Fix downgrades in migration scripts 016 and 035

Downgrade behavior was not checked in Heat, so there are a few mistakes
in migration scripts. Fixed these scripts, set test attributes
`snake_walk` and `downgrade` to True due to allow migration testing and
avoid of regression.

Change-Id: I30cfb282cf7884d8b2748c6486a68d05072957af

heat/db/sqlalchemy/migrate_repo/versions/016_timeout_nullable.py

@@ -27,4 +27,7 @@ def downgrade(migrate_engine):
     meta.bind = migrate_engine
 
     stack = sqlalchemy.Table('stack', meta, autoload=True)
+    # NOTE(viktors): We must be sure, that there are no nullable columns in
+    #                `stack` table before we alter it.
+    migrate_engine.execute('UPDATE stack set timeout=60 WHERE timeout IS NULL')
     stack.c.timeout.alter(nullable=False)

heat/db/sqlalchemy/migrate_repo/versions/035_event_uuid_to_id.py

@@ -146,6 +146,9 @@ def downgrade(migrate_engine):
     cons.create()
 
     event_table.c.tmp_id.alter('id', default=lambda: str(uuid.uuid4))
+    if migrate_engine.name == 'postgresql':
+        sequence = sqlalchemy.Sequence('evt')
+        sqlalchemy.schema.DropSequence(sequence, bind=migrate_engine).execute()
 
 
 def downgrade_sqlite(migrate_engine):
@@ -178,7 +181,7 @@ def downgrade_sqlite(migrate_engine):
     event_table.create()
 
     prev_event_table = sqlalchemy.Table('event', meta, autoload=True)
-    event_list = prev_event_table.select().execute()
+    event_list = prev_event_table.select().execute().fetchall()
     for event in event_list:
         values = {
             'id': event.uuid,

heat/db/sqlalchemy/migrate_repo/versions/045_stack_backup.py

@@ -39,4 +39,36 @@ def downgrade(migrate_engine):
     meta = sqlalchemy.MetaData(bind=migrate_engine)
 
     stack = sqlalchemy.Table('stack', meta, autoload=True)
+    if migrate_engine.name == 'sqlite':
+        _downgrade_045_sqlite(migrate_engine, meta, stack)
+    else:
         stack.c.backup.drop()
+
+
+def _downgrade_045_sqlite(migrate_engine, metadata, table):
+
+    table_name = table.name
+
+    constraints = [
+        c.copy() for c in table.constraints
+        if not isinstance(c, sqlalchemy.CheckConstraint)
+    ]
+    columns = [c.copy() for c in table.columns if c.name != "backup"]
+
+    new_table = sqlalchemy.Table(table_name + "__tmp__", metadata,
+                                 *(columns + constraints))
+    new_table.create()
+
+    migrate_data = """
+        INSERT INTO stack__tmp__
+            SELECT id, created_at, updated_at, name, raw_template_id,
+                   user_creds_id, username, owner_id, status, status_reason,
+                   parameters, timeout, tenant, disable_rollback, action,
+                   deleted_at, stack_user_project_id
+            FROM stack;"""
+
+    migrate_engine.execute(migrate_data)
+
+    table.drop()
+
+    new_table.rename(table_name)

heat/tests/db/test_migrations.py

@@ -39,8 +39,8 @@ class HeatMigrationsCheckers(test_migrations.WalkVersionsMixin,
                              common.FakeLogMixin):
     """Test sqlalchemy-migrate migrations."""
 
-    snake_walk = False
-    downgrade = False
+    snake_walk = True
+    downgrade = True
 
     @property
     def INIT_VERSION(self):
@@ -206,7 +206,7 @@ class HeatMigrationsCheckers(test_migrations.WalkVersionsMixin,
 
     def _pre_upgrade_045(self, engine):
         raw_template = utils.get_table(engine, 'raw_template')
-        templ = [dict(id=5, template='{}')]
+        templ = [dict(id=5, template='{}', files='{}')]
         engine.execute(raw_template.insert(), templ)
 
         user_creds = utils.get_table(engine, 'user_creds')