fix: Make transactions atomic / AutoLogout

lib/app/app.dart

@@ -2,7 +2,9 @@ import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:provider/provider.dart';
 
+import 'app_lifecycle_lock.dart';
 import '../models/settings.dart';
+import '../viewmodels/pin_lock_view_model.dart';
 import '../viewmodels/settings_view_model.dart';
 import '../theme.dart';
 import '../l10n/app_localizations.dart';
@@ -17,15 +19,28 @@ class KoolTabApp extends StatefulWidget {
 }
 
 class _KoolTabAppState extends State<KoolTabApp> {
+  late final AppLifecycleLockObserver _lifecycleLockObserver;
+
   @override
   void initState() {
     super.initState();
 
+    _lifecycleLockObserver = AppLifecycleLockObserver(
+      onLock: () => context.read<PinLockViewModel>().lock(),
+    );
+    WidgetsBinding.instance.addObserver(_lifecycleLockObserver);
+
     WidgetsBinding.instance.addPostFrameCallback((_) {
       context.read<SettingsViewModel>().ensureLoaded();
     });
   }
 
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(_lifecycleLockObserver);
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     final settings = context.watch<SettingsViewModel>().settings;

lib/app/app_lifecycle_lock.dart

@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+
+class AppLifecycleLockObserver extends WidgetsBindingObserver {
+  final VoidCallback onLock;
+
+  AppLifecycleLockObserver({required this.onLock});
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    switch (state) {
+      case AppLifecycleState.inactive:
+      case AppLifecycleState.hidden:
+      case AppLifecycleState.paused:
+      case AppLifecycleState.detached:
+        onLock();
+      case AppLifecycleState.resumed:
+        break;
+    }
+  }
+}

lib/services/bar_tab_service.dart

@@ -8,6 +8,7 @@ import '../models/closed_tab_item.dart';
 import '../models/payment_method.dart';
 import '../models/product.dart';
 import '../models/tab_item.dart';
+import 'product_service.dart';
 
 abstract class BarTabService {
   Future<List<BarTab>> getOpenTabs();
@@ -19,18 +20,19 @@ abstract class BarTabService {
   Future<void> addProductToTab({
     required String tabId,
     required Product product,
-    Future<void> Function()? stockAdjustment,
   });
 
-  Future<void> updateTabItemQuantity({
+  Future<int> adjustTabItemQuantity({
     required String tabItemId,
-    required int quantity,
+    required int delta,
-    Future<void> Function()? stockAdjustment,
   });
 
   /// Archives the tab's current items into history and clears them.
   /// The tab itself stays open under the same customer name.
-  Future<void> closeTab(String tabId, {PaymentMethod paymentMethod = PaymentMethod.cash});
+  Future<void> closeTab(
+    String tabId, {
+    PaymentMethod paymentMethod = PaymentMethod.cash,
+  });
 
   Future<List<ClosedTab>> getClosedTabs();
 
@@ -94,6 +96,54 @@ class DriftBarTabService implements BarTabService {
     return rows.map(_mapItemRow).toList();
   }
 
