Add verifiable atomic server updates

scripts/rollback-native.sh

@@ -0,0 +1,70 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ $# -ne 0 ]]; then
+	echo "Usage: $0" >&2
+	exit 2
+fi
+if [[ ${EUID} -ne 0 ]]; then
+	echo "Run this rollback tool as root." >&2
+	exit 2
+fi
+
+install_root="${NETFISHING_INSTALL_ROOT:-/opt/netfishing-server}"
+service_name="${NETFISHING_SERVICE_NAME:-netfishing-server.service}"
+releases_dir="${install_root}/releases"
+
+if [[ "${install_root}" != /* || "${install_root}" == "/" ]]; then
+	echo "NETFISHING_INSTALL_ROOT must be a specific absolute directory." >&2
+	exit 2
+fi
+if [[ ! -L "${install_root}/current" || ! -L "${install_root}/previous" ]]; then
+	echo "Both current and previous release links are required." >&2
+	exit 1
+fi
+
+current_target="$(readlink -f -- "${install_root}/current")"
+previous_target="$(readlink -f -- "${install_root}/previous")"
+for target in "${current_target}" "${previous_target}"; do
+	if [[ ! -d "${target}" || "${target}" != "${releases_dir}/"* ]]; then
+		echo "Refusing to use a release outside ${releases_dir}." >&2
+		exit 1
+	fi
+	(cd -- "${target}" && sha256sum -c SHA256SUMS)
+done
+
+replace_symlink() {
+	local target="$1"
+	local link_path="$2"
+	local temporary_link="${install_root}/.$(basename -- "${link_path}").new.$$"
+	ln -s -- "${target}" "${temporary_link}"
+	mv -Tf -- "${temporary_link}" "${link_path}"
+}
+
+service_active=false
+if systemctl is-active --quiet "${service_name}"; then
+	service_active=true
+	systemctl stop "${service_name}"
+fi
+
+replace_symlink "${previous_target}" "${install_root}/current"
+replace_symlink "${current_target}" "${install_root}/previous"
+
+if ${service_active}; then
+	if ! systemctl start "${service_name}"; then
+		start_succeeded=false
+	else
+		sleep 1
+		start_succeeded=true
+		systemctl is-active --quiet "${service_name}" || start_succeeded=false
+	fi
+	if ! ${start_succeeded}; then
+		echo "Rollback target failed to start; restoring the original release." >&2
+		replace_symlink "${current_target}" "${install_root}/current"
+		replace_symlink "${previous_target}" "${install_root}/previous"
+		systemctl start "${service_name}" || true
+		exit 1
+	fi
+fi
+
+echo "Rolled back to $(basename -- "${previous_target}")."