When unshelve instance, the instance object not sync up with nova db:
1. During unshelving, user(3rd party software) may want store some information in instance system metadata, but after unshelve operation done, those information will be overwrite because conductor code will save original instance object to nova db;
2. When create new instance, the conductor code will refresh instance object; The unshelve operation need do same thing to ensure the instance object keep consistent with nova db.
Reproduce steps:
1. Create instance by nova CLI or horizon dashboard;
2. Shelve instance by nova CLI or horizon dashboard;
3. Unshelve instance by nova CLI or horizon dashboard;
4. During unshelving, store some information by call function 'db.instance_system_metadata_update';
5. After unshelve operation done, check nova db by SQL "use nova;select * from instance_system_metadata where instance_uuid=xxx",
You will see the information you stored in instance_system_metadata table be overwrite.
Fix proposed to branch: master
Review: https:/