+  Future<void> _decreaseProductStock(String productId, int amount) async {
+    if (amount <= 0) {
+      throw ArgumentError.value(amount, 'amount', 'Must be greater than zero');
+    }
+
+    final updatedRows = await database.customUpdate(
+      'UPDATE products '
+      'SET stock_quantity = stock_quantity - ? '
+      'WHERE id = ? AND stock_quantity >= ?',
+      variables: [
+        Variable.withInt(amount),
+        Variable.withString(productId),
+        Variable.withInt(amount),
+      ],
+      updates: {database.products},
+    );
+
+    if (updatedRows == 1) return;
+
+    final product = await (database.select(
+      database.products,
+    )..where((row) => row.id.equals(productId))).getSingleOrNull();
+
+    if (product == null) {
+      throw Exception('Product not found');
+    }
+
+    throw const InsufficientStockException();
+  }
+
+  Future<void> _increaseProductStock(String productId, int amount) async {
+    if (amount <= 0) {
+      throw ArgumentError.value(amount, 'amount', 'Must be greater than zero');
+    }
+
+    final updatedRows = await database.customUpdate(
+      'UPDATE products '
+      'SET stock_quantity = stock_quantity + ? '
+      'WHERE id = ?',
+      variables: [Variable.withInt(amount), Variable.withString(productId)],
+      updates: {database.products},
+    );
+
+    if (updatedRows == 1) return;
+
+    throw Exception('Product not found');
+  }
+
   @override
   Future<List<BarTab>> getOpenTabs() async {
     final query = database.select(database.barTabs)
@@ -157,9 +207,10 @@ class DriftBarTabService implements BarTabService {
   Future<void> addProductToTab({
     required String tabId,
     required Product product,
-    Future<void> Function()? stockAdjustment,
   }) async {
     await database.transaction(() async {
+      await _decreaseProductStock(product.id, 1);
+
       final existingItemQuery = database.select(database.tabItems)
         ..where(
           (item) =>
@@ -176,10 +227,6 @@ class DriftBarTabService implements BarTabService {
           TabItemsCompanion(quantity: Value(existingItem.quantity + 1)),
         );
 
-        if (stockAdjustment != null) {
-          await stockAdjustment();
-        }
-
         return;
       }
 
@@ -196,46 +243,60 @@ class DriftBarTabService implements BarTabService {
               createdAt: DateTime.now(),
             ),
           );
-
-      if (stockAdjustment != null) {
-        await stockAdjustment();
-      }
     });
   }
 
   @override
-  Future<void> updateTabItemQuantity({
+  Future<int> adjustTabItemQuantity({
     required String tabItemId,
-    required int quantity,
+    required int delta,
-    Future<void> Function()? stockAdjustment,
   }) async {
-    await database.transaction(() async {
+    return database.transaction(() async {
-      if (quantity <= 0) {
+      final item = await (database.select(
-        final deleteQuery = database.delete(database.tabItems)
+        database.tabItems,
-          ..where((item) => item.id.equals(tabItemId));
+      )..where((row) => row.id.equals(tabItemId))).getSingleOrNull();
 
-        await deleteQuery.go();
+      if (item == null || delta == 0) return 0;
 
-        if (stockAdjustment != null) {
+      final requestedQuantity = item.quantity + delta;
-          await stockAdjustment();
+      final actualDelta = requestedQuantity <= 0 ? -item.quantity : delta;
+
+      if (actualDelta > 0) {
+        await _decreaseProductStock(item.productId, actualDelta);
+      } else {
+        await _increaseProductStock(item.productId, -actualDelta);
       }
 
-        return;
+      if (requestedQuantity <= 0) {
+        final deletedRows = await (database.delete(
+          database.tabItems,
+        )..where((row) => row.id.equals(tabItemId))).go();
+
+        if (deletedRows != 1) {
+          throw StateError('Tab item was changed before it could be deleted');
+        }
+      } else {
+        final updatedRows =
+            await (database.update(database.tabItems)
+                  ..where((row) => row.id.equals(tabItemId)))
+                .write(TabItemsCompanion(quantity: Value(requestedQuantity)));
+
+        if (updatedRows != 1) {
+          throw StateError(
+            'Tab item was changed before its quantity was updated',
+          );
+        }
       }
 
-      final updateQuery = database.update(database.tabItems)
+      return actualDelta;
-        ..where((item) => item.id.equals(tabItemId));
-
-      await updateQuery.write(TabItemsCompanion(quantity: Value(quantity)));
-
-      if (stockAdjustment != null) {
-        await stockAdjustment();
-      }
     });
   }
 
