test(ktuner): pin rollback keep-previous invariant#1338
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The rollback-merge keep-previous rule was an untested pure decision path in the tuner: across repeated tune/fix/import runs the ORIGINAL pre-ktuner value must survive so rollback restores pristine state. Extract merge_entries as a pure (no-I/O) fn with no behavior change, covered by a discriminating test — a merge that clobbered previous on re-tune fails the assert.
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What
Extract
merge_entriesfrommerge_rollbackas a pure (no-I/O) fn with zero behavior change, and pin the rollback keep-previous invariant with a discriminating test: across repeated tune/fix/import runs the ORIGINAL pre-ktuner value must survive so rollback restores pristine state.Why discriminating
A merge that clobbered
previouson re-tune would silently make rollback restore to a post-tune value instead of the pristine one. The test fails ifand_modifyalso setsprevious(mutation-verified).Testing
cargo fmt --check,clippy --all-targets -- -D warnings,cargo test(192 pass).Note
Slimmed from an earlier version: dropped the
is_metric_degradedextraction and its test. That covered the benchmark degradation direction, butverify_and_report/auto_rollback_on_degradationhave zero callers in the shipped CLI (ktuner tuneverifies via staticeval.score()), so testing them hardened dead code. Kept only the live rollback-merge coverage.