avahi: fix device list not shown when avahi is not running

This adds a D-Bus call in the avahi backend getDaemon() function. This will cause an exception to be thrown if avahi is not installed or running, in which case the extension will fall back to the native js implementation of bonjour.

Issue #83

Author: dlech

src/dnssd/avahi.ts

@@ -8,19 +8,25 @@ import * as events from 'events';
 import * as dnssd from '../dnssd';
 
 
-let _daemon: avahi.Daemon;
+let cachedDaemon: avahi.Daemon;
 
 async function getDaemon(): Promise<avahi.Daemon> {
-    if (_daemon) {
-        return Promise.resolve(_daemon);
+    if (cachedDaemon) {
+        return Promise.resolve(cachedDaemon);
     }
 
     return new Promise((resolve, reject) => {
         const bus = dbus.systemBus();
         bus.connection.on('connect', () => {
-            _daemon = new avahi.Daemon(bus);
-            resolve(_daemon);
-
+            const daemon = new avahi.Daemon(bus);
+            daemon.GetAPIVersion((err, _version) => {
+                if (err) {
+                    reject(err);
+                }
+                else {
+                    resolve(cachedDaemon = daemon);
+                }
+            });
         });
         bus.connection.on('error', err => {
             reject(err);