Add backport label check workflow

Author: flobernd

.github/workflows/check-backport-labels.yml

@@ -0,0 +1,126 @@
+name: Check
+
+on:
+  pull_request:
+    branches:
+      - main
+    types: [labeled, unlabeled, opened, reopened, synchronize]
+
+permissions:
+  pull-requests: "read"
+
+jobs:
+  check-backport-label:
+    name: backport label 
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: "Check backport label"
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          json_pr_labels='${{ toJSON(github.event.pull_request.labels) }}'
+          readarray -t pr_labels < <(echo "${json_pr_labels}" | jq -r -c '.[].name')
+
+          json_all_labels="$(gh label list --repo ${{ github.repository }} --json name --search "backport" --limit 1000)"
+          readarray -t all_labels < <(echo "${json_all_labels}" | jq -r -c '.[].name')
+
+          declare -A all_backport_labels=()
+          declare -A all_floating_majors=()
+
+          backport_regex="^backport ([0-9])+\.([0-9]+|x)$"
+
+          echo "::group::Available Labels"
+          echo "skip-backport"
+
+          for label in "${all_labels[@]}"; do
+            if [[ "${label}" =~ ${backport_regex} ]]; then
+              major="${BASH_REMATCH[1]}"
+              minor="${BASH_REMATCH[2]}"
+              all_backport_labels["${label}"]=1
+              echo "${label}"
+
+              if [ "${minor}" = "x" ]; then
+                all_floating_majors["${major}"]=1
+              fi
+            fi
+          done
+
+          echo "::endgroup::"
+
+          has_exact_backport_label=false
+          declare -A pr_exact_majors=()
+          declare -A pr_floating_majors=()
+
+          echo "::group::Detected Labels"
+
+          for pr_label in "${pr_labels[@]}"; do
+            if [ "${pr_label}" = "skip-backport" ]; then
+              has_exact_backport_label=true
+              echo "${pr_label}"
+              continue
+            fi
+
+            if [ -z "${all_backport_labels[${pr_label}]}" ]; then
+              continue
+            fi
+
+            if [[ "${pr_label}" =~ ${backport_regex} ]]; then
+              major="${BASH_REMATCH[1]}"
+              minor="${BASH_REMATCH[2]}"
+              if [ "${minor}" != "x" ]; then
+                pr_exact_majors["${major}"]=1
+                has_exact_backport_label=true
+              else
+                pr_floating_majors["${major}"]=1
+              fi
+            fi
+
+            echo "${pr_label}"
+          done
+
+          echo "::endgroup::"
+
+          if [ "${has_exact_backport_label}" != true ]; then
+            echo "::error::No exact backport label found. Please add at least one of the"\
+                  "'backport {major}.{minor}' labels or use 'skip-backport',"\
+                  "if this PR should not be backported."
+            exit 1
+          fi
+
+          # Validate that a floating backport label exists for each exact backport label major
+          # version.
+
+          has_required_floating_labels=true
+
+          for pr_major in "${!pr_exact_majors[@]}"; do
+            if [ -z "${all_floating_majors[${pr_major}]}" ]; then
+              # There is no floating version branch for the given major version.
+              continue
+            fi
+
+            if [ -z "${pr_floating_majors[${pr_major}]}" ]; then
+              has_required_floating_labels=false
+              echo "::error::Missing floating backport label for '${pr_major}.x'"
+            fi
+          done
+
+          if [ "${has_required_floating_labels}" != true ]; then
+            exit 1
+          fi
+
+          # Validate that an exact backport label exists for each floating backport label major
+          # version.
+
+          has_required_exact_labels=true
+
+          for pr_floating_major in "${!pr_floating_majors[@]}"; do
+            if [ -z "${pr_exact_majors[${pr_floating_major}]}" ]; then
+              has_required_exact_labels=false
+              echo "::error::Missing exact backport label for '${pr_floating_major}.x'"
+            fi
+          done
+
+          if [ "${has_required_exact_labels}" != true ]; then
+            exit 1
+          fi