Acquire the global DDL lock if BDR is enabled

This should allow users to run Alembic in "online" mode with a Postgres
BDR system. It will also emit the correct SQL for offline migrations.

It introduces a new boolean setting in alembic.ini, "bdr", which
defaults to false.

Signed-off-by: Jeremy Cline <[email protected]>

alembic.ini

@@ -14,6 +14,10 @@ script_location = alembic
 # Don't bother, this is obtained from the Bodhi config file
 sqlalchemy.url = sqlite://bodhi.db
 
+# Set to true to aquire the global DDL lock for BDR
+# See http://bdr-project.org/docs/stable/ddl-replication-advice.html
+bdr = false
+
 
 # Logging configuration
 [loggers]

alembic/env.py

@@ -2,7 +2,7 @@
 from logging.config import fileConfig
 
 from alembic import context
-from sqlalchemy import engine_from_config, pool
+from sqlalchemy import engine_from_config, pool, event
 
 from bodhi.server.config import config as bodhi_config
 from bodhi.server.models import Base
@@ -43,6 +43,8 @@ def run_migrations_offline():
     context.configure(url=url)
 
     with context.begin_transaction():
+        if config.get_main_option('bdr').strip().lower() == 'true':
+            context.execute('SET LOCAL bdr.permit_ddl_locking = true')
         context.run_migrations()
 
 
@@ -58,6 +60,12 @@ def run_migrations_online():
         prefix='sqlalchemy.',
         poolclass=pool.NullPool)
 
+    if config.get_main_option('bdr').strip().lower() == 'true':
+        def enable_bdr(connection, connection_record):
+            with connection.cursor() as cursor:
+                cursor.execute('SET LOCAL bdr.permit_ddl_locking = true')
+        event.listen(engine, 'connect', enable_bdr)
+
     connection = engine.connect()
     context.configure(
         connection=connection,