[PATCH 0/1 conductor] BZ 784499 add check to ensure task exists before saving

[Tzu-Mainn Chen]

Here's the original situation:

begin
      .
      .
      .
      task.instance.state = dcloud_to_instance_state(dcloud_instance.state)
      task.instance.save!
      task.state = Task::STATE_FINISHED
    rescue Exception => ex
      task.state = Task::STATE_FAILED
      task.message = ex.message
    ensure
      task.save!
    end

Due to optimistic locking, the above code gives stale object exceptions if the task is successfully deleted.  My understanding is that this is because the after_save method in instance_observer.rb will (under certain circumstances), delete itself and its tasks, meaning task.save! will fail.

The patch replaces

task.save!

with
      task.save! if Task.exists?(task.id)

If there's a better way to handle this (or if my logic is wrong), please let me know!

Mainn