implement native finalizers and use them to auto-close query and property-query (#240)

Author: vaind

objectbox/CHANGELOG.md

@@ -3,6 +3,8 @@
 This is a 1.0 release candidate - we encourage everyone to try it out and provide any last-minute feedback,
 especially to new/changed APIs.
 
+* Query now supports auto-closing. You can still call `close()` manually if you want to free native resources sooner  
+  than they would be by Dart's garbage collector, but it's not mandatory anymore.
 * Change the "meta-model" fields to provide completely type-safe query building.
   Conditions you specify are now checked at compile time to match the queried entity.
 * Make property queries fully typed, `PropertyQuery.find()` now returns the appropriate `List<...>` type without casts.

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

@@ -8,6 +8,7 @@ import 'objectbox-c.dart';
 export 'objectbox-c.dart';
 
 // ignore_for_file: public_member_api_docs
+// ignore_for_file: non_constant_identifier_names
 
 /// Tries to use an already loaded objectbox dynamic library. This is the only
 /// option for macOS and iOS and is ~5 times faster than loading from file so
@@ -18,7 +19,8 @@ ObjectBoxC? _tryObjectBoxLibProcess() {
 
   ObjectBoxC? obxc;
   try {
-    obxc = ObjectBoxC(DynamicLibrary.process());
+    _lib = DynamicLibrary.process();
+    obxc = ObjectBoxC(_lib!);
     _isSupportedVersion(obxc); // may throw in case symbols are not found
   } catch (_) {
     // ignore errors (i.e. symbol not found)
@@ -27,19 +29,19 @@ ObjectBoxC? _tryObjectBoxLibProcess() {
 }
 
 ObjectBoxC? _tryObjectBoxLibFile() {
-  DynamicLibrary? lib;
+  _lib = null;
   var libName = 'objectbox';
   if (Platform.isWindows) {
     libName += '.dll';
     try {
-      lib = DynamicLibrary.open(libName);
+      _lib = DynamicLibrary.open(libName);
     } on ArgumentError {
       libName = 'lib/' + libName;
     }
   } else if (Platform.isMacOS) {
     libName = 'lib' + libName + '.dylib';
     try {
-      lib = DynamicLibrary.open(libName);
+      _lib = DynamicLibrary.open(libName);
     } on ArgumentError {
       libName = '/usr/local/lib/' + libName;
     }
@@ -50,8 +52,8 @@ ObjectBoxC? _tryObjectBoxLibFile() {
   } else {
     return null;
   }
-  lib ??= DynamicLibrary.open(libName);
-  return ObjectBoxC(lib);
+  _lib ??= DynamicLibrary.open(libName);
+  return ObjectBoxC(_lib!);
 }
 
 bool _isSupportedVersion(ObjectBoxC obxc) => obxc.version_is_at_least(
@@ -77,9 +79,8 @@ ObjectBoxC loadObjectBoxLib() {
   return obxc;
 }
 
-ObjectBoxC? _cachedBindings;
-
-ObjectBoxC get C => _cachedBindings ??= loadObjectBoxLib();
+DynamicLibrary? _lib;
+late final ObjectBoxC C = loadObjectBoxLib();
 
 /// Init DartAPI in C for async callbacks.
 ///
@@ -111,3 +112,12 @@ void initializeDartAPI() {
 // -1 => failed to initialize - incompatible Dart version
 int _dartAPIInitialized = 0;
 Object? _dartAPIInitException;
+
+/// A couple of native functions we need as callbacks to pass back to native.
+/// Unfortunately, ffigen keeps those private.
+typedef _native_close = Int32 Function(Pointer<Void> ptr);
+
+late final native_query_close =
+    _lib!.lookup<NativeFunction<_native_close>>('obx_query_close');
+late final native_query_prop_close =
+    _lib!.lookup<NativeFunction<_native_close>>('obx_query_prop_close');

objectbox/lib/src/native/query/builder.dart

@@ -47,10 +47,7 @@ class QueryBuilder<T> extends _QueryBuilder<T> {
         onListen: subscribe,
         onResume: subscribe,
         onPause: () => subscription.pause(),
-        onCancel: () {
-          subscription.cancel();
-          query.close();
-        });
+        onCancel: () => subscription.cancel());
     if (triggerImmediately) controller.add(query);
     return controller.stream;
   }

objectbox/lib/src/native/query/property.dart

@@ -3,17 +3,36 @@ part of query;
 /// Property query base.
 class PropertyQuery<T> {
   final Pointer<OBX_query_prop> _cProp;
+  late final Pointer<OBX_dart_finalizer> _cFinalizer;
+  bool _closed = false;
   final int _type;
   bool _distinct = false;
   bool _caseSensitive = false;
 
