Fix local gold output for flutter/flutter (#66139)

packages/flutter_goldens/lib/flutter_goldens.dart

@@ -9,6 +9,7 @@ import 'dart:typed_data' show Uint8List;
 
 import 'package:file/file.dart';
 import 'package:file/local.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:meta/meta.dart';
 import 'package:platform/platform.dart';
@@ -670,7 +671,7 @@ class FlutterLocalFileComparator extends FlutterGoldenFileComparator with LocalC
     if (result.passed)
       return true;
 
-    generateFailureOutput(result, golden, basedir);
-    return false;
+    final String error = await generateFailureOutput(result, golden, basedir);
+    throw FlutterError(error);
   }
 }

packages/flutter_test/lib/src/_goldens_io.dart

@@ -8,11 +8,13 @@ import 'dart:math' as math;
 import 'dart:typed_data';
 import 'dart:ui';
 
+import 'package:flutter/foundation.dart';
 import 'package:path/path.dart' as path;
 // ignore: deprecated_member_use
 import 'package:test_api/test_api.dart' as test_package show TestFailure;
 
 import 'goldens.dart';
+import 'test_async_utils.dart';
 
 /// The default [GoldenFileComparator] implementation for `flutter test`.
 ///
@@ -102,7 +104,8 @@ class LocalFileComparator extends GoldenFileComparator with LocalComparisonOutpu
     );
 
     if (!result.passed) {
-      await generateFailureOutput(result, golden, basedir);
+      final String error = await generateFailureOutput(result, golden, basedir);
+      throw FlutterError(error);
     }
     return result.passed;
   }
@@ -125,16 +128,15 @@ class LocalComparisonOutput {
   /// Writes out diffs from the [ComparisonResult] of a golden file test.
   ///
   /// Will throw an error if a null result is provided.
-  Future<void> generateFailureOutput(
+  Future<String> generateFailureOutput(
     ComparisonResult result,
     Uri golden,
     Uri basedir, {
     String key = '',
-  }) async {
+  }) async => TestAsyncUtils.guard<String>(() async {
     String additionalFeedback = '';
     if (result.diffs != null) {
-      additionalFeedback = '\nFailure feedback can be found at '
-        '${path.join(basedir.path, 'failures')}';
+      additionalFeedback = '\nFailure feedback can be found at ${path.join(basedir.path, 'failures')}';
       final Map<String, Image> diffs = result.diffs!.cast<String, Image>();
       for (final MapEntry<String, Image> entry in diffs.entries) {
         final File output = getFailureFile(
@@ -143,15 +145,12 @@ class LocalComparisonOutput {
           basedir,
         );
         output.parent.createSync(recursive: true);
-        final ByteData? pngBytes =
-            await entry.value.toByteData(format: ImageByteFormat.png);
+        final ByteData? pngBytes = await entry.value.toByteData(format: ImageByteFormat.png);
         output.writeAsBytesSync(pngBytes!.buffer.asUint8List());
       }
     }
-    throw test_package.TestFailure(
-      'Golden "$golden": ${result.error}$additionalFeedback'
-    );
-  }
+    return 'Golden "$golden": ${result.error}$additionalFeedback';
+  });
 
   /// Returns the appropriate file for a given diff from a [ComparisonResult].
   File getFailureFile(String failure, Uri golden, Uri basedir) {

packages/flutter_test/test/goldens_test.dart

@@ -9,6 +9,7 @@ import 'dart:io' as io;
 import 'dart:typed_data';
 
 import 'package:file/memory.dart';
+import 'package:flutter/foundation.dart' show DiagnosticLevel, DiagnosticsNode, DiagnosticPropertiesBuilder, FlutterError;
 import 'package:flutter_test/flutter_test.dart' hide test;
 import 'package:flutter_test/flutter_test.dart' as test_package;
 
@@ -102,6 +103,36 @@ void main() {
       expect(comparator.basedir, Uri.parse('./'));
     });
 
+    test('throws if local output is not awaited', () {
+      try {
+        comparator.generateFailureOutput(
+          ComparisonResult(passed: false),
+          Uri.parse('foo_test.dart'),
+          Uri.parse('/foo/bar/'),
+        );
+        TestAsyncUtils.verifyAllScopesClosed();
+        throw 'unexpectedly did not throw';
+      } on FlutterError catch (e) {
+        final List<String> lines = e.message.split('\n');
+        expectSync(lines[0], 'Asynchronous call to guarded function leaked.');
+        expectSync(lines[1], 'You must use "await" with all Future-returning test APIs.');
+        expectSync(
+          lines[2],
+          matches(r'^The guarded method "generateFailureOutput" from class '
+            r'LocalComparisonOutput was called from .*goldens_test.dart on line '
+            r'[0-9]+, but never completed before its parent scope closed\.$'),
+        );
+        expectSync(lines.length, 3);
+        final DiagnosticPropertiesBuilder propertiesBuilder = DiagnosticPropertiesBuilder();
+        e.debugFillProperties(propertiesBuilder);
+        final List<DiagnosticsNode> information = propertiesBuilder.properties;
+        expectSync(information.length, 3);
+        expectSync(information[0].level, DiagnosticLevel.summary);
+        expectSync(information[1].level, DiagnosticLevel.hint);
+        expectSync(information[2].level, DiagnosticLevel.info);
+      }
+    });
+
     group('compare', () {
       Future<bool> doComparison([ String golden = 'golden.png' ]) {
         final Uri uri = fs.file(fix(golden)).uri;
@@ -157,7 +188,7 @@ void main() {
           try {
             await doComparison();
             fail('TestFailure expected but not thrown.');
-          } on TestFailure catch (error) {
+          } on FlutterError catch (error) {
             expect(error.message, contains('% diff detected'));
             final io.File master = fs.file(
               fix('/failures/golden_masterImage.png')
@@ -186,7 +217,7 @@ void main() {
           try {
             await doComparison('subdir/golden.png');
             fail('TestFailure expected but not thrown.');
-          } on TestFailure catch (error) {
+          } on FlutterError catch (error) {
             expect(error.message, contains('% diff detected'));
             final io.File master = fs.file(
               fix('/failures/golden_masterImage.png')
@@ -221,7 +252,7 @@ void main() {
           try {
             await doComparison();
             fail('TestFailure expected but not thrown.');
-          } on TestFailure catch (error) {
+          } on FlutterError catch (error) {
             expect(error.message, contains('image sizes do not match'));
           }
         });
@@ -231,7 +262,7 @@ void main() {
           try {
             await doComparison();
             fail('TestFailure expected but not thrown.');
-          } on TestFailure catch (error) {
+          } on FlutterError catch (error) {
             expect(error.message, contains('% diff detected'));
           }
         });
@@ -241,7 +272,7 @@ void main() {
           try {
             await doComparison();
             fail('TestFailure expected but not thrown.');
-          } on TestFailure catch (error) {
+          } on FlutterError catch (error) {
             expect(error.message, contains('null image provided'));
           }
         });