Fix Java and Kotlin

packages/pigeon/lib/src/java/java_generator.dart

@@ -835,15 +835,21 @@ if (wrapped == null) {
       indent.addScoped('{', '}', () {
         indent.writeln(
             'messageChannelSuffix = messageChannelSuffix.isEmpty() ? "" : "." + messageChannelSuffix;');
+        String? serialBackgroundQueue;
+        if (api.methods.any((Method m) =>
+            m.taskQueueType == TaskQueueType.serialBackgroundThread)) {
+          serialBackgroundQueue = 'taskQueue';
+          indent.writeln(
+              'BinaryMessenger.TaskQueue $serialBackgroundQueue = binaryMessenger.makeBackgroundTaskQueue();');
+        }
         for (final Method method in api.methods) {
-          _writeMethodSetUp(
-            generatorOptions,
-            root,
-            indent,
-            api,
-            method,
-            dartPackageName: dartPackageName,
-          );
+          _writeHostMethodMessageHandler(
+              generatorOptions, root, indent, api, method,
+              dartPackageName: dartPackageName,
+              serialBackgroundQueue:
+                  method.taskQueueType == TaskQueueType.serialBackgroundThread
+                      ? serialBackgroundQueue
+                      : null);
         }
       });
     });
@@ -887,34 +893,27 @@ if (wrapped == null) {
     indent.writeln('$returnType ${method.name}(${argSignature.join(', ')});');
   }
 
-  /// Write a static setUp function in the interface.
-  /// Example:
-  ///   static void setUp(BinaryMessenger binaryMessenger, Foo api) {...}
-  void _writeMethodSetUp(
+  /// Write a single method's handler for the setUp function.
+  void _writeHostMethodMessageHandler(
     JavaOptions generatorOptions,
     Root root,
     Indent indent,
     Api api,
     final Method method, {
     required String dartPackageName,
+    String? serialBackgroundQueue,
   }) {
     final String channelName = makeChannelName(api, method, dartPackageName);
     indent.write('');
     indent.addScoped('{', '}', () {
-      String? taskQueue;
-      if (method.taskQueueType != TaskQueueType.serial) {
-        taskQueue = 'taskQueue';
-        indent.writeln(
-            'BinaryMessenger.TaskQueue taskQueue = binaryMessenger.makeBackgroundTaskQueue();');
-      }
       indent.writeln('BasicMessageChannel<Object> channel =');
       indent.nest(2, () {
         indent.writeln('new BasicMessageChannel<>(');
         indent.nest(2, () {
           indent.write(
               'binaryMessenger, "$channelName" + messageChannelSuffix, getCodec()');
-          if (taskQueue != null) {
-            indent.addln(', $taskQueue);');
+          if (serialBackgroundQueue != null) {
+            indent.addln(', $serialBackgroundQueue);');
           } else {
             indent.addln(');');
           }

packages/pigeon/lib/src/kotlin/kotlin_generator.dart

@@ -654,6 +654,14 @@ if (wrapped == null) {
         indent.addScoped('{', '}', () {
           indent.writeln(
               r'val separatedMessageChannelSuffix = if (messageChannelSuffix.isNotEmpty()) ".$messageChannelSuffix" else ""');
+          String? serialBackgroundQueue;
+          if (api.methods.any((Method m) =>
+              m.taskQueueType == TaskQueueType.serialBackgroundThread)) {
+            serialBackgroundQueue = 'taskQueue';
+            serialBackgroundQueue = 'taskQueue';
+            indent.writeln(
+                'val $serialBackgroundQueue = binaryMessenger.makeBackgroundTaskQueue()');
+          }
           for (final Method method in api.methods) {
             _writeHostMethodMessageHandler(
               indent,
@@ -664,6 +672,10 @@ if (wrapped == null) {
               parameters: method.parameters,
               returnType: method.returnType,
               isAsynchronous: method.isAsynchronous,
+              serialBackgroundQueue:
+                  method.taskQueueType == TaskQueueType.serialBackgroundThread
+                      ? serialBackgroundQueue
+                      : null,
             );
           }
         });
@@ -1071,8 +1083,8 @@ if (wrapped == null) {
           fun error(errorCode: String, errorMessage: String?, errorDetails: Any?) {
             sink.error(errorCode, errorMessage, errorDetails)
           }
-  
-          fun endOfStream() { 
+
+          fun endOfStream() {
             sink.endOfStream()
           }
         }
@@ -1249,24 +1261,18 @@ if (wrapped == null) {
     required TypeDeclaration returnType,
     String setHandlerCondition = 'api != null',
     bool isAsynchronous = false,
+    String? serialBackgroundQueue,
     String Function(List<String> safeArgNames, {required String apiVarName})?
         onCreateCall,
   }) {
     indent.write('run ');
     indent.addScoped('{', '}', () {
-      String? taskQueue;
-      if (taskQueueType != TaskQueueType.serial) {
-        taskQueue = 'taskQueue';
-        indent.writeln(
-            'val $taskQueue = binaryMessenger.makeBackgroundTaskQueue()');
-      }
-
       indent.write(
         'val channel = BasicMessageChannel<Any?>(binaryMessenger, "$channelName", codec',
       );
 
-      if (taskQueue != null) {
-        indent.addln(', $taskQueue)');
+      if (serialBackgroundQueue != null) {
+        indent.addln(', $serialBackgroundQueue)');
       } else {
         indent.addln(')');
       }

packages/pigeon/platform_tests/alternate_language_test_plugin/android/src/main/java/com/example/alternate_language_test_plugin/CoreTests.java

@@ -3352,6 +3352,7 @@ static void setUp(
         @NonNull String messageChannelSuffix,
         @Nullable HostIntegrationCoreApi api) {
       messageChannelSuffix = messageChannelSuffix.isEmpty() ? "" : "." + messageChannelSuffix;
+      BinaryMessenger.TaskQueue taskQueue = binaryMessenger.makeBackgroundTaskQueue();
       {
         BasicMessageChannel<Object> channel =
             new BasicMessageChannel<>(
@@ -6145,7 +6146,6 @@ public void error(Throwable error) {
         }
       }
       {
-        BinaryMessenger.TaskQueue taskQueue = binaryMessenger.makeBackgroundTaskQueue();
         BasicMessageChannel<Object> channel =
             new BasicMessageChannel<>(
                 binaryMessenger,

packages/pigeon/platform_tests/test_plugin/android/src/main/kotlin/com/example/test_plugin/CoreTests.gen.kt

@@ -1109,6 +1109,7 @@ interface HostIntegrationCoreApi {
     ) {
       val separatedMessageChannelSuffix =
           if (messageChannelSuffix.isNotEmpty()) ".$messageChannelSuffix" else ""
+      val taskQueue = binaryMessenger.makeBackgroundTaskQueue()
       run {
         val channel =
             BasicMessageChannel<Any?>(
@@ -3404,7 +3405,6 @@ interface HostIntegrationCoreApi {
         }
       }
       run {
-        val taskQueue = binaryMessenger.makeBackgroundTaskQueue()
         val channel =
             BasicMessageChannel<Any?>(
                 binaryMessenger,