db: Enable auto-generation of API DB migrations

Change I18846a5c7557db45bb63b97c7e8be5c4367e4547 enabled auto-generation
of migrations for the main database. Let's now extend this to the API
database using the same formula. While we're here, we also enable
"batch" migrations for SQLite [1] by default, which allow us to work
around SQLite's inability to support the ALTER statement for all but a
limited set of cases. As noted in the documentation [2], this will have
no impact on other backends where "we'd see the usual 'ALTER' statements
done as though there were no batch directive".

[1] https://stackoverflow.com/a/31140916/613428
[2] https://alembic.sqlalchemy.org/en/latest/batch.html

Change-Id: I51c3a53286a0eced4bf57ad4fc13ac5f3616f7eb
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.gitignore

@@ -49,5 +49,9 @@ tools/conf/nova.conf*
 doc/source/_static/nova.conf.sample
 doc/source/_static/nova.policy.yaml.sample
 
- # Files created by releasenotes build
+# Files created by releasenotes build
 releasenotes/build
+
+# Files created by alembic
+/nova.db
+/nova_api.db

nova/db/api/migrations/env.py

@@ -12,10 +12,11 @@
 
 from logging.config import fileConfig
 
+from alembic import context
 from sqlalchemy import engine_from_config
 from sqlalchemy import pool
 
-from alembic import context
+from nova.db.api import models
 
 # this is the Alembic Config object, which provides
 # access to the values within the .ini file in use.
@@ -26,11 +27,49 @@ config = context.config
 if config.attributes.get('configure_logger', True):
     fileConfig(config.config_file_name)
 
-# add your model's MetaData object here
-# for 'autogenerate' support
-# from myapp import mymodel
-# target_metadata = mymodel.Base.metadata
-target_metadata = None
+# this is the MetaData object for the various models in the API database
+target_metadata = models.BASE.metadata
+
+
+def include_name(name, type_, parent_names):
+    if type_ == 'column':
+        # NOTE(stephenfin): This is a list of fields that have been removed
+        # from various SQLAlchemy models but which still exist in the
+        # underlying tables. Our upgrade policy dictates that we remove fields
+        # from models at least one cycle before we remove the column from the
+        # underlying table. Not doing so would prevent us from applying the
+        # new database schema before rolling out any of the new code since the
+        # old code could attempt to access data in the removed columns. Alembic
+        # identifies this temporary mismatch between the models and underlying
+        # tables and attempts to resolve it. Tell it instead to ignore these
+        # until we're ready to remove them ourselves.
+        return (parent_names['table_name'], name) not in {
+            ('build_requests', 'request_spec_id'),
+            ('build_requests', 'user_id'),
+            ('build_requests', 'display_name'),
+            ('build_requests', 'instance_metadata'),
+            ('build_requests', 'progress'),
+            ('build_requests', 'vm_state'),
+            ('build_requests', 'task_state'),
+            ('build_requests', 'image_ref'),
+            ('build_requests', 'access_ip_v4'),
+            ('build_requests', 'access_ip_v6'),
+            ('build_requests', 'info_cache'),
+            ('build_requests', 'security_groups'),
+            ('build_requests', 'config_drive'),
+            ('build_requests', 'key_name'),
+            ('build_requests', 'locked_by'),
+            ('build_requests', 'reservation_id'),
+            ('build_requests', 'launch_index'),
+            ('build_requests', 'hostname'),
+            ('build_requests', 'kernel_id'),
+            ('build_requests', 'ramdisk_id'),
+            ('build_requests', 'root_device_name'),
+            ('build_requests', 'user_data'),
+            ('resource_providers', 'can_host'),
+        }
+
+    return True
 
 
 def run_migrations_offline():
@@ -46,6 +85,8 @@ def run_migrations_offline():
     context.configure(
         url=url,
         target_metadata=target_metadata,
+        render_as_batch=True,
+        include_name=include_name,
         literal_binds=True,
         dialect_opts={"paramstyle": "named"},
     )
@@ -81,7 +122,10 @@ def run_migrations_online():
 
     with connectable.connect() as connection:
         context.configure(
-            connection=connection, target_metadata=target_metadata
+            connection=connection,
+            target_metadata=target_metadata,
+            render_as_batch=True,
+            include_name=include_name,
         )
 
         with context.begin_transaction():

nova/db/main/migrations/env.py

@@ -27,18 +27,28 @@ config = context.config
 if config.attributes.get('configure_logger', True):
     fileConfig(config.config_file_name)
 
-# add your model's MetaData object here
-# for 'autogenerate' support
-# from myapp import mymodel
-# target_metadata = mymodel.Base.metadata
+# this is the MetaData object for the various models in the main database
 target_metadata = models.BASE.metadata
 
 
 def include_name(name, type_, parent_names):
     if type_ == 'table':
+        # NOTE(stephenfin): We don't have models corresponding to the various
+        # shadow tables. Alembic doesn't like this. Tell Alembic to look the
+        # other way. Good Alembic.
         return not name.startswith('shadow_')
 
     if type_ == 'column':
+        # NOTE(stephenfin): This is a list of fields that have been removed
+        # from various SQLAlchemy models but which still exist in the
+        # underlying tables. Our upgrade policy dictates that we remove fields
+        # from models at least one cycle before we remove the column from the
+        # underlying table. Not doing so would prevent us from applying the
+        # new database schema before rolling out any of the new code since the
+        # old code could attempt to access data in the removed columns. Alembic
+        # identifies this temporary mismatch between the models and underlying
+        # tables and attempts to resolve it. Tell it instead to ignore these
+        # until we're ready to remove them ourselves.
         return (parent_names['table_name'], name) not in {
             ('instances', 'internal_id'),
             ('instance_extra', 'vpmems'),
@@ -60,6 +70,7 @@ def run_migrations_offline():
     context.configure(
         url=url,
         target_metadata=target_metadata,
+        render_as_batch=True,
         include_name=include_name,
         literal_binds=True,
         dialect_opts={"paramstyle": "named"},
@@ -98,6 +109,7 @@ def run_migrations_online():
         context.configure(
             connection=connection,
             target_metadata=target_metadata,
+            render_as_batch=True,
             include_name=include_name,
         )