Test Model Switch Mode Series

This ticket is part 5 of 5 in one staged body of work. The series adds generated single-node leader/follower switch-mode testing to test/model.

Delivery Order

• WT-18867 - Follower workloads
• WT-18868 - Single step-up
• WT-18869 - Synchronous step-down
• WT-18870 - Random role cycles
• WT-17938 - Asynchronous step-down (this ticket)

Summary

Extend test/model so generated disaggregated role cycles can use the format-style asynchronous step-down protocol.

Motivation

Asynchronous step-down is the final stage of single-node role-switch testing. It verifies straddler rollback, post-boundary writes, checkpoint boundaries, timestamp rules, and final data correctness against the model.

Scope

• Add a set_step_down_timestamp workload operation to the model, parser, serializer, and WiredTiger runner.
• Let the generator randomly select a synchronous or asynchronous leader-to-follower transition in each role cycle.
• Select a valid boundary timestamp and leave selected write transactions open when the boundary is set.
• Verify that straddling write transactions receive WT_ROLLBACK.
• Generate leader writes with timestamps above the boundary before completing the transition.
• Set stable to exactly the boundary, take the step-down checkpoint, and reconfigure to follower in the required order.
• Continue generated read and write operations in follower mode.
• Enforce the step-down timestamp and commit timestamp rules in workload validation.
• Exclude prepared transactions that span the transition until WiredTiger supports them.
• Add fixed regression workloads before enabling randomized asynchronous step-down cycles.

Evergreen Coverage

All five tickets extend one Evergreen task named model-test-switch. This follows existing model task names such as model-test-long and model-test-long-disagg.

Definition of Done

• Fixed asynchronous step-down workloads pass against the model and WiredTiger.
• The generator produces valid randomized role cycles with synchronous and asynchronous step-down transitions.
• Straddler and timestamp error codes match between the model and WiredTiger.
• Final durable table contents match between the model and WiredTiger.
• The asynchronous step-down workloads run in model-test-switch.