[Fix] Mutation scope changes when main advances

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What changed

The mutation-diff workflow now compares a pull request's synthetic merge result with that merge commit's first parent, the exact main snapshot GitHub used to create it. A regression test models a stale pull-request base followed by an unrelated executable change on main and verifies only the pull-request file is selected.

Why this change was made

PR #1505 changed from 499 to 624 reported executable lines after main advanced even though its latest source commit added only 6 executable lines. The stale event base caused 119 executable lines from the already-merged coverage cache verifier to be charged to the PR.

Impact

Mutation scope remains stable when main advances and still uses merge-result line coordinates. The 500-line limit is unchanged: #1505's corrected scope is 505 executable lines, so it remains legitimately blocked by 5 lines rather than the erroneous 124-line overage.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9973630 and b7177b2.

📒 Files selected for processing (2)

• .github/workflows/mutation-testing.yml
• scripts/stryker-diff.test.mjs

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• GitHub Check: platform-unit-test (windows-latest)
• GitHub Check: e2e-mock

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⚙️ CodeRabbit configuration file

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• scripts/stryker-diff.test.mjs

Require full commit SHA pins, least-privilege permissions, safe expression and shell interpolation, and trusted metadata handling.

⚙️ CodeRabbit configuration file

Files:

• .github/workflows/mutation-testing.yml

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⚙️ CodeRabbit configuration file

Files:

• scripts/stryker-diff.test.mjs

🔇 Additional comments (2)

.github/workflows/mutation-testing.yml (1)

53-55: LGTM!

scripts/stryker-diff.test.mjs (1)

60-61: LGTM!

Also applies to: 450-491

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Mutation testing now compares changes against the pull request’s actual base parent, improving accuracy when the target branch has advanced.
  • Prevented unrelated target-branch changes from being included in pull request mutation testing.

• Tests

  • Added coverage to verify correct base commit selection and change isolation.

Walkthrough

The mutation-testing workflow now derives BASE_SHA from the first parent of HEAD_SHA. Tests verify this workflow configuration and confirm that stale base references do not include unrelated base-branch changes.

Changes

Mutation testing base selection

Layer / File(s)	Summary
Derive the base commit from HEAD_SHA .github/workflows/mutation-testing.yml	The workflow removes the remote base-fetch step and computes BASE_SHA with git rev-parse "$HEAD_SHA^1".
Validate stale base behavior scripts/stryker-diff.test.mjs	Tests require first-parent base resolution and verify that selecting from the merge result's first parent reports only the pull request change.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: edelauna

Merge Risk: ⚪ Minimal · up to b7177

The mutation scope now compares the merge result against its actual base snapshot, preventing unrelated main-branch changes from being included. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 7 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains what changed, why it changed, and the expected impact, but it omits the required Related GitHub Issue, Test Procedure, Pre-Submission Checklist, Documentation Updates, and Add…	Use the repository template. Add an approved issue reference, explicit test steps and environment details, complete the pre-submission checklist, state whether documentation updates are required, and include any relevant additional notes.

✅ Passed checks (7 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Regression Evidence	✅ Passed	The changed mutation-scope behavior has focused coverage. The regression test builds a synthetic merge history with an advanced base branch, verifies the stale base selects both files, and verifies th…
Security Boundaries	✅ Passed	No changed path meets the stated security failure conditions. The workflow change only derives BASE_SHA from the platform-provided github.sha in HEAD_SHA using quoted shell parameters (`.github/…
Persistence Integrity	✅ Passed	No changed persistence path exists. The workflow change only derives BASE_SHA with git rev-parse and passes it to selectFromGit; selectFromGit reads files and runs Git diffs without writing ap…
Lifecycle Resource Cleanup	✅ Passed	PASS: The changed workflow path only computes BASE_SHA with git rev-parse and invokes the existing mutation command. It does not add a listener, watcher, timer, provider, or long-lived task. The a…
Title check	✅ Passed	The title clearly identifies the main change: fixing mutation scope when the main branch advances.

Full details: Description check

Explanation

The description explains what changed, why it changed, and the expected impact, but it omits the required Related GitHub Issue, Test Procedure, Pre-Submission Checklist, Documentation Updates, and Additional Notes sections.

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• Commit unit tests in branch fix/mutation-diff-merge-base-0i9xkg3qnqf1s

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Review status

This PR was opened by an automated account. A human maintainer must verify the change intent, provenance, and validation before merging.

Current step: The required review sequence passed. Remaining merge requirements apply.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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