RFC: Automated test coverage PR comments for easier code review

[skasti]

Proposal: Automated PR Test Coverage Comments

Context

Reviewers currently have to check the CI/CD command output to verify test coverage for new code in PRs. Automatically posting coverage summaries will make it easier to understand the quality of contributions and improve code confidence without manual checks.

Motivation

• Faster review: Reviewers can immediately see test coverage impact for a PR without digging into artifacts or logs.
• Better insight: Ensures test gaps are visible during code review, helping prevent untested changes being merged by accident.
• Non-intrusive: No required coverage threshold by default; focus on reporting, not enforcing. Could allow teams to configure thresholds if desired.
• Performance: Should not meaningfully slow down CI, as code coverage is typically already measured.

Scope & Implementation

• Start with posting a summary for total repository coverage.
• Ideally, progress toward showing coverage only for changed files in each PR.
• Implementation can leverage uploading coverage.out with actions/upload-artifact and then a bot/script posting a PR comment with a summary (see below for an example outline).
• Make it generic so other repositories using mage can follow the same approach.

Example workflow steps

    - name: Upload coverage artifact
      if: ${{ github.event_name == 'pull_request' && hashFiles('coverage.out') != '' }}
      uses: actions/upload-artifact@v4
      with:
        name: go-coverage
        path: coverage.out

    - name: Comment PR with coverage
      if: ${{ github.event_name == 'pull_request' && hashFiles('coverage.out') != '' }}
      uses: actions/github-script@v7
      with:
        script: |
          const { execSync } = require('child_process');

          const summary = execSync('go tool cover -func=coverage.out', { encoding: 'utf8' });

          const body =
            '## Go coverage\n\n' +
            '```text\n' + summary + '\n```\n' +
            '<!-- go-coverage-comment -->';

          const issue_number = context.payload.pull_request.number;

          const { data: comments } = await github.rest.issues.listComments({
            owner: context.repo.owner,
            repo: context.repo.repo,
            issue_number,
          });

          const existing = comments.find(c =>
            c.body.includes('<!-- go-coverage-comment -->')
          );

          if (existing) {
            await github.rest.issues.updateComment({
              owner: context.repo.owner,
              repo: context.repo.repo,
              comment_id: existing.id,
              body,
            });
          } else {
            await github.rest.issues.createComment({
              owner: context.repo.owner,
              repo: context.repo.repo,
              issue_number,
              body,
            });
          }

---

Open for discussion on best practices or ideas for improving how coverage is measured and reported within mage-based Go projects.

[skasti]

tried googling how to only get coverage for changed files from go cover, and google ai suggests grep:

# Example snippet to filter using grep and sed
grep ".go" coverage.out | grep -f <(git diff --name-only origin/main) > changed_coverage.out

and then passing that filtered coverage report to go cover instead of the full report 😅

[AtzeDeVries]

This is an good idea., I think this might also be nicely implemented with a combination of

https://github.com/reviewdog/reviewdog

and

https://github.com/marketplace/actions/code-coverage-with-reviewdog

[skasti]

@AtzeDeVries That would be really nice. it is always a bit frustrating to parse the linter output! 😅