action.yml

name: is-tag-reachable-from-default-branch
description: Determines whether a tag is reachable from the default branch

inputs:
  tag:
    description: 'The tag to be checked'
    required: true
  default-branch:
    description: 'The default branch for the repository.  Defaults to main.'
    required: true
    default: 'main'
  ref:
    description: 'Required when a `ref` arg was used with `actions/checkout` or if a non-default branch was selected when starting the workflow. Specify the same ref here so the action can return the repository to that ref.'
    required: false
  error-if-not-reachable:
    description: 'Throw an error if the tag is not reachable from the default branch'
    required: true
    default: 'true'

outputs:
  reachable:
    description: 'Whether the tag is reachable from the default branch.'
    value: '${{ steps.check-tag.outputs.reachable }}'

runs:
  using: 'composite'
  steps:
    - name: Check if tag is reachable from the default branch
      id: check-tag
      shell: bash
      run: |
        errorIfNotReachable=${{ inputs.error-if-not-reachable }}
        tag=${{ inputs.tag }}
        defaultBranch=${{ inputs.default-branch }}
        ref=${{ inputs.ref }}

        echo "Checking tag: $tag"
        echo "ErrorIfNotReachable: $errorIfNotReachable"

        if [ $(echo ${#ref}) -gt 0 ]; then
          echo "::group::Switching to the default branch"
          git checkout $defaultBranch -f
          echo "::endgroup::"
        fi

        # Store the list of tags reachable from the default branch (--merged)
        # and sort them by ascending version number, interpreting the tag name as a version number (--sort="version:refname")
        mergedTags=$(git tag --list --sort="version:refname" --merged)

        echo "Checking tag: $tag"
        echo "::group::The repository contains the following tags which are reachable from $defaultBranch"
        printf '%s\n' "${mergedTags[@]}"
        echo "::endgroup::"

        if [ $(echo ${#ref}) -gt 0 ]; then
          echo "::group::Switching back to $ref"
          git checkout $ref -f
          echo "::endgroup::"
        fi       

        if printf '%s\n' "${mergedTags[@]}" | grep -q -w $tag ; 
        then
          echo "The tag appears in the list of reachable tags"
          echo "reachable=true" >> $GITHUB_OUTPUT
        else
          echo "reachable=false" >> $GITHUB_OUTPUT
          if [ "$errorIfNotReachable" == "true" ]; then
            echo "::error::The tag does not appear to be reachable from $defaultBranch."
            exit 1
          else
            echo "::warning::The tag does not appear to be reachable from $defaultBranch."    
          fi
        fi