First shot at splitting off evaluator

.github/actions/changelog_evaluator/action.yml

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+---
+name: Changelog based PR evaluator
+author: Mark Chappell (tremble)
+branding:
+  icon: git-branch
+  color: gray-dark
+description: |
+  This action evaluates the contents of changelog fragments in "changelogs/fragments/" to assess
+  which branches it may be appropriate to backport a change to.
+
+  A PR is evaluated as needing a "major release" if it includes at least one of "major_changes",
+  "breaking_changes", or "removed_features".
+
+  A PR is evaluated as needing a "minor release" if it includes at least one of "minor_changes" or
+  "deprecated_features".
+
+  A PR is evaluated as being a "bugfix" PR if it includes at least one of "bugfixes" or
+  "security_fixes".
+
+  The output values of this action are "bash-ian" booleans ("0" == True, anything else == False)
+
+outputs:
+  major_release:
+    description: Whether the changelogs indicate that a major release would be needed.
+    value: ${{ steps.evaluate.outputs.major }}
+  minor_release:
+    description: Whether the changelogs indicate that a minor release would be needed.
+    value: ${{ steps.evaluate.outputs.minor }}
+  bugfix_release:
+    description: Whether the changelogs indicate that a the PR includes bugfixes.
+    value: ${{ steps.evaluate.outputs.bugfix }}
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@v2
+      id: checkout
+    - name: Fetch change types from changelog fragments
+      id: evaluate
+      shell: bash {0}
+      run: |
+        gh pr -R "${GITHUB_REPOSITORY}" diff "${{ github.event.pull_request.number }}" --name-only | \
+          grep -E '^changelogs/fragments/' | \
+          while read -r line
+            do cat "${line}" | \
+              python -c 'import sys, yaml; change = yaml.safe_load(sys.stdin.read()) ; print("\n".join(change.keys()));' \
+              | tee -a all-changelog-types
+          done
+        # Beware, these are bash-ian booleans: "true == 0"
+        grep -qE '(release_summary|breaking_changes|major_changes|removed_features)' all-changelog-types ; echo "major=${?}" >>${GITHUB_OUTPUT}
+        grep -qE '(deprecated_features|minor_changes)' all-changelog-types ; echo "minor=${?}" >>${GITHUB_OUTPUT}
+        grep -qE '(bugfixes|security_fixes)' all-changelog-types ; echo "bugfix=${?}" >>${GITHUB_OUTPUT}
+      env:
+        GH_TOKEN: ${{ github.token }}

.github/actions/changelog_labeller/action.yml

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+---
+name: Apply backporting labels
+author: Mark Chappell (tremble)
+branding:
+  icon: git-branch
+  color: gray-dark
+description: |
+  Applies backporting labels to a PR.
+
+inputs:
+  purge_labels:
+    description: Whether to purge existing labels
+    required: false
+    default: false
+  purge_prefix:
+    description: The prefix used when purging labels
+    required: false
+    default: 'backport-'
+  label_to_add:
+    description: The label(s) to be applied to the PR
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Strip existing labels and add new labels
+      id: label-strip-add
+      # If breaking_changes or major_changes are pushed, then we always apply do_not_backport
+      # and strip any existing backport-* labels
+      if: ${{ inputs.purge_labels }}
+      shell: bash {0}
+      run: |
+        # If this includes breaking changes, then set the do_not_backport label and remove all
+        # labels starting with "backport-".
+        CURRENT_LABELS=$(
+          gh pr -R "${GITHUB_REPOSITORY}" view "${{ github.event.pull_request.number }}" \
+            --json labels \
+            --jq '[.labels[] | select(.name | startswith("${{ inputs.purge_prefix }}"))] | map(.name) | join(",")'
+        )
+        echo "Apply '${{ inputs.label_to_add }}' (remove '${CURRENT_LABELS}')"
+        if [[ -n ${CURRENT_LABELS} ]] ; then
+          gh pr -R "${GITHUB_REPOSITORY}" edit "${{ github.event.pull_request.number }}" \
+            --add-label ${{ inputs.label_to_add }} \
+            --remove-label "${CURRENT_LABELS}"
+        else
+          gh pr -R "${GITHUB_REPOSITORY}" edit "${{ github.event.pull_request.number }}" \
+            --add-label ${{ inputs.label_to_add }}
+        fi
+      env:
+        GH_TOKEN: ${{ github.token }}
+
+    - name: Apply labels
+      id: label-add
+      if: ${{ ! inputs.purge_labels }}
+      shell: bash {0}
+      run: |
+        echo "Apply '${{ inputs.label_to_add }}'"
+        gh pr -R "${GITHUB_REPOSITORY}" edit "${{ github.event.pull_request.number }}" \
+          --add-label ${{ inputs.label_to_add }}
+      env:
+        GH_TOKEN: ${{ github.token }}

