[shared_preferences] Fix concurrent modification of the shared preferences on Android (flutter#3684)

Uses an actual queue, rather than a one-element handoff "queue", for the single-thread background processing of commits.

Author: konstantintuev

packages/shared_preferences/shared_preferences/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.0.4
+
+* Fix a regression with simultaneous writes on Android.
+
 ## 2.0.3
 
 * Android: don't create additional Handler when method channel is called.

packages/shared_preferences/shared_preferences/android/src/main/java/io/flutter/plugins/sharedpreferences/MethodCallHandlerImpl.java

@@ -23,7 +23,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.SynchronousQueue;
+import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 
@@ -53,7 +53,7 @@ class MethodCallHandlerImpl implements MethodChannel.MethodCallHandler {
   MethodCallHandlerImpl(Context context) {
     preferences = context.getSharedPreferences(SHARED_PREFERENCES_NAME, Context.MODE_PRIVATE);
     executor =
-        new ThreadPoolExecutor(0, 1, 30L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>());
+        new ThreadPoolExecutor(0, 1, 30L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
     handler = new Handler(Looper.getMainLooper());
   }
 

packages/shared_preferences/shared_preferences/example/integration_test/shared_preferences_test.dart

@@ -91,5 +91,18 @@ void main() {
       expect(preferences.getDouble('double'), null);
       expect(preferences.getStringList('List'), null);
     });
+
+    testWidgets('simultaneous writes', (WidgetTester _) async {
+      final List<Future<bool>> writes = <Future<bool>>[];
+      final int writeCount = 100;
+      for (int i = 1; i <= writeCount; i++) {
+        writes.add(preferences.setInt('int', i));
+      }
+      List<bool> result = await Future.wait(writes, eagerError: true);
+      // All writes should succeed.
+      expect(result.where((element) => !element), isEmpty);
+      // The last write should win.
+      expect(preferences.getInt('int'), writeCount);
+    });
   });
 }

packages/shared_preferences/shared_preferences/pubspec.yaml

@@ -2,7 +2,7 @@ name: shared_preferences
 description: Flutter plugin for reading and writing simple key-value pairs.
   Wraps NSUserDefaults on iOS and SharedPreferences on Android.
 homepage: https://github.com/flutter/plugins/tree/master/packages/shared_preferences/shared_preferences
-version: 2.0.3
+version: 2.0.4
 
 flutter:
   plugin: