Use GitHub checks API rather than labels to trigger CI

With the GitHub Checks API, it is possible to add a neutral/failing commit status that includes a button to trigger an arbitrary action.

For example, this might look something like:

Clicking "details" will take you to the checks tab of the PR, which might show something like:

Clicking the button will trigger a check_run.requested_action event. Then this app can respond to the action_requested webhook and trigger a CI run.

I think this is preferable than the label-based approach for a few reasons:

Adding/removing labels creates noise in the PR
Because checks apply to a specific commit, it avoids the current potential race conditions where a user pushes new commits right before a label is added
Having an explicit status check and associated action button is more intuitive than a convention regarding adding/removing labels

If this app used probot, the code might look something like this:

context.github.checks.create(
  context.repo({
    name: "CI Gate",
    head_branch,
    head_sha,
    status: "completed",
    conclusion: "action_required",
    completed_at: new Date(),
    output: {
      title: "CI run not yet authorized",
      summary:
        "Click the above <kbd>Trigger CI run</kbd> button to trigger a CI run."
    },
    actions: [
      {
        label: "Trigger CI run",
        description: "Trigger a CI run",
        identifier: "trigger_ci"
      }
    ]
  })
);

If that sounds good to you, I could probably help implement this.

mvines / ci-gate

Use GitHub checks API rather than labels to trigger CI #84