   @override
-  Future<void> closeTab(String tabId, {PaymentMethod paymentMethod = PaymentMethod.cash}) async {
+  Future<void> closeTab(
+    String tabId, {
+    PaymentMethod paymentMethod = PaymentMethod.cash,
+  }) async {
     await database.transaction(() async {
       final tabQuery = database.select(database.barTabs)
         ..where((tab) => tab.id.equals(tabId));
@@ -307,8 +368,7 @@ class DriftBarTabService implements BarTabService {
       ..limit(limit, offset: offset);
 
     if (customerName != null && customerName.isNotEmpty) {
-      query = query
+      query = query..where((tab) => tab.customerName.equals(customerName));
-        ..where((tab) => tab.customerName.equals(customerName));
     }
 
     final closedTabRows = await query.get();
@@ -355,10 +415,6 @@ class DriftBarTabService implements BarTabService {
   @override
   Future<List<String>> getDistinctCustomerNames() async {
     final rows = await database.select(database.closedTabs).get();
-    return rows
+    return rows.map((row) => row.customerName).toSet().toList()..sort();
-        .map((row) => row.customerName)
-        .toSet()
-        .toList()
-      ..sort();
   }
 }

lib/services/product_service.dart

@@ -130,31 +130,48 @@ class DriftProductService implements ProductService {
 
   @override
   Future<void> decreaseStock(String productId, int amount) async {
-    final product = await getProductById(productId);
+    _validateStockAmount(amount);
 
-    if (product == null) {
+    final updatedRows = await database.customUpdate(
-      throw Exception('Product not found');
+      'UPDATE products '
-    }
+      'SET stock_quantity = stock_quantity - ? '
-
+      'WHERE id = ? AND stock_quantity >= ?',
-    if (product.stockQuantity < amount) {
+      variables: [
-      throw const InsufficientStockException();
+        Variable.withInt(amount),
-    }
+        Variable.withString(productId),
-
+        Variable.withInt(amount),
-    await updateProduct(
+      ],
-      product.copyWith(stockQuantity: product.stockQuantity - amount),
+      updates: {database.products},
     );
+
+    if (updatedRows == 1) return;
+
+    final product = await getProductById(productId);
+    if (product == null) throw Exception('Product not found');
+
+    throw const InsufficientStockException();
   }
 
   @override
   Future<void> increaseStock(String productId, int amount) async {
-    final product = await getProductById(productId);
+    _validateStockAmount(amount);
+
+    final updatedRows = await database.customUpdate(
+      'UPDATE products '
+      'SET stock_quantity = stock_quantity + ? '
+      'WHERE id = ?',
+      variables: [Variable.withInt(amount), Variable.withString(productId)],
+      updates: {database.products},
+    );
+
+    if (updatedRows == 1) return;
 
-    if (product == null) {
     throw Exception('Product not found');
   }
 
-    await updateProduct(
+  void _validateStockAmount(int amount) {
-      product.copyWith(stockQuantity: product.stockQuantity + amount),
+    if (amount <= 0) {
-    );
+      throw ArgumentError.value(amount, 'amount', 'Must be greater than zero');
+    }
   }
 }

lib/viewmodels/bar_screen_view_model.dart

@@ -95,16 +95,9 @@ class BarScreenViewModel extends ChangeNotifier {
 
     if (tab == null) return;
 
-    if (product.stockQuantity <= 0) {
-      throw const InsufficientStockException();
-    }
-
     try {
-      await barTabService.addProductToTab(
+      await barTabService.addProductToTab(tabId: tab.id, product: product);
-        tabId: tab.id,
+      inventory.applyStockDelta(product.id, -1);
-        product: product,
-        stockAdjustment: () => inventory.decreaseStock(product.id, 1),
-      );
 
       await _reloadTabs();
     } catch (e, stack) {
@@ -118,35 +111,15 @@ class BarScreenViewModel extends ChangeNotifier {
     }
   }
 
