RDART-1020: Fix writeAsync behaviour (#1666)

* Add test that illustrates problem with async callback to writeAsync

* Fix async callback issue

* Update CHANGELOG

* Add assert that callback to write is not async

* Handle Never, which is a subtype of any type, but not a future

* Disallow async callbacks even on writeAsync

* Drop new async callback tests again

CHANGELOG.md

@@ -4,6 +4,7 @@
 * None
 
 ### Fixed
+* `Realm.writeAsync` did not handle async callbacks (`Future<T> Function()`) correctly. (Issue [#1667](https://github.com/realm/realm-dart/issues/1667)) 
 * Fixed an issue that would cause macOS apps to be rejected with `Invalid Code Signing Entitlements` error. (Issue [#1679](https://github.com/realm/realm-dart/issues/1679))
 * Fixed a regression that makes it inconvenient to run unit tests using realm. (Issue [#1619](https://github.com/realm/realm-dart/issues/1619))
 

packages/realm_dart/lib/src/realm_class.dart

@@ -355,11 +355,15 @@ class Realm {
   /// Checks whether the `Realm` is in write transaction.
   bool get isInTransaction => handle.isWritable;
 
+  bool _isSubtype<S, T>() => <S>[] is List<T>;
+  bool _isFuture<T>() => T != Never && _isSubtype<T, Future>();
+
   /// Synchronously calls the provided callback inside a write transaction.
   ///
   /// If no exception is thrown from within the callback, the transaction will be committed.
   /// It is more efficient to update several properties or even create multiple objects in a single write transaction.
   T write<T>(T Function() writeCallback) {
+    assert(!_isFuture<T>(), 'writeCallback must be synchronous');
     final transaction = beginWrite();
 
     try {
@@ -388,8 +392,8 @@ class Realm {
   /// Executes the provided [writeCallback] in a temporary write transaction. Both acquiring the write
   /// lock and committing the transaction will be done asynchronously.
   Future<T> writeAsync<T>(T Function() writeCallback, [CancellationToken? cancellationToken]) async {
+    assert(!_isFuture<T>(), 'writeCallback must be synchronous');
     final transaction = await beginWriteAsync(cancellationToken);
-
     try {
       T result = writeCallback();
       await transaction.commitAsync(cancellationToken);

packages/realm_dart/test/realm_test.dart

@@ -1071,7 +1071,7 @@ void main() {
     expect(realm2.all<Person>().length, 0);
   });
 
-  test("Realm.writeAsync with multiple transactions doesnt't deadlock", () async {
+  test("Realm.writeAsync with multiple transactions doesn't deadlock", () async {
     final realm = getRealm(Configuration.local([Person.schema]));
     final t1 = await realm.beginWriteAsync();
     realm.add(Person('Marco'));
@@ -1200,6 +1200,16 @@ void main() {
     expect(realm.isInTransaction, false);
   });
 
+  test('Realm.writeAsync with async callback fails with assert', () async {
+    final realm = getRealm(Configuration.local([Person.schema]));
+    await expectLater(realm.writeAsync(() async {}), throwsA(isA<AssertionError>()));
+  });
+
+  test('Realm.write with async callback', () {
+    final realm = getRealm(Configuration.local([Person.schema]));
+    expect(() => realm.write(() async {}), throwsA(isA<AssertionError>()));
+  });
+
   test('Transaction.commitAsync with a canceled token throws', () async {
     final realm = getRealm(Configuration.local([Person.schema]));