[__single_binary_service] Support customisation of systemd units

Requested by pedro

manifest

@@ -60,6 +60,8 @@ if [ -z "${SERVICE_DESCRIPTION}" ]; then
 	SERVICE_DESCRIPTION="cdist-managed '${SERVICE_NAME}' service"
 fi
 
+SERVICE_DEFINITION="$(cat "${__object}/parameter/service-definition" 2>/dev/null || true)"
+
 DOWNLOAD_URL="$(cat "${__object}/parameter/url")"
 CHECKSUM="$(cat "${__object}/parameter/checksum")"
 SHOULD_VERSION="$(cat "${__object}/parameter/version")"
@@ -102,10 +104,8 @@ INIT="$(cat "${__global}/explorer/init")"
 # TODO: Support non-systemd
 case "${INIT}" in
 	systemd)
-		__systemd_unit "${SERVICE_NAME}.service" \
-			--source "-" \
-			--state "${STATE}" \
-			--enablement-state "enabled" <<EOF
+		if [ -z "${SERVICE_DEFINITION}" ]; then
+			SERVICE_DEFINITION="$(cat <<EOF
 [Unit]
 Description=${SERVICE_DESCRIPTION}
 After=network.target
@@ -121,6 +121,12 @@ Restart=always
 [Install]
 WantedBy=multi-user.target
 EOF
+)"
+		fi
+		echo ${SERVICE_DEFINITION} | __systemd_unit "${SERVICE_NAME}.service" \
+			--source "-" \
+			--state "${STATE}" \
+			--enablement-state "enabled"
 	service_require="${service_require} __systemd_unit/${SERVICE_NAME}.service"
 
   ;;

parameter/optional

@@ -6,4 +6,5 @@ binary
 service-args
 service-exec
 service-description
+service-definition
 unpack-args