ci: auto-assign milestones to pull requests (#1301)

Fixes #1296.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

.github/workflows/dev_pr.yml

@@ -48,3 +48,10 @@ jobs:
           script: |
             const script = require(`${process.env.GITHUB_WORKSPACE}/.github/workflows/dev_pr/title_check.js`);
             await script({github, context});
+
+      - name: Assign milestone
+        if: always()
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          ./.github/workflows/dev_pr/milestone.sh "${GITHUB_REPOSITORY}" ${{github.event.number}}

.github/workflows/dev_pr/milestone.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# Assign a milestone to the given PR based on the open milestones and known
+# releases.
+
+set -euo pipefail
+
+main() {
+    local -r repo="${1}"
+    local -r pr_number="${2}"
+    echo "On ${repo} pull ${pr_number}"
+
+    local -r existing_milestone=$(gh pr view "${pr_number}" \
+                                  --json milestone \
+                                  -t '{{if .milestone}}{{.milestone.title}}{{end}}')
+
+    if [[ -n "${existing_milestone}" ]]; then
+        echo "PR has milestone: ${existing_milestone}"
+        return 0
+    fi
+
+    local -r latest_version=$(git ls-remote --heads origin |
+                                  grep -o '[0-9.]*$' |
+                                  sort --version-sort |
+                                  tail -n1)
+
+    local -r milestone=$(gh api "/repos/${repo}/milestones" |
+                             jq -r '.[] | .title' |
+                             grep -E '^ADBC Libraries' |
+                             grep -v "${latest_version}" |
+                             head -n1)
+
+    echo "Latest tagged version: ${latest_version}"
+    echo "Assigning milestone: ${milestone}"
+
+    gh pr edit "${pr_number}" -m "${milestone}"
+}
+
+main "$@"