installer: add a naive systemd service state backup and restore

Add support for backing up and restoring explicitly disabled and enabled
services by checking their symlinks and restoring or removing them when
applying the backup.

It does naive enabled/disabled services heuristic:

- if the .preset does not say enabled, and there is a symlink in
  multi-user.target.wants, recreate the symink on restore
- if the .preset says enabled, and there was no symlink in
  multi-user.target.wants, remove the symlink on restore

Additionally, it only checks certain services:
- Skip any oneshot services (these are expected to be disabled with
  enabled preset after first boot)
- Skip any services not targeting multi-user

It does support parameterized services though, as supported by frr (via
frr@something).

This allows e.g. docker to continue to be enabled after an update to a
newer version. It should now also keep the choice of baseboxd vs ryu on
upgrade (untested though).

Signed-off-by: Jonas Gorski <[email protected]>

scripts/installer/lib/backup.sh

@@ -117,6 +117,69 @@ parse_file() {
 	done < $1
 }
 
+backup_systemd_state() {
+	local enabled_services disabled_services service_files service_links
+	local service enabled preset oneshot
+
+	if [ -d "$1/lib/systemd/system" ]; then
+		service_files=$(ls $1/lib/systemd/system)
+	fi
+	if [ -d "$1/etc/systemd/system/multi-user.target.wants" ]; then
+		service_links=$(ls $1/etc/systemd/system/multi-user.target.wants)
+	fi
+
+	# collect explicitly disabled services
+	for service in $service_files; do
+		case "$service" in
+			*@.service)
+				# always disabled by default
+				;;
+			*.service)
+				preset=$(grep "$service" $1/lib/systemd/system-preset/*.preset | cut -d ':' -f 2 | cut -d ' ' -f 1)
+				# check only services enabled by default
+				[ "$preset" = "enable" ] || continue
+
+				wantedby=$(grep "^WantedBy=" $1/lib/systemd/system/$service || true)
+
+				# skip oneshots, these got automatically disabled
+				grep -q "^Type=oneshot" $1/lib/systemd/system/$service && continue
+
+				# for now we only support multi-user services
+				[ "$wantedby" = "WantedBy=multi-user.target" ] || continue
+
+				# check for symlink in multi-user.target.wants
+				enabled=$(find $1/etc/systemd/system/multi-user.target.wants/ -name $service)
+
+				# nothing to do if still enabled
+				[ -z "$enabled" ] || continue
+
+				[ -n "$DEBUG" ] && echo "DEBUG: service disabled by user: $service"
+				disabled_services="$disabled_services ${service%.service}"
+				;;
+		esac
+	done
+
+	# collect explicitly enabled services
+	for service in $service_links; do
+		case "$service" in
+			*.service)
+				service_file=$(readlink $1/etc/systemd/system/multi-user.target.wants/$service)
+				base=$(basename $service_file)
+				preset=$(grep "$base" $1/lib/systemd/system-preset/*.preset | cut -d ':' -f 2 | cut -d ' ' -f 1)
+
+				# check only services disabled by default
+				[ "$preset" != "enable" ] || continue
+
+				[ -n "$DEBUG" ] && echo "DEBUG: service enabled by user: $service"
+				enabled_services="$enabled_services ${service%.service}"
+				;;
+		esac
+	done
+
+	echo "$disabled_services" > $2/.SERVICES_DISABLED
+	echo "$enabled_services" > $2/.SERVICES_ENABLED
+}
+
 # $1 src $2 dst
 apply_fixups() {
 	# releases pre 4.7.0 are missing gshadow in the backup list
@@ -150,20 +213,60 @@ create_backup()
 		parse_file "$1/$USER_BACKUP_FILE" "-" $1 $2
 	fi
 
+	# step 3 - backup changed systemd service states
+	backup_systemd_state $1 $2
+
 	apply_fixups $1 $2
 
-	# step 3 - check if anything is left
+	# step 4 - check if anything is left
 	[ -n "$(find $2 -type f)" ] || return 0
 
-	# step 4 - remove empty directories
+	# step 5 - remove empty directories
 	find $2 -depth -type d -exec rmdir -p --ignore-fail-on-non-empty {} \;
 
 	DO_RESTORE=true
 }
 
+restore_systemd_state()
+{
+	local enabled_services disabled_service service baseservice
+
+	if [ -f "$1/.SERVICES_ENABLED" ]; then
+		enabled_services=$(cat $1/.SERVICES_ENABLED)
+	fi
+	if [ -f "$1/.SERVICES_DISABLED" ]; then
+		disabled_services=$(cat $1/.SERVICES_DISABLED)
+	fi
+
+	# for any services to enable, create symlinks
+	for service in $enabled_services; do
+		# strip foo from service@foo
+		baseservice=${service/@*/@}
+		# check that the service exists in the target system
+		if [ ! -f $2/lib/systemd/system/$baseservice.service ]; then
+			echo "WARNING: cannot enable service $service.service: $baseservice.service not found" >&2
+			continue
+		fi
+		[ -n "$DEBUG" ] && echo "DEBUG: enabling service: $service.service"
+		ln -fs /lib/systemd/system/$baseservice.service \
+			$2/etc/systemd/system/multi-user.target.wants/$service.service
+	done
+
+	# for any services to disable, remove their symlinks
+	for service in $disabled_services; do
+		[ -n "$DEBUG" ] && echo "DEBUG: disabling service: $service.service"
+		rm -f $2/etc/systemd/system/multi-user.target.wants/$service.service
+	done
+
+	rm -f $1/.SERVICES_ENABLED $2/.SERVICES_DISABLED
+}
+
 # $1 backup storage dir $2 restore target
 restore_backup()
 {
+    # restore systemd services state
+    restore_systemd_state $1 $2
+
     # rename existing target files if different
     for file in $(find $1 -type f); do
         basefile="${file#${1}/}"