-  Future<void> changeItemQuantity(TabItem item, int quantity) async {
+  Future<void> changeItemQuantity(TabItem item, int delta) async {
-    final difference = quantity - item.quantity;
+    if (delta == 0) return;
-
-    if (difference > 0) {
-      Product? product;
-      for (final candidate in inventory.products) {
-        if (candidate.id == item.productId) {
-          product = candidate;
-          break;
-        }
-      }
-
-      if (product != null && product.stockQuantity < difference) {
-        throw const InsufficientStockException();
-      }
-    }
 
     try {
-      await barTabService.updateTabItemQuantity(
+      final actualDelta = await barTabService.adjustTabItemQuantity(
         tabItemId: item.id,
-        quantity: quantity,
+        delta: delta,
-        stockAdjustment: () async {
-          if (difference > 0) {
-            await inventory.decreaseStock(item.productId, difference);
-          } else if (difference < 0) {
-            await inventory.increaseStock(item.productId, -difference);
-          }
-        },
       );
+      inventory.applyStockDelta(item.productId, -actualDelta);
 
       await _reloadTabs();
     } catch (e, stack) {
@@ -160,7 +133,9 @@ class BarScreenViewModel extends ChangeNotifier {
     }
   }
 
-  Future<void> closeSelectedTab({PaymentMethod paymentMethod = PaymentMethod.cash}) async {
+  Future<void> closeSelectedTab({
+    PaymentMethod paymentMethod = PaymentMethod.cash,
+  }) async {
     final tab = selectedTab;
 
     if (tab == null) return;
@@ -177,7 +152,10 @@ class BarScreenViewModel extends ChangeNotifier {
     }
   }
 
-  Future<void> closeTab(String tabId, {PaymentMethod paymentMethod = PaymentMethod.cash}) async {
+  Future<void> closeTab(
+    String tabId, {
+    PaymentMethod paymentMethod = PaymentMethod.cash,
+  }) async {
     try {
       await barTabService.closeTab(tabId, paymentMethod: paymentMethod);
       await _reloadTabs();

lib/viewmodels/inventory_view_model.dart

@@ -65,4 +65,16 @@ class InventoryViewModel extends ChangeNotifier {
       rethrow;
     }
   }
+
+  void applyStockDelta(String productId, int delta) {
+    if (delta == 0) return;
+
+    final index = _products.indexWhere((product) => product.id == productId);
+    if (index == -1) return;
+
+    _products[index] = _products[index].copyWith(
+      stockQuantity: _products[index].stockQuantity + delta,
+    );
+    notifyListeners();
+  }
 }

lib/views/bar_screen_view.dart

@@ -343,7 +343,7 @@ class _TabPanel extends StatefulWidget {
   final Map<String, int> stockByProductId;
   final VoidCallback onNewTabPressed;
   final ValueChanged<String> onTabSelected;
-  final Future<void> Function(TabItem item, int quantity) onItemQuantityChanged;
+  final Future<void> Function(TabItem item, int delta) onItemQuantityChanged;
   final VoidCallback onCloseTabPressed;
   final Future<void> Function(String) onTabClosed;
 
@@ -634,7 +634,7 @@ class _OpenTabsList extends StatelessWidget {
 class _SelectedTabDetails extends StatelessWidget {
   final BarTab tab;
   final Map<String, int> stockByProductId;
-  final Future<void> Function(TabItem item, int quantity) onItemQuantityChanged;
+  final Future<void> Function(TabItem item, int delta) onItemQuantityChanged;
   final VoidCallback onCloseTabPressed;
 
   const _SelectedTabDetails({
@@ -747,7 +747,7 @@ class _SelectedTabDetails extends StatelessWidget {
 class _TabItemRow extends StatelessWidget {
   final TabItem item;
   final Map<String, int> stockByProductId;
-  final Future<void> Function(TabItem item, int quantity) onQuantityChanged;
+  final Future<void> Function(TabItem item, int delta) onQuantityChanged;
 
   const _TabItemRow({
     required this.item,
@@ -796,7 +796,7 @@ class _TabItemRow extends StatelessWidget {
                   visualDensity: VisualDensity.compact,
                   onPressed: () async {
                     try {
-                      await onQuantityChanged(item, item.quantity - 1);
+                      await onQuantityChanged(item, -1);
                     } catch (e, stack) {
                       Sentry.captureException(e, stackTrace: stack);
                       if (context.mounted) {
@@ -821,7 +821,7 @@ class _TabItemRow extends StatelessWidget {
                   onPressed: canIncrease
                       ? () async {
                           try {
-                            await onQuantityChanged(item, item.quantity + 1);
+                            await onQuantityChanged(item, 1);
                           } catch (e, stack) {
                             if (e is! InsufficientStockException) {
                               Sentry.captureException(e, stackTrace: stack);

