android: improved error handling for missing multicast permission

- startDiscovery() and stopDiscovery() will now throw proper NsdError if multicast locking is not available
- extended NsdError for security issues
- moved the permission to the main manifest in the example application
- now converting PlatformError back to NsdError on platform side

nsd/example/android/app/src/debug/AndroidManifest.xml

@@ -4,5 +4,4 @@
          to allow setting breakpoints, to provide hot reload, etc.
     -->
     <uses-permission android:name="android.permission.INTERNET"/>
-    <uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE"/>
 </manifest>

nsd/example/android/app/src/main/AndroidManifest.xml

@@ -1,6 +1,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.haberey.flutter.example">
 
+    <uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE"/>
     <application
         android:icon="@mipmap/ic_launcher"
         android:label="example">

nsd_android/android/src/main/kotlin/com/haberey/flutter/nsd_android/NsdAndroidPlugin.kt

@@ -1,18 +1,21 @@
 package com.haberey.flutter.nsd_android
 
+import android.Manifest
+import android.content.Context
+import android.content.pm.PackageManager
 import android.net.nsd.NsdManager
 import android.net.nsd.NsdServiceInfo
 import android.net.wifi.WifiManager
 import android.os.Handler
 import android.os.Looper
 import androidx.annotation.NonNull
+import androidx.core.content.ContextCompat
 import androidx.core.content.ContextCompat.getSystemService
 import io.flutter.embedding.engine.plugins.FlutterPlugin
 import io.flutter.plugin.common.MethodCall
 import io.flutter.plugin.common.MethodChannel
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler
 import io.flutter.plugin.common.MethodChannel.Result
-import java.lang.Error
 import java.util.concurrent.Semaphore
 import kotlin.collections.HashMap
 import kotlin.concurrent.thread
@@ -23,23 +26,26 @@ class NsdAndroidPlugin : FlutterPlugin, MethodCallHandler {
 
     private lateinit var nsdManager: NsdManager
     private lateinit var wifiManager: WifiManager
-    private lateinit var multicastLock: WifiManager.MulticastLock
     private lateinit var methodChannel: MethodChannel
 
+    private var multicastLock: WifiManager.MulticastLock? = null
+
     private val discoveryListeners = HashMap<String, NsdManager.DiscoveryListener>()
     private val resolveListeners = HashMap<String, NsdManager.ResolveListener>()
     private val registrationListeners = HashMap<String, NsdManager.RegistrationListener>()
 
     private val resolveSemaphore = Semaphore(1)
 
     override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
-        nsdManager =
-            getSystemService(flutterPluginBinding.applicationContext, NsdManager::class.java)!!
-        wifiManager =
-            getSystemService(flutterPluginBinding.applicationContext, WifiManager::class.java)!!
+        val context = flutterPluginBinding.applicationContext
+
+        nsdManager = getSystemService(context, NsdManager::class.java)!!
+        wifiManager = getSystemService(context, WifiManager::class.java)!!
 
-        multicastLock = wifiManager.createMulticastLock("nsdMulticastLock")
-        multicastLock.setReferenceCounted(true)
+        if (multicastPermissionGranted(context)) {
+            multicastLock = wifiManager.createMulticastLock("nsdMulticastLock")
+            multicastLock?.setReferenceCounted(true)
+        }
 
         methodChannel = MethodChannel(flutterPluginBinding.binaryMessenger, CHANNEL_NAME)
         methodChannel.setMethodCallHandler(this)
@@ -79,7 +85,14 @@ class NsdAndroidPlugin : FlutterPlugin, MethodCallHandler {
             "Cannot start discovery: expected handle"
         )
 
-        multicastLock.acquire()
+        if (multicastLock == null) {
+            throw NsdError(
+                ErrorCause.SECURITY_ISSUE,
+                "Missing required permission CHANGE_WIFI_MULTICAST_STATE"
+            );
+        }
+
+        multicastLock?.acquire()
 
         try {
 
@@ -95,7 +108,7 @@ class NsdAndroidPlugin : FlutterPlugin, MethodCallHandler {
             result.success(null)
 
