I've discovered that this action actually fails for external pull requests. I'm in the process of reviewing how this action should work, but it takes some time. The problem is described here and here.

Later update: Since then, I've created python-coverage-comment-action which works for external PRs but only supports Python. I have no plan to port the "external PR" part to this action.

Publish diff coverage report as PR comment, and create a coverage badge to display on the readme.

See example at: https://github.com/ewjoachim/coverage-comment-action-example

This action operates on an already generated coverage.xml (Cobertura) file as generated by most tools, accross languages.

It has two main modes of operation:

If acting on a PR, it will analyze the XML file, and produce a comment that will be posted to the PR. If a comment had already previously be written, it will be updated. The comment contains information on the evolution of coverage rate attributed to this PR, as well as the rate of coverage for lines that this PR introduces. There's also a small analysis for each file in a collapsed block.

See: ewjoachim/coverage-comment-action-example#3 (comment)

If acting on the repository's default branch, it will extract the coverage rate and create a small JSON file that will be stored on the repository's wiki. This file will then have a stable URL, which means you can create a shields.io badge from it.

See: https://github.com/ewjoachim/coverage-comment-action-example

Please ensure that the repository wiki has been initialized with at least a single page created. Once it's done, you can disable the wiki for the repository.

- name: Display coverage
  uses: ewjoachim/coverage-comment-action@v1
  with:
    GITHUB_TOKEN: ${{ github.token }}

- name: Display coverage
  uses: ewjoachim/coverage-comment-action@v1
  with:
    GITHUB_TOKEN: ${{ github.token }}

    # Path and filename of the coverage XML file to analyze.
    COVERAGE_FILE: "coverage.xml"

    # Whether or not a badge will be generated and stored.
    BADGE_ENABLED: "true"

    # Name of the json file containing badge informations stored in the repo wiki.
    BADGE_FILENAME: coverage-comment-badge.json

    # If the coverage percentage is above or equal to this value, the badge will be green.
    MINIMUM_GREEN: 100

    # Same with orange. Below is red.
    MINIMUM_ORANGE: 70

    # [Advanced] Specify a different template for the comments that will be written on the PR.
    COMMENT_TEMPLATE: ""

    # [Advanced] Additional args to pass to diff cover (one per line)
    DIFF_COVER_ARGS: ""

On the examples above, the version was set to v1 (a branch). You can also pin a specific version such as v1.0.3 (a tag). There are still things left to figure out in how to manage releases and version. If you're interested, there's a corresponding issue.

There is no automated test and the dependencies are not frozen, so it's possible that it fails at some point if a dependency breaks compatibility. If this happens, we'll fix it and put better checks in place.

It's probably usable as-is, but you're welcome to offer feedback and, if you want, contributions.

COMMENT_TEMPLATE and DIFF_COVER_ARGS are somewhat experimental. They haven't been thouroughly tested.