Description

Remove eventlet dependency from Barbican codebase and migrate to using threading-based execution instead of eventlet.

Implementation Details

This change removes eventlet dependency from Barbican and migrates to using threading-based execution:

• Remove eventlet from requirements.txt
• Remove eventlet imports and monkey_patch calls from:
• barbican/cmd/worker.py
• barbican/cmd/keystone_listener.py
• barbican/cmd/retry_scheduler.py
• barbican/tests/queue/test_retry_scheduler.py
• Replace 'eventlet' executor with 'threading' executor in:
• get_server() function for RPC server
• get_notification_server() function for notification server

Testing

• Tested with tox environment to ensure functionality is preserved
• Queue module imports successfully
• All components work correctly with threading executor

Upstream Review

The implementation has been submitted for upstream review: https://review.opendev.org/c/openstack/barbican/+/955301

Related Work

This aligns with the OpenStack community goal for eventlet removal:

• https://governance.openstack.org/tc/goals/selected/remove-eventlet.html
• https://wiki.openstack.org/wiki/Eventlet-removal

The threading executor provides the same functionality as eventlet for oslo.messaging but without the eventlet dependency. This change modernizes the codebase and removes the need for monkey patching.