         } catch (e: Throwable) {
-            multicastLock.release()
+            multicastLock?.release()
             throw e
         }
 
@@ -107,7 +120,14 @@ class NsdAndroidPlugin : FlutterPlugin, MethodCallHandler {
             "Cannot stop discovery: expected handle"
         )
 
-        multicastLock.release()
+        if (multicastLock == null) {
+            throw NsdError(
+                ErrorCause.SECURITY_ISSUE,
+                "Missing required permission CHANGE_WIFI_MULTICAST_STATE"
+            );
+        }
+
+        multicastLock?.release()
 
         nsdManager.stopServiceDiscovery(discoveryListeners[handle])
         result.success(null)
@@ -290,4 +310,10 @@ class NsdAndroidPlugin : FlutterPlugin, MethodCallHandler {
     override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
         methodChannel.setMethodCallHandler(null)
     }
+
+    private fun multicastPermissionGranted(context: Context) =
+        ContextCompat.checkSelfPermission(
+            context,
+            Manifest.permission.CHANGE_WIFI_MULTICAST_STATE
+        ) == PackageManager.PERMISSION_GRANTED
 }

nsd_android/android/src/main/kotlin/com/haberey/flutter/nsd_android/NsdError.kt

@@ -27,6 +27,7 @@ internal enum class ErrorCause(val code: String) {
     ALREADY_ACTIVE("alreadyActive"),
     MAX_LIMIT("maxLimit"),
     INTERNAL_ERROR("internalError"),
+    SECURITY_ISSUE("securityIssue"),
 }
 
 internal class NsdError(val errorCause: ErrorCause, val errorMessage: String) :

nsd_platform_interface/lib/src/method_channel_nsd_platform.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'dart:io';
 
 import 'package:flutter/services.dart';
+import 'package:nsd_platform_interface/src/utilities.dart';
 import 'package:uuid/uuid.dart';
 
 import 'logging.dart';
@@ -195,7 +196,9 @@ class MethodChannelNsdPlatform extends NsdPlatformInterface {
 
   Future<void> invoke(String method, [dynamic arguments]) {
     log(this, LogTopic.calls, () => 'Call: $method $arguments');
-    return _methodChannel.invokeMethod(method, arguments);
+    return _methodChannel
+        .invokeMethod(method, arguments)
+        .catchError((e) => throw toNsdError(e));
   }
 
   void setHandler(String handle, String method, _Handler handler) {
@@ -268,3 +271,18 @@ InternetAddressType? getInternetAddressType(IpLookupType ipLookupType) {
 bool isIpLookupEnabled(IpLookupType ipLookupType) {
   return ipLookupType != IpLookupType.none;
 }
+
+NsdError toNsdError(Exception e) {
+  if (e is! PlatformException) {
+    return NsdError(ErrorCause.internalError, e.toString());
+  }
+
+  final message = e.message ?? '';
+  final errorCode = enumValueFromString(ErrorCause.values, e.code);
+
+  if (errorCode == null) {
+    return NsdError(ErrorCause.internalError, message);
+  }
+
+  return NsdError(errorCode, message);
+}

nsd_platform_interface/lib/src/nsd_platform_interface.dart

@@ -88,17 +88,20 @@ enum ErrorCause {
   maxLimit,
 
   /// This error occurs on Android (seen on API 30) if too many resolve
-  /// operations are requested simultanously. It should be prevented by the
+  /// operations are requested simultaneously. It should be prevented by the
   /// semaphore on the native side.
   alreadyActive,
 
-  /// An error in platform or native code that cannot be adressed by the client.
+  /// An error in platform or native code that cannot be addressed by the client.
   internalError,
+
+  /// A security issue, for example a missing permission.
+  securityIssue,
 }
 
 /// Represents an error that occurred during an NSD operation.
 ///
-/// Examine the [ErrorCause] to see wether or not the error can be adressed by the client.
+/// Examine the [ErrorCause] to see whether or not the error can be addressed by the client.
 class NsdError extends Error {
   final ErrorCause cause;
   final String message;