Add code coverage reporting to GitHub Actions #305

Summary

Add code coverage reporting to the GitHub Actions CI pipeline to track test coverage metrics and identify untested code paths.

Current State

The pr-check.yml workflow runs tests across multiple platforms and configurations but does not collect or report code coverage.

Proposed Implementation

1. Add coverage tooling

Use cargo-llvm-cov (or tarpaulin as alternative) to collect coverage data:

- name: Install cargo-llvm-cov
  uses: taiki-e/install-action@cargo-llvm-cov

- name: Run tests with coverage
  run: cargo llvm-cov --all-features --workspace --lcov --output-path lcov.info

2. Upload to coverage service

Options:

• Codecov (recommended): Free for open source, integrates well with PRs
• Coveralls: Alternative coverage service
• Artifact upload: Just store coverage reports as build artifacts

Example with Codecov:

- name: Upload coverage to Codecov
  uses: codecov/codecov-action@v4
  with:
    files: lcov.info
    fail_ci_if_error: false

3. Add coverage badge to README

After setup, add a coverage badge to README.md:

[![Coverage](https://codecov.io/gh/microsoft/python-environment-tools/branch/main/graph/badge.svg)](https://codecov.io/gh/microsoft/python-environment-tools)

Considerations

• Single platform for coverage: Coverage can be collected on one platform (Ubuntu) to avoid duplicated uploads
• Feature flags: May need to run coverage with specific feature flags (ci) to get meaningful coverage of conditional code
• Performance: Coverage collection adds ~20-30% overhead to test runtime
• Threshold: Consider setting a minimum coverage threshold (e.g., 60%) as a PR check

Tasks

• Choose coverage tool (cargo-llvm-cov or tarpaulin)
• Choose coverage service (Codecov, Coveralls, or artifacts-only)
• Add coverage step to pr-check.yml or create dedicated coverage.yml workflow
• Configure coverage service (if using Codecov/Coveralls)
• Add coverage badge to README
• (Optional) Set up coverage thresholds for PRs

References

• cargo-llvm-cov
• Codecov GitHub Action
• tarpaulin