switch-branch: prevent downgrades

Downgrades between versions is dangerous and can break router. It is not supported and all fixes and migration scripts are just one way, there is no way to revert changes done by them on downgrade. Thus doing downgrade can cause pretty much any component to become invalidly configured and thus broken.

Fixes: #741 (closed)

updater/switch-branch/files/switch-branch

@@ -108,6 +108,22 @@ fetch_existing_versions() {
 	LATEST_VERSION="$(echo "$EXISTING_VERSIONS" | head -1)"
 }
 
+fetch_target_version() (
+	. /etc/os-release
+	case "$OPENWRT_DEVICE_PRODUCT" in
+		"Turris 1.x")
+			board="turris1x"
+			;;
+		"Turris Omnia")
+			board="omnia"
+			;;
+		"Turris Mox")
+			board="mox"
+			;;
+	esac
+	curl "https://repo.turris.cz/$1/$board/lists/turris-version"
+)
+
 # We allows some branch alternatives. This function replaces them with canonical variant
 branch_alts() {
 	case "$target" in
@@ -338,6 +354,22 @@ if [ "$mode" != "version" ]; then
 	esac
 fi
 
+# Verify that this is not a downgrade
+if [ "$force" != "y" ]; then
+	target_version="$target"
+	[ "$mode" = "branch" ] \
+		&& target_version="$(fetch_target_version "$target")"
+	if [ "$(printf "%s\n%s" "$target_version" "$CUR_VERSION" | sort -V | head -1)" != "$CUR_VERSION" ]; then
+		cat <<-EOF
+			$CUR_VERSION -> $target_version
+			You are attempting to do downgrade while only upgrades are supported.
+			Downgrade can cause unexpected issues because of migration steps performed on upgrade.
+			You can use schnapps to rollback to a previous version or perform a factory reset.
+		EOF
+		exit 1
+	fi
+fi
+
 ###################################
 
 set -x