Add workflow to automatically cherry-pick on merge

When a PR is merged that has a cherry-pick label such as
`process/cherry-pick/<branch>`, this workflow will kick in and attempt
to open a backport PR.
If the commits do not apply cleanly it will just error out.

This requires a settings change to allow github actions to approve and
open PR's.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

.github/workflows/backport.yml

@@ -0,0 +1,27 @@
+name: Backport
+on:
+  pull_request:
+    types: [closed]
+    branches:
+      - master
+
+jobs:
+  backport:
+    if: ${{ github.event.pull_request.merged == true && contains(join(github.event.pull_request.labels.*.name, ' '), 'process/cherry-pick/') }}
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+      contents: write
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: cherry-pick
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          git config --global user.email "github-actions[bot]@users.noreply.github.com"
+          git config --global user.name "github-actions[bot]"
+          ./hack/backport-pr.sh ${{ github.event.pull_request.number }}
+

hack/backport-pr.sh

@@ -0,0 +1,79 @@
+#!/usr/bin/env bash
+
+set -u -o pipefail
+
+log_error() {
+	if [ -n "${GITHUB_ACTIONS:-}" ]; then
+		echo "::error::${@}"
+		return
+	fi
+	echo "$@" >&2
+}
+
+PR="${1}"
+
+# Some round about stuff to run either locally or in github actions
+: "${GITHUB_SERVER_URL:=https://github.com}"
+if [ -v 2 ]; then
+	GITHUB_REPO="${2}"
+fi
+: "${GITHUB_REPO:=${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}}"
+
+content="$(gh pr view -R "${GITHUB_REPO}" --json title --json labels --json commits "${PR}")"
+if [ $? -ne 0 ]; then
+	log_error "Error getting PR #${PR} to backport"
+	exit 1
+fi
+
+labels="$(jq -r '.labels[].name' <<<"${content}" | grep cherry-pick/)"
+commits="$(jq -r '.commits[].oid' 2>&1 <<<"${content}")"
+title="$(jq -r '.title' <<<"${content}")"
+
+if [ $? -ne 0 ] || [ -z "${commits}" ]; then
+	log_error "Error getting commits for PR #${PR} to backport: ${commits}"
+	exit 1
+fi
+
+dir="$(mktemp -d)"
+remote="cherry_pick_origin_for_${PR}"
+cleanup() {
+	rm -rf "${dir}"
+	git remote remove "${remote}"
+}
+
+trap cleanup EXIT
+
+git remote add "${remote}" "${GITHUB_REPO}.git"
+
+backport() {
+	(
+		branch="${1}"
+
+		new_branch="cherry_pick_pr${PR}_to_${branch}"
+		trap "cd -; git worktree remove -f ${dir}; git branch -D ${new_branch}" EXIT
+		git fetch "${remote}" $branch:$branch >/dev/null || return 2
+		git worktree add -b "${new_branch}" "${dir}" $branch >/dev/null || return 3
+
+		cd "${dir}"
+		for i in ${commits}; do
+			git cherry-pick -x -s $i >/dev/null || return 3
+		done
+
+		git push -f "${remote}" "${new_branch}" >/dev/null || return 4
+		gh pr create -R "${GITHUB_REPO}" --base "${branch}" --head "${new_branch}" \
+			--title "[${branch}] ${title}" \
+			--body "Backport PR #${PR} to $branch"
+	)
+}
+
+for label in $labels; do
+	branch="${label#process/cherry-pick/}"
+	out="$(backport "${branch}" 2>&1)"
+	if [ $? -eq 0 ]; then
+		gh pr comment -R "${GITHUB_REPO}" "${PR}" -b "Backport this change to $branch: ${out}"
+		continue
+	fi
+	log_error "Error cherry-picking PR #${PR} to $branch: ${out}"
+	gh pr comment -R "${GITHUB_REPO}" "${PR}" -b "Failed to backport this change to $branch: ${out}"
+	continue
+done