[shared_preferences] Fix initialization race (#4159)

During the NNBD transition, the structure of the completer logic in the initialization flow was incorrectly changed to set the field after an `await` instead of immediately.

Also updates the error handling to handle `Error` the same way it currently handles `Exception`, which this change surfaced.

Fixes https://github.com/flutter/flutter/issues/42407

packages/shared_preferences/shared_preferences/CHANGELOG.md

@@ -1,5 +1,7 @@
-## NEXT
+## 2.1.2
 
+* Fixes singleton initialization race condition introduced during NNBD
+  transition.
 * Updates minimum supported macOS version to 10.14.
 * Updates minimum supported SDK version to Flutter 3.3/Dart 2.18.
 

packages/shared_preferences/shared_preferences/lib/shared_preferences.dart

@@ -62,11 +62,12 @@ class SharedPreferences {
     if (_completer == null) {
       final Completer<SharedPreferences> completer =
           Completer<SharedPreferences>();
+      _completer = completer;
       try {
         final Map<String, Object> preferencesMap =
             await _getSharedPreferencesMap();
         completer.complete(SharedPreferences._(preferencesMap));
-      } on Exception catch (e) {
+      } catch (e) {
         // If there's an error, explicitly return the future with an error.
         // then set the completer to null so we can retry.
         completer.completeError(e);
@@ -74,7 +75,6 @@ class SharedPreferences {
         _completer = null;
         return sharedPrefsFuture;
       }
-      _completer = completer;
     }
     return _completer!.future;
   }

packages/shared_preferences/shared_preferences/pubspec.yaml

@@ -3,7 +3,7 @@ description: Flutter plugin for reading and writing simple key-value pairs.
   Wraps NSUserDefaults on iOS and SharedPreferences on Android.
 repository: https://github.com/flutter/packages/tree/main/packages/shared_preferences/shared_preferences
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+shared_preferences%22
-version: 2.1.1
+version: 2.1.2
 
 environment:
   sdk: ">=2.18.0 <4.0.0"

packages/shared_preferences/shared_preferences/test/shared_preferences_test.dart

@@ -10,7 +10,6 @@ import 'package:shared_preferences_platform_interface/shared_preferences_platfor
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
-  group('SharedPreferences', () {
   const String testString = 'hello world';
   const bool testBool = true;
   const int testInt = 42;
@@ -147,8 +146,7 @@ void main() {
     await preferences.setString('String', testString);
     expect(preferences.getString('String'), testString);
 
-      SharedPreferences.setMockInitialValues(
-          testValues2.cast<String, Object>());
+    SharedPreferences.setMockInitialValues(testValues2.cast<String, Object>());
     expect(preferences.getString('String'), testString);
 
     await preferences.reload();
@@ -203,7 +201,6 @@ void main() {
 
     expect(preferences.getStringList('myList'), <String>[]);
   });
-  });
 
   test('calling mock initial values with non-prefixed keys succeeds', () async {
     SharedPreferences.setMockInitialValues(<String, Object>{
@@ -214,6 +211,16 @@ void main() {
     expect(value, 'foo');
   });
 
+  test('getInstance always returns the same instance', () async {
+    SharedPreferencesStorePlatform.instance = SlowInitSharedPreferencesStore();
+
+    final Future<SharedPreferences> firstFuture =
+        SharedPreferences.getInstance();
+    final Future<SharedPreferences> secondFuture =
+        SharedPreferences.getInstance();
+    expect(identical(await firstFuture, await secondFuture), true);
+  });
+
   test('calling setPrefix after getInstance throws', () async {
     const String newPrefix = 'newPrefix';
 
@@ -367,6 +374,15 @@ class UnimplementedSharedPreferencesStore
   }
 }
 
+class SlowInitSharedPreferencesStore
+    extends UnimplementedSharedPreferencesStore {
+  @override
+  Future<Map<String, Object>> getAll() async {
+    await Future<void>.delayed(const Duration(seconds: 1));
+    return <String, Object>{};
+  }
+}
+
 class ThrowingSharedPreferencesStore extends SharedPreferencesStorePlatform {
   @override
   Future<bool> clear() {