[pigeon] Fixes potential race condition caused by a ProxyApi constructor message call being made in an async method (flutter#8656)

Host api calls now declare a `Future` variable from `channel.send` first. Then it `await`s the future immedieately or inside of a async method.

Part of fixing webview_flutter_wkwebview issue: flutter/flutter#162437

Author: bparrishMines

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 24.2.1
+
+* [dart] Fixes potential race condition caused by a ProxyApi constructor message call being made in
+  an async method.
+
 ## 24.2.0
 
 * Adjusts task queues to use a shared task queue for all methods in a single

packages/pigeon/example/app/lib/src/messages.g.dart

@@ -126,8 +126,9 @@ class ExampleHostApi {
       pigeonChannelCodec,
       binaryMessenger: pigeonVar_binaryMessenger,
     );
+    final Future<Object?> pigeonVar_sendFuture = pigeonVar_channel.send(null);
     final List<Object?>? pigeonVar_replyList =
-        await pigeonVar_channel.send(null) as List<Object?>?;
+        await pigeonVar_sendFuture as List<Object?>?;
     if (pigeonVar_replyList == null) {
       throw _createConnectionError(pigeonVar_channelName);
     } else if (pigeonVar_replyList.length > 1) {
@@ -155,8 +156,10 @@ class ExampleHostApi {
       pigeonChannelCodec,
       binaryMessenger: pigeonVar_binaryMessenger,
     );
+    final Future<Object?> pigeonVar_sendFuture =
+        pigeonVar_channel.send(<Object?>[a, b]);
     final List<Object?>? pigeonVar_replyList =
-        await pigeonVar_channel.send(<Object?>[a, b]) as List<Object?>?;
+        await pigeonVar_sendFuture as List<Object?>?;
     if (pigeonVar_replyList == null) {
       throw _createConnectionError(pigeonVar_channelName);
     } else if (pigeonVar_replyList.length > 1) {
@@ -184,8 +187,10 @@ class ExampleHostApi {
       pigeonChannelCodec,
       binaryMessenger: pigeonVar_binaryMessenger,
     );
+    final Future<Object?> pigeonVar_sendFuture =
+        pigeonVar_channel.send(<Object?>[message]);
     final List<Object?>? pigeonVar_replyList =
-        await pigeonVar_channel.send(<Object?>[message]) as List<Object?>?;
+        await pigeonVar_sendFuture as List<Object?>?;
     if (pigeonVar_replyList == null) {
       throw _createConnectionError(pigeonVar_channelName);
     } else if (pigeonVar_replyList.length > 1) {

packages/pigeon/lib/src/dart/dart_generator.dart

@@ -1115,6 +1115,7 @@ if (wrapped == null) {
     required Iterable<Parameter> parameters,
     required TypeDeclaration returnType,
     required bool addSuffixVariable,
+    bool insideAsyncMethod = true,
   }) {
     String sendArgument = 'null';
     if (parameters.isNotEmpty) {
@@ -1152,9 +1153,21 @@ if (wrapped == null) {
     }
     returnStatement = '$returnStatement;';
 
+    const String sendFutureVar = '${varNamePrefix}sendFuture';
+    indent.writeln(
+      'final Future<Object?> $sendFutureVar = ${varNamePrefix}channel.send($sendArgument);',
+    );
+
+    // If the message call is not made inside of an async method, this creates
+    // an anonymous function to handle the send future.
+    if (!insideAsyncMethod) {
+      indent.writeln('() async {');
+      indent.inc();
+    }
+
     indent.format('''
 final List<Object?>? ${varNamePrefix}replyList =
-\t\tawait ${varNamePrefix}channel.send($sendArgument) as List<Object?>?;
+\t\tawait $sendFutureVar as List<Object?>?;
 if (${varNamePrefix}replyList == null) {
 \tthrow _createConnectionError(${varNamePrefix}channelName);
 } else if (${varNamePrefix}replyList.length > 1) {
@@ -1178,6 +1191,11 @@ if (${varNamePrefix}replyList == null) {
 } else {
 \t$returnStatement
 }''');
+
+    if (!insideAsyncMethod) {
+      indent.dec();
+      indent.writeln('}();');
+    }
   }
 
   void _writeFlutterMethodMessageHandler(
@@ -1398,6 +1416,7 @@ if (${varNamePrefix}replyList == null) {
                   Indent(messageCallSink),
                   addSuffixVariable: false,
                   channelName: channelName,
+                  insideAsyncMethod: false,
                   parameters: <Parameter>[
                     Parameter(
                       name: '${varNamePrefix}instanceIdentifier',
@@ -1426,9 +1445,7 @@ if (${varNamePrefix}replyList == null) {
                   cb.Code(
                     'final BinaryMessenger? ${varNamePrefix}binaryMessenger = ${binaryMessengerParameter.name};',
                   ),
-                  const cb.Code('() async {'),
                   cb.Code(messageCallSink.toString()),
-                  const cb.Code('}();'),
                 ]);
               },
             );

packages/pigeon/lib/src/generator_tools.dart

@@ -14,7 +14,7 @@ import 'ast.dart';
 /// The current version of pigeon.
 ///
 /// This must match the version in pubspec.yaml.
-const String pigeonVersion = '24.2.0';
+const String pigeonVersion = '24.2.1';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {

packages/pigeon/platform_tests/shared_test_plugin_code/lib/src/generated/background_platform_channels.gen.dart

@@ -64,8 +64,12 @@ class BackgroundApi2Host {
       pigeonChannelCodec,
       binaryMessenger: pigeonVar_binaryMessenger,
     );
+    final Future<Object?> pigeonVar_sendFuture =
+        pigeonVar_channel.send(<Object?>[
+      <Object?>[x, y]
+    ]);
     final List<Object?>? pigeonVar_replyList =
-        await pigeonVar_channel.send(<Object?>[x, y]) as List<Object?>?;
+        await pigeonVar_sendFuture as List<Object?>?;
     if (pigeonVar_replyList == null) {
       throw _createConnectionError(pigeonVar_channelName);
     } else if (pigeonVar_replyList.length > 1) {