Attach Dart finalizer to Store to close on Flutter hot restart.

Then ensure Store is kept alive until methods accessing its pointer have completed.

Author: greenrobot-team

objectbox/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## latest
 
+* Resolve "another store is still open" issue after Flutter hot restart (hot reload continues to work). #387
+* Add `Store.isClosed()`. #390
 * Add note to `objectbox.g.dart` on how to re-generate (update) it.
 
 ## 1.4.0 (2022-02-22)

objectbox/lib/src/native/bindings/bindings.dart

@@ -118,6 +118,8 @@ Object? _dartAPIInitException;
 /// Unfortunately, ffigen keeps those private.
 typedef _NativeClose = Int32 Function(Pointer<Void> ptr);
 
+late final native_store_close =
+    _lib!.lookup<NativeFunction<_NativeClose>>('obx_store_close');
 late final native_query_close =
     _lib!.lookup<NativeFunction<_NativeClose>>('obx_query_close');
 late final native_query_prop_close =

objectbox/lib/src/native/observable.dart

@@ -85,6 +85,7 @@ extension ObservableStore on Store {
       observer.cObserver =
           C.dartc_observe_single_type(_ptr, entityId, observer.nativePort);
     });
+    reachabilityFence(this);
 
     return observer.stream;
   }
@@ -125,6 +126,7 @@ extension ObservableStore on Store {
     observer.init(() {
       observer.cObserver = C.dartc_observe(_ptr, observer.nativePort);
     }, broadcast: broadcast);
+    reachabilityFence(this);
 
     if (broadcast) {
       _onClose[observer] = observer.close;

objectbox/lib/src/native/store.dart

@@ -35,11 +35,11 @@ class Store {
   /// This meant for tests only; do not enable for releases!
   static bool debugLogs = false;
 
-  late final Pointer<OBX_store> _cStore;
+  late Pointer<OBX_store> _cStore;
+  late final Pointer<OBX_dart_finalizer> _cFinalizer;
   HashMap<int, Type>? _entityTypeById;
   final _boxes = HashMap<Type, Box>();
   final ModelDefinition _defs;
-  bool _closed = false;
   Stream<List<Type>>? _entityChanges;
   final _reader = ReaderWithCBuffer();
   Transaction? _tx;
@@ -154,6 +154,8 @@ class Store {
       _reference.setUint64(1 * _int64Size, _ptr.address);
 
       _openStoreDirectories.add(_absoluteDirectoryPath);
+
+      _attachFinalizer();
     } catch (e) {
       _reader.clear();
       rethrow;
@@ -261,6 +263,8 @@ class Store {
 
       // Not setting _reference as this is a replacement for obtaining a store
       // via reference.
+
+      _attachFinalizer();
     } catch (e) {
       _reader.clear();
       rethrow;
@@ -296,6 +300,24 @@ class Store {
     }
   }
 
+  /// Attach a finalizer (using Dart C API) so when garbage collected, most
+  /// importantly on Flutter's hot restart (not hot reload), the native Store is
+  /// properly closed.
+  ///
+  /// During regular use it's still recommended to explicitly call
+  /// close() and not rely on garbage collection [to avoid out-of-memory
+  /// errors](https://github.com/dart-lang/language/issues/1847#issuecomment-1002751632).
+  void _attachFinalizer() {
+    initializeDartAPI();
+    // Keep the finalizer so it can be detached when close() is called.
+    _cFinalizer = C.dartc_attach_finalizer(
+        this, native_store_close, _cStore.cast(), 1024 * 1024);
+    if (_cFinalizer == nullptr) {
+      close();
+      throwLatestNativeError(context: 'attach store finalizer');
+    }
+  }
+
   /// Returns if an open store (i.e. opened before and not yet closed) was found
   /// for the given [directoryPath] (or if null the [defaultDirectoryPath]).
   static bool isOpen(String? directoryPath) {
@@ -312,12 +334,14 @@ class Store {
   /// a single underlying native store. See [Store.fromReference] for more details.
   ByteData get reference => _reference;
 
+  /// Returns if this store is already closed and can no longer be used.
+  bool isClosed() => _cStore.address == 0;
+
   /// Closes this store.
   ///
   /// Don't try to call any other ObjectBox methods after the store is closed.
   void close() {
-    if (_closed) return;
-    _closed = true;
+    if (isClosed()) return;
 
     _boxes.values.forEach(InternalBoxAccess.close);
     _boxes.clear();
@@ -331,8 +355,14 @@ class Store {
 
     if (!_weak) {
       _openStoreDirectories.remove(_absoluteDirectoryPath);
-      checkObx(C.store_close(_cStore));
+      final errors = List.filled(2, 0);
+      if (_cFinalizer != nullptr) {
+        errors[0] = C.dartc_detach_finalizer(_cFinalizer, this);
+      }
+      errors[1] = C.store_close(_cStore);
+      errors.forEach(checkObx);
     }
+    _cStore = nullptr;
   }
 
   /// Returns a cached Box instance.
@@ -459,7 +489,11 @@ class Store {
   /// not started; false if shutting down (or an internal error occurred).
   ///
   /// Use to wait until all puts by [Box.putQueued] have finished.
-  bool awaitAsyncCompletion() => C.store_await_async_submitted(_ptr);
+  bool awaitAsyncCompletion() {
+    final result = C.store_await_async_submitted(_ptr);
+    reachabilityFence(this);
+    return result;
+  }
 
   /// Await for previously submitted async operations to be completed
   /// (the async queue does not have to become idle).
@@ -468,14 +502,16 @@ class Store {
   /// not started; false if shutting down (or an internal error occurred).
   ///
   /// Use to wait until all puts by [Box.putQueued] have finished.
-  bool awaitAsyncSubmitted() => C.store_await_async_submitted(_ptr);
+  bool awaitAsyncSubmitted() {
+    final result = C.store_await_async_submitted(_ptr);
+    reachabilityFence(this);
+    return result;
+  }
 
   /// The low-level pointer to this store.
   @pragma('vm:prefer-inline')
-  Pointer<OBX_store> get _ptr {
-    if (_closed) throw StateError('Cannot access a closed store pointer');
-    return _cStore;
-  }
+  Pointer<OBX_store> get _ptr =>
+      isClosed() ? throw StateError('Store is closed') : _cStore;
 }
 
 /// Internal only.

objectbox/test/basics_test.dart

@@ -4,7 +4,6 @@ import 'dart:io';
 import 'dart:isolate';
 
 import 'package:async/async.dart';
-import 'package:meta/meta.dart';
 import 'package:objectbox/internal.dart';
 import 'package:objectbox/src/native/bindings/bindings.dart';
 import 'package:objectbox/src/native/bindings/helpers.dart';
@@ -114,8 +113,10 @@ void main() {
     expect(false, Store.isOpen(''));
     expect(false, Store.isOpen('testdata-basics'));
     final env = TestEnv('basics');
+    expect(false, env.store.isClosed());
     expect(true, Store.isOpen('testdata-basics'));
     env.closeAndDelete();
+    expect(true, env.store.isClosed());
     expect(false, Store.isOpen('testdata-basics'));
   });