alembic: Fix compatibility with SQLAlchemy 2.0+.

SQLAlchemy 2.0 changed the way that commits/rollbacks are handled
causing the final `UPDATE` to our `alembic_version_<whatever>` tables
to be rolled back instead of committed.

We now use one connection to determine which
`alembic_version_<whatever>` table to use and another to run the
actual migrations. This prevents the erroneous rollback.

This change is compatible with both SQLAlchemy 1.4 and 2.0.

contrib/ast-db-manage/env.py

@@ -67,6 +67,8 @@ def run_migrations_online():
     and associate a connection with the context.
 
     """
+    script_location = config.get_main_option('script_location')
+
     engine = engine_from_config(
                 config.get_section(config.config_ini_section),
                 prefix='sqlalchemy.',
@@ -74,76 +76,72 @@ def run_migrations_online():
 
     logger.info('Testing for an old alembic_version table.')
 
-    connection = engine.connect()
-    context.configure(
-                connection=connection,
-                target_metadata=target_metadata,
-                version_table='alembic_version'
-                )
-
-    script_location = config.get_main_option('script_location')
-    found = False
-    mc = context.get_context()
-    current_db_revision = mc.get_current_revision()
-    script = ScriptDirectory.from_config(config)
-    """ If there was an existing alembic_version table, we need to
-    check that it's current revision is in the history for the tree
-    we're working with.
-    """
-    for x in script.iterate_revisions('head', 'base'):
-        if x.revision == current_db_revision:
-            """ An alembic_versions table was found and it belongs to
-            this alembic tree
-            """
-            logger.info(
-                ('An old alembic_version table at revision %s was '
-                 'found for %s.  Renaming to alembic_version_%s.'),
-                        current_db_revision, script_location,
-                        script_location)
-            op = Operations(mc)
-            try:
-                with context.begin_transaction():
-                    op.rename_table(
-                        'alembic_version', 'alembic_version_%s'
-                        % script_location)
-                found = True
-            except:
-                logger.error(('Unable to rename alembic_version to '
-                             'alembic_version_%s.'),
-                             script_location)
-                connection.close()
-                return
-
-            break
-
-    if not found:
-        logger.info('Didn\'t find an old alembic_version table.')
-    logger.info('Trying alembic_version_%s.' % script_location)
+    with engine.connect() as connection:
+        context.configure(
+            connection=connection,
+            target_metadata=target_metadata
+        )
+
+        found = False
+        mc = context.get_context()
+        current_db_revision = mc.get_current_revision()
+        script = ScriptDirectory.from_config(config)
+        """ If there was an existing alembic_version table, we need to
+        check that it's current revision is in the history for the tree
+        we're working with.
+        """
+        for x in script.iterate_revisions('head', 'base'):
+            if x.revision == current_db_revision:
+                """ An alembic_versions table was found and it belongs to
+                this alembic tree
+                """
+                logger.info(
+                    ('An old alembic_version table at revision %s was '
+                     'found for %s.  Renaming to alembic_version_%s.'),
+                            current_db_revision, script_location,
+                            script_location)
+                op = Operations(mc)
+                try:
+                    with context.begin_transaction():
+                        op.rename_table(
+                            'alembic_version', 'alembic_version_%s'
+                            % script_location)
+                    found = True
+                except:
+                    logger.error(('Unable to rename alembic_version to '
+                                'alembic_version_%s.'),
+                                script_location)
+                    connection.close()
+                    return
+
+                break
+
+        if not found:
+            logger.info('Didn\'t find an old alembic_version table.')
+        logger.info('Trying alembic_version_%s.' % script_location)
 
     """ We MAY have an alembic_version table that doesn't belong to
     this tree but if we still don't have an alembic_version_<tree>
     table, alembic will create it.
     """
-    context.configure(
-                connection=connection,
-                target_metadata=target_metadata,
-                version_table='alembic_version_' + script_location
-                )
-    mc = context.get_context()
-    current_db_revision = mc.get_current_revision()
-    if current_db_revision:
-        logger.info(
-            'Using the alembic_version_%s table at revision %s.',
-            script_location, current_db_revision)
-    else:
-        logger.info('Creating new alembic_version_%s table.',
-                    script_location)
-
-    try:
+    with engine.connect() as connection:
+        context.configure(
+            connection=connection,
+            target_metadata=target_metadata,
+            version_table='alembic_version_' + script_location
+        )
+        mc = context.get_context()
+        current_db_revision = mc.get_current_revision()
+        if current_db_revision:
+            logger.info(
+                'Using the alembic_version_%s table at revision %s.',
+                script_location, current_db_revision)
+        else:
+            logger.info('Creating new alembic_version_%s table.',
+                        script_location)
+
         with context.begin_transaction():
             context.run_migrations()
-    finally:
-        connection.close()
 
 
 if context.is_offline_mode():