.github/workflows/backport-labeller.yml

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+---
+name: Apply labels for backporting
+
+on:
+  workflow_call:
+    inputs:
+      label_major_release:
+        default: 'do_not_backport'
+        required: false
+        type: string
+        description: |
+          The label to apply if the PR includes a change that would necessitate a major release.
+      label_minor_release:
+        required: true
+        type: string
+        description: |
+          The label to apply if the PR only includes a change that would necessitate a minor
+          release.  This will also be applied by default if the PR doesn't necessitate a major
+          release.
+      label_bugfix_release:
+        required: true
+        type: string
+        description: |
+          The label to apply if the PR only includes a bugfix or security release.
+      label_skip:
+        default: 'do_not_backport'
+        required: false
+        type: string
+        description: |
+          If this label has been applied, then the PR will not be re-assessed.
+      label_mergeit:
+        default: 'mergeit'
+        required: false
+        type: string
+        description: |
+          Which label will be used to trigger labelling for minor/bugfix backporting.
+          We look for major releases when a change is pushed, and minor/bugfixes when the PR
+          has been approved for merging and the mergeit label has been applied to trigger
+          the merge.
+
+jobs:
+  changelog-types:
+    # We always skip if do_not_backport has previously been applied.
+    # Otherwise, if someone applies 'mergeit', opens the PR, or pushes a new commit
+    # we'll examine the contents of changelog fragments to try to guess the best backport strategy.
+    if: ${{
+        ! contains(github.event.pull_request.labels.*.name, inputs.label_skip)
+        && (
+          (github.event.action == 'labeled' && github.event.label.name == inputs.label_mergeit)
+          || (github.event.action == 'synchronize')
+          || (github.event.action == 'opened')
+        )
+      }}
+    permissions:
+      pull-requests: read
+    runs-on: ubuntu-latest
+    outputs:
+      no_backport: ${{ steps.evaluate.outputs.major_release }}
+      bugfix: ${{ steps.evaluate.outputs.bugfix_release }}
+      minor_only: ${{ steps.evaluate.outputs.minor_release }}
+    steps:
+      - name: Evaluate change types
+        id: evaluate
+        uses: ansible-network/github_actions/.github/actions/changelog_evaluator@main
+
+  changelog-labeling:
+    permissions:
+      pull-requests: write
+    runs-on: ubuntu-latest
+    needs:
+      - changelog-types
+    steps:
+      - name: Strip tags for backporting and apply do_not_backport
+        id: no-backport
+        # If breaking_changes or major_changes are pushed, then we always apply do_not_backport
+        # and strip any existing backport-* labels
+        if: ${{ needs.changelog-types.outputs.no_backport == '0' }}
+        uses: ansible-network/github_actions/.github/actions/changelog_labeller@main
+        with:
+          purge_labels: true
+          label_to_add: ${{ inputs.label_major_release }}
+
+      - name: Apply tag for backporting to at least the most recent major release
+        id: minor-only
+        if: ${{
+          (github.event.action == 'labeled' && github.event.label.name == inputs.label_mergeit )
+          && ! ( needs.changelog-types.outputs.no_backport == '0' )
+          && (
+            ( needs.changelog-types.outputs.minor_only == '0' )
+            || ! (needs.changelog-types.outputs.bugfix == '0' )
+          )
+          }}
+        uses: ansible-network/github_actions/.github/actions/changelog_labeller@main
+        with:
+          label_to_add: ${{ inputs.label_minor_release }}
+
+      - name: Apply tag for backporting to at least the two most recent major releases
+        id: security-or-bugfix
+        if: ${{
+          (github.event.action == 'labeled' && github.event.label.name == inputs.label_mergeit )
+          && ! ( needs.changelog-types.outputs.no_backport == '0' )
+          && ! ( needs.changelog-types.outputs.minor_only == '0' )
+          && ( needs.changelog-types.outputs.bugfix == '0' )
+          }}
+        uses: ansible-network/github_actions/.github/actions/changelog_labeller@main
+        with:
+          label_to_add: ${{ inputs.label_minor_release }},${{ inputs.label_bugfix_release }}