[12] - Update module issues on 24/7 production environment with table locks

[Nimja]

The upgrade process in Odoo currently has a major issue with how it updates the DB.

Setup:

• 2 modules: module_parent and module_child
• module_child is dependent on module_parent
• module_parent has a model: user, with role, not required=False (default).
• module_child updates that model: user, with role required=True (explicitly set).

If you update PARENT, the following happens:

• Load modules to update
• Update module_parent
• Update the DB, removing the "not null" requirement from user.role (locking the table!)
• Commit the changes???
• Update module_child
• Update the DB, re-adding the "not null" requirement from user.role

The major issue with this is that it becomes impossible to update modules on a large production environment. Because it's very likely (and actually happening) that:

• A: The table lock on user causes issues with other running processes at the same time.
• B: There is a period of time where the not null constrained is removed from the DB. If there are many modules to update, this is quite serious!

[cymitearacil]

This is happening also on Odoo 10 branch. Yesterday I could not deploy on production server because of this issue.