lib/views/settings_view.dart

@@ -44,7 +44,7 @@ class _SettingsScreenViewState extends State<SettingsScreenView> {
 
   void _startVersionHold() {
     _versionHoldTimer?.cancel();
-    _versionHoldTimer = Timer(const Duration(seconds: 3), () {
+    _versionHoldTimer = Timer(const Duration(seconds: 1), () {
       _versionHoldTimer = null;
       if (mounted) context.push('/dev');
     });

make_release.bat

@@ -0,0 +1,52 @@
+@echo off
+setlocal EnableExtensions DisableDelayedExpansion
+
+cd /d "%~dp0"
+
+echo.
+powershell -NoProfile -ExecutionPolicy Bypass -Command ^
+    "$content = [IO.File]::ReadAllText('pubspec.yaml'); " ^
+    "$match = [regex]::Match($content, '(?m)(?:\A|\r?\n)version:[ \t]*([^\r\n]*)'); " ^
+    "if (-not $match.Success) { Write-Error 'Could not find the version line in pubspec.yaml.'; exit 1 }; " ^
+    "Write-Output ('Current version: ' + $match.Groups[1].Value.Trim())"
+
+if errorlevel 1 (
+    echo Could not read the current version. Release cancelled.
+    exit /b 1
+)
+
+set "APP_VERSION="
+set /p "APP_VERSION=Enter release version (for example 1.0.10 or 1.0.10+2): "
+
+if not defined APP_VERSION (
+    echo No version entered. Release cancelled.
+    exit /b 1
+)
+
+powershell -NoProfile -ExecutionPolicy Bypass -Command ^
+    "$version = $env:APP_VERSION; " ^
+    "if ($version -notmatch '\A[0-9]+\.[0-9]+\.[0-9]+(\+[0-9]+)?\z') { " ^
+    "    Write-Error 'Use a version like 1.0.10 or 1.0.10+2.'; exit 1 " ^
+    "}; " ^
+    "$path = 'pubspec.yaml'; " ^
+    "$content = [IO.File]::ReadAllText($path); " ^
+    "$updated = ([regex]::new('(?m)(\A|\r?\n)version:[^\r\n]*')).Replace($content, ('${1}version: ' + $version), 1); " ^
+    "if ($updated -eq $content) { Write-Error 'Could not find the version line in pubspec.yaml.'; exit 1 }; " ^
+    "[IO.File]::WriteAllText($path, $updated)"
+
+if errorlevel 1 (
+    echo Version update failed. Release cancelled.
+    exit /b 1
+)
+
+echo Updated pubspec.yaml to version %APP_VERSION%.
+echo Building release APK...
+flutter build apk --release
+
+if errorlevel 1 (
+    echo Release APK build failed.
+    exit /b 1
+)
+
+echo Release APK created at build\app\outputs\flutter-apk\app-release.apk
+endlocal

pubspec.yaml

@@ -16,7 +16,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
 # In Windows, build-name is used as the major, minor, and patch parts
 # of the product and file versions while build-number is used as the build suffix.
-version: 1.0.9+1
+version: 1.0.11
 
 environment:
   sdk: ^3.12.2

test/app_lifecycle_lock_test.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:kooltab2/app/app_lifecycle_lock.dart';
+
+void main() {
+  test('locks when the app is no longer active', () {
+    var lockCount = 0;
+    final observer = AppLifecycleLockObserver(onLock: () => lockCount++);
+
+    for (final state in [
+      AppLifecycleState.inactive,
+      AppLifecycleState.hidden,
+      AppLifecycleState.paused,
+      AppLifecycleState.detached,
+    ]) {
+      observer.didChangeAppLifecycleState(state);
+    }
+
+    expect(lockCount, 4);
+  });
+
+  test('does not lock while the app is active', () {
+    var lockCount = 0;
+    final observer = AppLifecycleLockObserver(onLock: () => lockCount++);
+
+    observer.didChangeAppLifecycleState(AppLifecycleState.resumed);
+
+    expect(lockCount, 0);
+  });
+}

