db: Remove 'cinder.database.migration_backend' entrypoint

We don't provide multiple backends in-tree and couldn't possibly support
those provided out-of-tree. Remove this knob.

Change-Id: Ia05eb6536bd4b2977358c0086293757091aa5e68
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

cinder/db/migration.py

@@ -17,22 +17,16 @@
 """Database setup and migration commands."""
 
 import os
-import threading
 
 from oslo_config import cfg
 from oslo_db import options
-from stevedore import driver
+from oslo_db.sqlalchemy import migration
 
 from cinder.db.sqlalchemy import api as db_api
 
-
-INIT_VERSION = 122
-
-_IMPL = None
-_LOCK = threading.Lock()
-
 options.set_defaults(cfg.CONF)
 
+INIT_VERSION = 122
 MIGRATE_REPO_PATH = os.path.join(
     os.path.abspath(os.path.dirname(__file__)),
     'sqlalchemy',
@@ -40,23 +34,14 @@ MIGRATE_REPO_PATH = os.path.join(
 )
 
 
-def get_backend():
+def db_sync(version=None, engine=None):
-    global _IMPL
-    if _IMPL is None:
-        with _LOCK:
-            if _IMPL is None:
-                _IMPL = driver.DriverManager(
-                    "cinder.database.migration_backend",
-                    cfg.CONF.database.backend).driver
-    return _IMPL
-
-
-def db_sync(version=None, init_version=INIT_VERSION, engine=None):
     """Migrate the database to `version` or the most recent version."""
 
     if engine is None:
         engine = db_api.get_engine()
-    return get_backend().db_sync(engine=engine,
+
+    return migration.db_sync(
+        engine=engine,
         abs_path=MIGRATE_REPO_PATH,
         version=version,
-                                 init_version=init_version)
+        init_version=INIT_VERSION)

releasenotes/notes/remove-configurable-migration-backend-aaee5a2f808c9b36.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Support for the ``cinder.database.migration_backend`` entrypoint, which
+    provided for configurable database migration backends, has been removed.
+    This was never exercised and was a source of unnecessary complexity.

setup.cfg

@@ -75,8 +75,6 @@ console_scripts =
     cinder-volume-usage-audit = cinder.cmd.volume_usage_audit:main
 wsgi_scripts =
     cinder-wsgi = cinder.wsgi.wsgi:initialize_application
-cinder.database.migration_backend =
-    sqlalchemy = oslo_db.sqlalchemy.migration
 
 [extras]
 all =