[ Widget Preview ] Fix crash when widget_preview_scaffold/.dart_tool doesn't exist (#178662)

If the `.dart_tool/widget_preview_scaffold/.dart_tool/` directory wasn't
created during the initial run of `flutter widget-preview start` due to
the command being interrupted or `pub` being disabled, `flutter
widget-preview start` would crash due to the `package_config.json` logic
walking up the directory structure looking for the nearest
`package_config.json`. This would point to the parent project's
`package_config.json`, which would not be compatible with the widget
preview scaffold project.

Fixes https://github.com/flutter/flutter/issues/178660 and
https://github.com/flutter/flutter/issues/177655

packages/flutter_tools/lib/src/commands/widget_preview.dart

@@ -318,8 +318,9 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
     // after we generate the scaffold project as invoking the getter triggers
     // lazy initialization of the preview scaffold's FlutterManifest before
     // the scaffold project's pubspec has been generated.
+    final FlutterProject widgetPreviewScaffoldProject = rootProject.widgetPreviewScaffoldProject;
     _previewCodeGenerator = PreviewCodeGenerator(
-      widgetPreviewScaffoldProject: rootProject.widgetPreviewScaffoldProject,
+      widgetPreviewScaffoldProject: widgetPreviewScaffoldProject,
       fs: fs,
     );
 
@@ -333,6 +334,12 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
       await _previewPubspecBuilder.populatePreviewPubspec(rootProject: rootProject);
     }
 
+    if (!widgetPreviewScaffoldProject.dartTool.existsSync()) {
+      await _previewPubspecBuilder.generatePackageConfig(
+        widgetPreviewScaffoldProject: widgetPreviewScaffoldProject,
+      );
+    }
+
     shutdownHooks.addShutdownHook(() async {
       await _widgetPreviewApp?.exitApp();
       await _previewDetector.dispose();
@@ -343,7 +350,7 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
 
     await configureDtd();
     final int result = await runPreviewEnvironment(
-      widgetPreviewScaffoldProject: rootProject.widgetPreviewScaffoldProject,
+      widgetPreviewScaffoldProject: widgetPreviewScaffoldProject,
     );
     if (result != 0) {
       throwToolExit('Failed to launch the widget previewer.', exitCode: result);

packages/flutter_tools/lib/src/widget_preview/preview_pubspec_builder.dart

@@ -91,6 +91,8 @@ class PreviewPubspecBuilder {
     );
   }
 
+  PubOutputMode get _outputMode => verbose ? PubOutputMode.all : PubOutputMode.failuresOnly;
+
   Future<void> populatePreviewPubspec({
     required FlutterProject rootProject,
     String? updatedPubspecPath,
@@ -126,7 +128,6 @@ class PreviewPubspecBuilder {
           }),
     };
 
-    final PubOutputMode outputMode = verbose ? PubOutputMode.all : PubOutputMode.failuresOnly;
     await pub.interactively(
       <String>[
         pubAdd,
@@ -146,7 +147,7 @@ class PreviewPubspecBuilder {
       context: PubContext.pubAdd,
       command: pubAdd,
       touchesPackageConfig: true,
-      outputMode: outputMode,
+      outputMode: _outputMode,
     );
 
     // Adds dependencies required by the widget preview scaffolding.
@@ -161,18 +162,22 @@ class PreviewPubspecBuilder {
       context: PubContext.pubAdd,
       command: pubAdd,
       touchesPackageConfig: true,
-      outputMode: outputMode,
+      outputMode: _outputMode,
     );
 
+    await generatePackageConfig(widgetPreviewScaffoldProject: widgetPreviewScaffoldProject);
+    previewManifest.updatePubspecHash(updatedPubspecPath: updatedPubspecPath);
+  }
+
+  /// Generates `widget_preview_scaffold/.dart_tool/package_config.json`.
+  Future<void> generatePackageConfig({required FlutterProject widgetPreviewScaffoldProject}) async {
     // Generate package_config.json.
     await pub.get(
       context: PubContext.create,
       project: widgetPreviewScaffoldProject,
       offline: offline,
-      outputMode: outputMode,
+      outputMode: _outputMode,
     );
-
-    previewManifest.updatePubspecHash(updatedPubspecPath: updatedPubspecPath);
   }
 
   void onPubspecChangeDetected(String path) {

packages/flutter_tools/test/integration.shard/widget_preview_test.dart

@@ -7,6 +7,7 @@ import 'dart:convert';
 
 import 'package:dtd/dtd.dart';
 import 'package:file/file.dart';
+import 'package:file_testing/file_testing.dart';
 import 'package:flutter_tools/src/base/io.dart';
 import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/commands/widget_preview.dart';
@@ -125,6 +126,36 @@ void main() {
       await runWidgetPreview(expectedMessages: firstLaunchMessagesWebServer, useWebServer: true);
     });
 
+    testWithoutContext('runs flutter pub get in widget_preview_scaffold if '
+        "widget_preview_scaffold/.dart_tool doesn't exist", () async {
+      // Regression test for https://github.com/flutter/flutter/issues/178660
+      // Generate the widget preview scaffold, but don't bother launching it.
+      processManager.runSync(<String>[
+        flutterBin,
+        'widget-preview',
+        'start',
+        '--no-${WidgetPreviewStartCommand.kLaunchPreviewer}',
+      ], workingDirectory: tempDir.path);
+
+      // Ensure widget_preview_scaffold/.dart_tool/package_config.json exists.
+      final Directory widgetPreviewScaffoldDartTool = tempDir
+          .childDirectory('.dart_tool')
+          .childDirectory('widget_preview_scaffold')
+          .childDirectory('.dart_tool');
+      expect(widgetPreviewScaffoldDartTool, exists);
+      expect(widgetPreviewScaffoldDartTool.childFile('package_config.json'), exists);
+
+      // Delete widget_preview_scaffold/.dart_tool/. This simulates an interrupted
+      // flutter widget-preview start where 'flutter pub get' wasn't run after
+      // the widget_preview_scaffold project was created.
+      widgetPreviewScaffoldDartTool.deleteSync(recursive: true);
+
+      // Ensure we don't crash due to the package_config.json lookup pointing to
+      // the parent project's package_config.json due to
+      // widget_preview_scaffold/.dart_tool/package_config.json not existing.
+      await runWidgetPreview(expectedMessages: subsequentLaunchMessagesWeb);
+    });
+
     testWithoutContext('does not recreate project on subsequent runs', () async {
       // The first run of 'flutter widget-preview start' should generate a new preview scaffold
       await runWidgetPreview(expectedMessages: firstLaunchMessagesWeb);