Fix StateError during hot reload when no Dart isolates found (#130537)

Fixes flutter/flutter#116262

Author: christopherfujino

packages/flutter_tools/lib/src/run_hot.dart

@@ -91,7 +91,7 @@ class HotRunner extends ResidentRunner {
     this.multidexEnabled = false,
     super.devtoolsHandler,
     StopwatchFactory stopwatchFactory = const StopwatchFactory(),
-    ReloadSourcesHelper reloadSourcesHelper = _defaultReloadSourcesHelper,
+    ReloadSourcesHelper reloadSourcesHelper = defaultReloadSourcesHelper,
     ReassembleHelper reassembleHelper = _defaultReassembleHelper,
   }) : _stopwatchFactory = stopwatchFactory,
        _reloadSourcesHelper = reloadSourcesHelper,
@@ -1174,7 +1174,8 @@ typedef ReloadSourcesHelper = Future<OperationResult> Function(
   String? reason,
 );
 
-Future<OperationResult> _defaultReloadSourcesHelper(
+@visibleForTesting
+Future<OperationResult> defaultReloadSourcesHelper(
   HotRunner hotRunner,
   List<FlutterDevice?> flutterDevices,
   bool? pause,
@@ -1186,18 +1187,21 @@ Future<OperationResult> _defaultReloadSourcesHelper(
 ) async {
   final Stopwatch vmReloadTimer = Stopwatch()..start();
   const String entryPath = 'main.dart.incremental.dill';
-  final List<Future<DeviceReloadReport>> allReportsFutures = <Future<DeviceReloadReport>>[];
+  final List<Future<DeviceReloadReport?>> allReportsFutures = <Future<DeviceReloadReport?>>[];
 
   for (final FlutterDevice? device in flutterDevices) {
     final List<Future<vm_service.ReloadReport>> reportFutures = await _reloadDeviceSources(
       device!,
       entryPath,
       pause: pause,
     );
-    allReportsFutures.add(Future.wait(reportFutures).then(
+    allReportsFutures.add(Future.wait(reportFutures).then<DeviceReloadReport?>(
       (List<vm_service.ReloadReport> reports) async {
         // TODO(aam): Investigate why we are validating only first reload report,
         // which seems to be current behavior
+        if (reports.isEmpty) {
+          return null;
+        }
         final vm_service.ReloadReport firstReport = reports.first;
         // Don't print errors because they will be printed further down when
         // `validateReloadReport` is called again.
@@ -1208,9 +1212,9 @@ Future<OperationResult> _defaultReloadSourcesHelper(
       },
     ));
   }
-  final List<DeviceReloadReport> reports = await Future.wait(allReportsFutures);
-  final vm_service.ReloadReport reloadReport = reports.first.reports[0];
-  if (!HotRunner.validateReloadReport(reloadReport)) {
+  final Iterable<DeviceReloadReport> reports = (await Future.wait(allReportsFutures)).whereType<DeviceReloadReport>();
+  final vm_service.ReloadReport? reloadReport = reports.isEmpty ? null : reports.first.reports[0];
+  if (reloadReport == null || !HotRunner.validateReloadReport(reloadReport)) {
     // Reload failed.
     HotEvent('reload-reject',
       targetPlatform: targetPlatform!,
@@ -1223,6 +1227,9 @@ Future<OperationResult> _defaultReloadSourcesHelper(
     // Reset devFS lastCompileTime to ensure the file will still be marked
     // as dirty on subsequent reloads.
     _resetDevFSCompileTime(flutterDevices);
+    if (reloadReport == null) {
+      return OperationResult(1, 'No Dart isolates found');
+    }
     final ReloadReportContents contents = ReloadReportContents.fromReloadReport(reloadReport);
     return OperationResult(1, 'Reload rejected: ${contents.notices.join("\n")}');
   }

packages/flutter_tools/test/general.shard/run_hot_test.dart

@@ -0,0 +1,62 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/devfs.dart';
+import 'package:flutter_tools/src/resident_runner.dart';
+import 'package:flutter_tools/src/run_hot.dart';
+import 'package:flutter_tools/src/vmservice.dart';
+import 'package:test/fake.dart';
+import 'package:vm_service/vm_service.dart' as vm_service;
+
+import '../src/context.dart';
+
+void main() {
+  testUsingContext('defaultReloadSourcesHelper() handles empty DeviceReloadReports)', () {
+    defaultReloadSourcesHelper(
+      _FakeHotRunner(),
+      <FlutterDevice?>[_FakeFlutterDevice()],
+      false,
+      const <String, dynamic>{},
+      'android',
+      'flutter-sdk',
+      false,
+      'test-reason',
+    );
+  });
+}
+
+class _FakeHotRunner extends Fake implements HotRunner {}
+
+class _FakeDevFS extends Fake implements DevFS {
+  @override
+  final Uri? baseUri = Uri();
+
+  @override
+  void resetLastCompiled() {}
+}
+
+class _FakeFlutterDevice extends Fake implements FlutterDevice {
+  @override
+  final DevFS? devFS = _FakeDevFS();
+
+  @override
+  final FlutterVmService? vmService = _FakeFlutterVmService();
+}
+
+class _FakeFlutterVmService extends Fake implements FlutterVmService {
+  @override
+  final vm_service.VmService service = _FakeVmService();
+}
+
+class _FakeVmService extends Fake implements vm_service.VmService {
+  @override
+  Future<_FakeVm> getVM() async => _FakeVm();
+}
+
+class _FakeVm extends Fake implements vm_service.VM {
+  final List<vm_service.IsolateRef> _isolates = <vm_service.IsolateRef>[];
+
+  @override
+  List<vm_service.IsolateRef>? get isolates => _isolates;
+}