Lite Spec: Introduce Glance Taskflow stopfile feature

Lite Spec: Introduce Glance Taskflow stopfile feature

problem:When preparing to take a Glance node down for maintenance or upgrade it is necessary to allow long-running operations to complete without allowing new operations to begin. The Taskflow engine does not have the capability to prevent individual executors from starting new jobs, and so attempting to take a Glance node out of the Taskflow processing pool risks a race condition with that executor starting a job just before the service is terminated which could cause the Task processing to fail.
solution:Introduce a disable_by_file_path feature to Glance Taskflow which will prevent the node from picking up new jobs. This allows an operator or automation engine to touch the appropriate file before terminating the service. This feature should depend on the oslo healthcheck middleware configuration to disable the taskflow engine. As an additional impact, this work will require Glance to instantiate a single taskflow engine as a singleton and reuse that engine instance on all subsequent taskflow operations.
impacts:None
timeline:Expected to have a fix merged in the Pike cycle before milestone 2.
link:https://docs.openstack.org/developer/oslo.middleware/healthcheck_plugins.html
assignee:Open
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