Radius server - fail gracefully if port in use

app/unit/RadiusUnit.js

@@ -1,6 +1,7 @@
 const { Unit } = require('libflitter')
 const fs = require('fs')
 const radius = require('radius')
+const net = require("net");
 const uuid = require('uuid').v4
 
 /**
@@ -12,6 +13,11 @@ class RadiusUnit extends Unit {
     }
 
     async go(app) {
+        if ( !(await this.port_free()) ) {
+            this.output.info('RADIUS server port is in use. Will not start!')
+            return
+        }
+
         const config = this.getConfig()
 
         // Overwrite radius-server's global config object with the user-provided values
@@ -122,7 +128,21 @@ class RadiusUnit extends Unit {
             // if message is accept or reject, we conside this as final message
             // this means we do not expect a reponse from the client again (acknowledgement for package)
             expectAcknowledgment: response.code === 'Access-Challenge',
-        };
+        }
+    }
+
+    async port_free() {
+        return new Promise((res, rej) => {
+            const server = net.createServer()
+            server.once('error', (e) => {
+                res(false)
+            })
+            server.once('listening', () => {
+                server.close()
+                res(true)
+            })
+            server.listen(this.getConfig().port)
+        })
     }
 
     getConfig() {