Optimize ParallelExecutor

[brfrn169]

Description

This PR optimizes ParallelExecutor to run a single task without parallel execution when asynchronous execution is not required.

Related issues and/or PRs

N/A

Changes made

• Optimized ParallelExecutor to run a single task without parallel execution when asynchronous execution is not required.

Checklist

The following is a best-effort checklist. If any items in this checklist are not applicable to this PR or are dependent on other, unmerged PRs, please still mark the checkboxes after you have read and understood each item.

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes.
• I have considered whether similar issues could occur in other products, components, or modules if this PR is for bug fixes.
• Any remaining open issues linked to this PR are documented and up-to-date (Jira, GitHub, etc.).
• Tests (unit, integration, etc.) have been added for the changes.
• My changes generate no new warnings.
• Any dependent changes in other PRs have been merged and published.

Additional notes (optional)

N/A

Release notes

N/A

[Copilot]

Pull Request Overview

This PR adds an optimization to ParallelExecutor so that single-task workloads run serially when waiting is required, and updates the test suite to verify this behavior across prepare, validate, commit, and rollback operations.

• Short-circuits parallel execution for one task when noWait is false.
• Adds unit tests for single-task scenarios under parallel/synchronous and parallel/asynchronous configurations.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
core/src/main/java/com/scalar/db/transaction/consensuscommit/ParallelExecutor.java	Added early return to run a single task serially.
core/src/test/java/com/scalar/db/transaction/consensuscommit/ParallelExecutorTest.java	Added tests for single-task behavior in all executor paths.

Comments suppressed due to low confidence (1)

core/src/test/java/com/scalar/db/transaction/consensuscommit/ParallelExecutorTest.java:104

• [nitpick] These single-task tests follow the same structure for prepare, validate, commit, and rollback; consider refactoring into a parameterized test or helper to reduce duplication.

@Test public void prepare_ParallelPreparationEnabled_SingleTaskGiven_ShouldExecuteTasksSerially()

[Copilot]

Using atMost(tasks.size()) allows zero invocations, so the test could pass even if task.run() was never called. Consider using verify(task, atLeastOnce()).run() or verify(task, timeout(...).times(1)).run() to ensure it actually executes.

Suggested change

	verify(task, atMost(tasks.size())).run();
	verify(task, atLeastOnce()).run();

[Copilot]

As above, atMost(tasks.size()) could allow zero runs. For the asynchronous rollback test, use atLeastOnce() or a timeout-based verification to confirm the task actually executes.

Suggested change

	verify(task, atMost(tasks.size())).run();
	verify(task, atLeastOnce()).run();

[Torch3333]

LGTM, thank you!

[komamitsu]

LGTM! 👍

[feeblefakie]

LGTM! Thank you!