ensure _futurize does not leak uncaught errors into the zone (flutter/engine#32070)

engine/src/flutter/lib/ui/fixtures/ui_test.dart

@@ -747,9 +747,98 @@ void hooksTests() {
     expectEquals(frameNumber, 2);
   });
 
+  test('_futureize handles callbacker sync error', () async {
+    String? callbacker(void Function(Object? arg) cb) {
+      return 'failure';
+    }
+    Object? error;
+    try {
+      await _futurize(callbacker);
+    } catch (err) {
+      error = err;
+    }
+    expectNotEquals(error, null);
+  });
+
+  test('_futureize does not leak sync uncaught exceptions into the zone', () async {
+    String? callbacker(void Function(Object? arg) cb) {
+      cb(null); // indicates failure
+    }
+    Object? error;
+    try {
+      await _futurize(callbacker);
+    } catch (err) {
+      error = err;
+    }
+    expectNotEquals(error, null);
+  });
+
+  test('_futureize does not leak async uncaught exceptions into the zone', () async {
+    String? callbacker(void Function(Object? arg) cb) {
+      Timer.run(() {
+        cb(null); // indicates failure
+      });
+    }
+    Object? error;
+    try {
+      await _futurize(callbacker);
+    } catch (err) {
+      error = err;
+    }
+    expectNotEquals(error, null);
+  });
+
+  test('_futureize successfully returns a value sync', () async {
+    String? callbacker(void Function(Object? arg) cb) {
+      cb(true);
+    }
+    final Object? result = await _futurize(callbacker);
+
+    expectEquals(result, true);
+  });
+
+  test('_futureize successfully returns a value async', () async {
+    String? callbacker(void Function(Object? arg) cb) {
+      Timer.run(() {
+        cb(true);
+      });
+    }
+    final Object? result = await _futurize(callbacker);
+
+    expectEquals(result, true);
+  });
+
   _finish();
 }
 
+typedef _Callback<T> = void Function(T result);
+typedef _Callbacker<T> = String? Function(_Callback<T?> callback);
+
+// This is an exact copy of the function defined in painting.dart. If you change either
+// then you must change both.
+Future<T> _futurize<T>(_Callbacker<T> callbacker) {
+  final Completer<T> completer = Completer<T>.sync();
+  // If the callback synchronously throws an error, then synchronously
+  // rethrow that error instead of adding it to the completer. This
+  // prevents the Zone from receiving an uncaught exception.
+  bool sync = true;
+  final String? error = callbacker((T? t) {
+    if (t == null) {
+      if (sync) {
+        throw Exception('operation failed');
+      } else {
+        completer.completeError(Exception('operation failed'));
+      }
+    } else {
+      completer.complete(t);
+    }
+  });
+  sync = false;
+  if (error != null)
+    throw Exception(error);
+  return completer.future;
+}
+
 void _callHook(
   String name, [
   int argCount = 0,

engine/src/flutter/lib/ui/painting.dart

@@ -5599,15 +5599,27 @@ typedef _Callbacker<T> = String? Function(_Callback<T?> callback);
 ///   return _futurize(_doSomethingAndCallback);
 /// }
 /// ```
+// Note: this function is not directly tested so that it remains private, instead an exact
+// copy of it has been inlined into the test at lib/ui/fixtures/ui_test.dart. if you change
+// this function, then you  must update the test.
 Future<T> _futurize<T>(_Callbacker<T> callbacker) {
   final Completer<T> completer = Completer<T>.sync();
+  // If the callback synchronously throws an error, then synchronously
+  // rethrow that error instead of adding it to the completer. This
+  // prevents the Zone from receiving an uncaught exception.
+  bool sync = true;
   final String? error = callbacker((T? t) {
     if (t == null) {
-      completer.completeError(Exception('operation failed'));
+      if (sync) {
+        throw Exception('operation failed');
+      } else {
+        completer.completeError(Exception('operation failed'));
+      }
     } else {
       completer.complete(t);
     }
   });
+  sync = false;
   if (error != null)
     throw Exception(error);
   return completer.future;

engine/src/flutter/lib/web_ui/lib/initialization.dart

@@ -139,15 +139,28 @@ final PlatformViewRegistry platformViewRegistry = PlatformViewRegistry();
 //                NNBD migration.
 typedef _Callback<T> = void Function(T result);
 typedef _Callbacker<T> = String? Function(_Callback<T> callback);
+
+// Note: this function is not directly tested so that it remains private, instead an exact
+// copy of it has been inlined into the test at lib/ui/fixtures/ui_test.dart. if you change
+// this function, then you  must update the test.
 Future<T> _futurize<T>(_Callbacker<T> callbacker) {
   final Completer<T> completer = Completer<T>.sync();
-  final String? error = callbacker((T t) {
+  // If the callback synchronously throws an error, then synchronously
+  // rethrow that error instead of adding it to the completer. This
+  // prevents the Zone from receiving an uncaught exception.
+  bool sync = true;
+  final String? error = callbacker((T? t) {
     if (t == null) {
-      completer.completeError(Exception('operation failed'));
+      if (sync) {
+        throw Exception('operation failed');
+      } else {
+        completer.completeError(Exception('operation failed'));
+      }
     } else {
       completer.complete(t);
     }
   });
+  sync = false;
   if (error != null) {
     throw Exception(error);
   }