0 # Apache-2.0
-oslo.config>=6.1.0 # Apache-2.0
+oslo.config>=6.7.0 # Apache-2.0
 oslo.context>=2.19.2 # Apache-2.0
 oslo.log>=3.36.0 # Apache-2.0
 oslo.serialization!=2.19.1,>=2.18.0 # Apache-2.0