   PropertyQuery._(Pointer<OBX_query> cQuery, ModelProperty property)
       : _type = property.type,
-        _cProp =
-            checkObxPtr(C.query_prop(cQuery, property.id.id), 'property query');
+        _cProp = checkObxPtr(
+            C.query_prop(cQuery, property.id.id), 'property query') {
+    _cFinalizer = C.dartc_attach_finalizer(
+        this, native_query_prop_close, _cProp.cast(), 64);
+    if (_cFinalizer == nullptr) {
+      close();
+      throwLatestNativeError();
+    }
+  }
 
   /// Close the property query, freeing its resources
-  void close() => checkObx(C.query_prop_close(_cProp));
+  void close() {
+    if (!_closed) {
+      _closed = true;
+      var err = 0;
+      if (_cFinalizer != nullptr) {
+        err = C.dartc_detach_finalizer(_cFinalizer, this);
+      }
+      checkObx(C.query_prop_close(_cProp));
+      checkObx(err);
+    }
+  }
 
   int _count() {
     final ptr = malloc<Uint64>();

objectbox/lib/src/native/query/query.dart

@@ -618,6 +618,7 @@ class _ConditionGroupAll<EntityT> extends _ConditionGroup<EntityT> {
 class Query<T> {
   bool _closed = false;
   final Pointer<OBX_query> _cQuery;
+  late final Pointer<OBX_dart_finalizer> _cFinalizer;
   final Store _store;
   final EntityDefinition<T> _entity;
 
@@ -631,7 +632,17 @@ class Query<T> {
   }
 
   Query._(this._store, Pointer<OBX_query_builder> cBuilder, this._entity)
-      : _cQuery = checkObxPtr(C.query(cBuilder), 'create query');
+      : _cQuery = checkObxPtr(C.query(cBuilder), 'create query') {
+    initializeDartAPI();
+
+    // Keep the finalizer so we can detach it when close() is called manually.
+    _cFinalizer =
+        C.dartc_attach_finalizer(this, native_query_close, _cQuery.cast(), 256);
+    if (_cFinalizer == nullptr) {
+      close();
+      throwLatestNativeError();
+    }
+  }
 
   /// Configure an [offset] for this query.
   ///
@@ -675,9 +686,15 @@ class Query<T> {
 
   /// Close the query and free resources.
   void close() {
-    if (_closed) return;
-    _closed = true;
-    checkObx(C.query_close(_cQuery));
+    if (!_closed) {
+      _closed = true;
+      var err = 0;
+      if (_cFinalizer != nullptr) {
+        err = C.dartc_detach_finalizer(_cFinalizer, this);
+      }
+      checkObx(C.query_close(_cQuery));
+      checkObx(err);
+    }
   }
 
   /// Finds Objects matching the query and returns the first result or null

objectbox/pubspec.yaml

@@ -21,7 +21,7 @@ dev_dependencies:
   objectbox_generator: any
   pedantic: ^1.11.0
   test: ^1.16.5
-  ffigen: ^2.2.2
+  ffigen: ^2.4.2
   # No null-safety compatible version yet and we only need it in tests.
   # flat_buffers: 1.12.0
 

objectbox/test/query_property_test.dart

@@ -63,6 +63,14 @@ void main() {
 
   final tString = TestEntity_.tString;
 
+  test('property query auto-close', () {
+    // Finalizer is executed after the query object goes out of scope.
+    // Note: only caught by valgrind - I've tested that it actually catches
+    // when the finalizer assignment was disabled. Now, this will only fail in
+    // CI when running valgrind.sh - if finalizer won't work properly.
+    box.query().build().property(TestEntity_.tString).find();
+  });
+
   test('.count (basic query)', () {
     box.putMany(integerList());
     box.putMany(stringList());

objectbox/test/query_test.dart

@@ -17,6 +17,14 @@ void main() {
   });
   tearDown(() => env.close());
 
+  test('Query auto-close', () {
+    // Finalizer is executed after the query object goes out of scope.
+    // Note: only caught by valgrind - I've tested that it actually catches
+    // when the finalizer assignment was disabled. Now, this will only fail in
+    // CI when running valgrind.sh - if finalizer won't work properly.
+    box.query().build().find();
+  });
+
   test('Query with no conditions, and order as desc ints', () {
     box.putMany(<TestEntity>[
       TestEntity(tInt: 0),

objectbox/test/stream_test.dart

@@ -91,17 +91,17 @@ void main() {
     await sub2.cancel();
   });
 
-  test('can only use query during listen()', () async {
+  test('can use query after subscription is canceled', () async {
+    // This subscribes, gets the first element and cancels immediately.
+    // We're testing that if user keeps the query instance, they can use it
+    // later. This is only possible because of query auto-close with finalizers.
     final query = await box
         .query()
         .watch(triggerImmediately: true)
         .first
         .timeout(defaultTimeout);
 
-    expect(
-        query.count,
-        throwsA(predicate(
-            (StateError e) => e.toString().contains('Query already closed'))));
+    expect(query.count(), 0);
   });
 
   test(