Make build_requests.instance MediumText

The build_requests.instance column is a serialized
instance object, and the instances.user_data column
is MediumText, so the build_requests.instance column
itself needs to be at least MediumText in size for MySQL.

Change-Id: I7d65df37c02750593037744543ad15e5bc64e913
Closes-Bug: #1635446

nova/db/sqlalchemy/api_migrations/migrate_repo/versions/028_build_requests_instance_mediumtext.py

@@ -0,0 +1,24 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from sqlalchemy import MetaData
+from sqlalchemy import Table
+
+from nova.db.sqlalchemy import api_models
+
+
+def upgrade(migrate_engine):
+    meta = MetaData()
+    meta.bind = migrate_engine
+
+    build_requests = Table('build_requests', meta, autoload=True)
+    build_requests.c.instance.alter(type=api_models.MediumText())

nova/db/sqlalchemy/api_models.py

@@ -241,7 +241,7 @@ class BuildRequest(API_BASE):
     id = Column(Integer, primary_key=True)
     instance_uuid = Column(String(36))
     project_id = Column(String(255), nullable=False)
-    instance = Column(Text)
+    instance = Column(MediumText())
     block_device_mappings = Column(MediumText())
     # TODO(alaski): Drop these from the db in Ocata
     # columns_to_drop = ['request_spec_id', 'user_id', 'display_name',