Replace custom lazy loading by stevedore

Now we have own implementation for loading
extensions. stevedore avoids creating yet
another extension mechanism by building on
top of setuptools entry points.

Change-Id: I7fcb981b58b20fb49d10eea3d477f962e4bf3565

glance/common/utils.py

@@ -477,41 +477,6 @@ def setup_remote_pydev_debug(host, port):
             LOG.exception(error_msg)
 
 
-class LazyPluggable(object):
-    """A pluggable backend loaded lazily based on some value."""
-
-    def __init__(self, pivot, config_group=None, **backends):
-        self.__backends = backends
-        self.__pivot = pivot
-        self.__backend = None
-        self.__config_group = config_group
-
-    def __get_backend(self):
-        if not self.__backend:
-            if self.__config_group is None:
-                backend_name = CONF[self.__pivot]
-            else:
-                backend_name = CONF[self.__config_group][self.__pivot]
-            if backend_name not in self.__backends:
-                msg = _('Invalid backend: %s') % backend_name
-                raise exception.GlanceException(msg)
-
-            backend = self.__backends[backend_name]
-            if isinstance(backend, tuple):
-                name = backend[0]
-                fromlist = backend[1]
-            else:
-                name = backend
-                fromlist = backend
-
-            self.__backend = __import__(name, None, None, fromlist)
-        return self.__backend
-
-    def __getattr__(self, key):
-        backend = self.__get_backend()
-        return getattr(backend, key)
-
-
 def validate_key_cert(key_file, cert_file):
     try:
         error_key_name = "private key"

glance/db/migration.py

@@ -20,21 +20,30 @@
 """Database setup and migration commands."""
 
 import os
+import threading
 
 from oslo.config import cfg
 from oslo import db
+from stevedore import driver
 
-from glance.common import utils
 from glance.db.sqlalchemy import api as db_api
 
 
+_IMPL = None
+_LOCK = threading.Lock()
+
 db.options.set_defaults(cfg.CONF)
 
 
-IMPL = utils.LazyPluggable(
-    'backend',
-    config_group='database',
-    sqlalchemy='oslo.db.sqlalchemy.migration')
+def get_backend():
+    global _IMPL
+    if _IMPL is None:
+        with _LOCK:
+            if _IMPL is None:
+                _IMPL = driver.DriverManager(
+                    "glance.database.migration_backend",
+                    cfg.CONF.database.backend).driver
+    return _IMPL
 
 INIT_VERSION = 0
 
@@ -47,9 +56,10 @@ MIGRATE_REPO_PATH = os.path.join(
 
 def db_sync(version=None, init_version=0, engine=None):
     """Migrate the database to `version` or the most recent version."""
+
     if engine is None:
         engine = db_api.get_engine()
-    return IMPL.db_sync(engine=engine,
-                        abs_path=MIGRATE_REPO_PATH,
-                        version=version,
-                        init_version=init_version)
+    return get_backend().db_sync(engine=engine,
+                                 abs_path=MIGRATE_REPO_PATH,
+                                 version=version,
+                                 init_version=init_version)

setup.cfg

@@ -43,6 +43,8 @@ oslo.config.opts =
     glance.scrubber = glance.opts:list_scrubber_opts
     glance.cache= glance.opts:list_cache_opts
     glance.manage = glance.opts:list_manage_opts
+glance.database.migration_backend =
+    sqlalchemy = oslo.db.sqlalchemy.migration
 
 [build_sphinx]
 all_files = 1