Event transition definition - BackgroundMigration::BatchedMigration

Currently, we don't have any transition rules. We can move a batched background migration from any state to any state. Also, we don't validate the batched background migration data before we move it to a new state. This could lead to unsuspected behavior.

Example: We found a migration in state 'finished' with failed jobs.

I suggest that we define the different transitions and we validate the data before we move to a new state.

(Please double-check the data below, I am not 100% sure)

flowchart LR
   active <--> paused
   active ----> finished
   active --> finalizing
   active <--> failed
   finalizing --> failed
   finalizing --> finished

States description:

• Active: Ready to be picked by the runner / Running background jobs
• Finalizing: Running background jobs
• Failed: Failed migration
• Finished: Completed migration
• Paused: Not visible to the runner

Transition rules:

• :active => [:paused, :finalizing, :finished, :failed]
• :paused => :active
• :finalizing => [:finished, :failed]
• :failed => :active
• :finished => nil

Validations:

• Failed: At least one background job failed
• Finished: All background jobs are completed