Eliminate circular dependency

backend/lib/ddns_resolver/ddns_resolver.js

@@ -0,0 +1,85 @@
+const error  = require('../error');
+const logger = require('../../logger').ddns;
+const utils  = require('../utils');
+
+const ddnsResolver = {
+	/**
+     * Checks whether the address requires resolution (i.e. starts with ddns:)
+     * @param {String} address 
+     * @returns {boolean}
+     */
+	requiresResolution: (address) => {
+		if (typeof address !== 'undefined' && address && address.toLowerCase().startsWith('ddns:')) {
+			return true;
+		}
+		return false;
+	},
+
+	/**
+     * Resolves the given address to its IP
+     * @param {String} address 
+     * @param {boolean} forceUpdate: whether to force resolution instead of using the cached value
+     */
+	resolveAddress: (address, forceUpdate=false) => {
+		if (!forceUpdate && ddnsResolver._cache.has(address)) {
+			// Check if it is still valid
+			const value       = ddnsResolver._cache.get(address);
+			const ip          = value[0];
+			const lastUpdated = value[1];
+			const nowSeconds  = Date.now();
+			const delta       = nowSeconds - lastUpdated;
+			if (delta < ddnsResolver._updateIntervalMs) {
+				return Promise.resolve(ip);
+			}
+		}
+		ddnsResolver._cache.delete(address);
+		// Reach here only if cache value doesn't exist or needs to be updated 
+		let host = address.toLowerCase();
+		if (host.startsWith('ddns:')) {
+			host = host.substring(5);
+		}
+		return ddnsResolver._queryHost(host)
+			.then((resolvedIP) => {
+				ddnsResolver._cache.set(address, [resolvedIP, Date.now()]);
+				return resolvedIP;
+			})
+			.catch((/*error*/) => {
+				// return input address in case of failure
+				return address;
+			});
+	},
+
+    
+	/** Private **/
+	// Properties
+	/**
+     * cache mapping host to (ip address, last updated time)
+     */
+	_cache: new Map(),
+
+	// Methods
+	/**
+     * 
+     * @param {String} host 
+     * @returns {Promise}
+     */
+	_queryHost: (host) => {
+		logger.info('Looking up IP for ', host);
+		return utils.execSafe('getent', ['hosts', host])
+			.then((result) => {
+				if (result.length < 8) {
+					logger.error('IP lookup returned invalid output: ', result);
+					throw error.ValidationError('Invalid output from getent hosts');
+				}
+				const out = result.split(/\s+/);
+				logger.info(`Resolved ${host} to ${out[0]}`);
+				return out[0];
+			},
+			(error) => {
+				logger.error('Error looking up IP for ' + host + ': ', error);
+				throw error; 
+			});
+	},
+};
+
+module.exports = ddnsResolver;