test/bar_screen_view_model_test.dart

@@ -1,5 +1,4 @@
 import 'package:flutter_test/flutter_test.dart';
-import 'package:kooltab2/models/product.dart';
 import 'package:kooltab2/models/tab_item.dart';
 import 'package:kooltab2/services/bar_tab_service.dart';
 import 'package:kooltab2/services/product_service.dart';
@@ -27,17 +26,7 @@ void main() {
     );
   });
 
-  test(
+  test('propagates an insufficient-stock error from the transaction', () async {
-    'does not increase a tab item when its product is out of stock',
-    () async {
-      const product = Product(
-        id: 'prod-1',
-        name: 'Chips',
-        category: 'Snacks',
-        stockQuantity: 0,
-        lowStockThreshold: 5,
-        priceInCents: 150,
-      );
     const item = TabItem(
       id: 'item-1',
       tabId: 'tab-1',
@@ -48,17 +37,17 @@ void main() {
     );
 
     when(
-        () => productService.getProducts(),
+      () => barTabService.adjustTabItemQuantity(tabItemId: 'item-1', delta: 1),
-      ).thenAnswer((_) async => [product]);
+    ).thenAnswer((_) async => throw const InsufficientStockException());
-      await inventory.load();
 
     await expectLater(
-        viewModel.changeItemQuantity(item, 21),
+      viewModel.changeItemQuantity(item, 1),
       throwsA(isA<InsufficientStockException>()),
     );
 
     expect(viewModel.errorMessage, isNull);
-      verifyNoMoreInteractions(barTabService);
+    verify(
-    },
+      () => barTabService.adjustTabItemQuantity(tabItemId: 'item-1', delta: 1),
-  );
+    ).called(1);
+  });
 }

test/bar_tab_service_test.dart

@@ -4,14 +4,28 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:kooltab2/database/app_database.dart';
 import 'package:kooltab2/models/product.dart';
 import 'package:kooltab2/services/bar_tab_service.dart';
+import 'package:kooltab2/services/product_service.dart';
 
 void main() {
   late AppDatabase database;
   late DriftBarTabService service;
 
-  setUp(() {
+  setUp(() async {
     database = AppDatabase(NativeDatabase.memory());
     service = DriftBarTabService(database: database);
+
+    await database
+        .into(database.products)
+        .insert(
+          ProductsCompanion.insert(
+            id: 'prod-1',
+            name: 'Test Beer',
+            category: 'Drinks',
+            stockQuantity: 100,
+            lowStockThreshold: 10,
+            priceInCents: 400,
+          ),
+        );
   });
 
   tearDown(() async {
@@ -59,7 +73,9 @@ void main() {
 
       test('returns only open tabs', () async {
         await service.createTab(customerName: 'Open Tab');
-        await database.into(database.barTabs).insert(
+        await database
+            .into(database.barTabs)
+            .insert(
               BarTabsCompanion.insert(
                 id: 'closed-tab',
                 customerName: 'Closed Tab',
@@ -76,7 +92,9 @@ void main() {
 
       test('tabs sorted by openedAt descending', () async {
         final now = DateTime.now();
-        await database.into(database.barTabs).insert(
+        await database
+            .into(database.barTabs)
+            .insert(
               BarTabsCompanion.insert(
                 id: 'tab-first',
                 customerName: 'First',
@@ -84,7 +102,9 @@ void main() {
                 openedAt: now.subtract(const Duration(hours: 1)),
               ),
             );
-        await database.into(database.barTabs).insert(
+        await database
+            .into(database.barTabs)
+            .insert(
               BarTabsCompanion.insert(
                 id: 'tab-second',
                 customerName: 'Second',
@@ -152,9 +172,38 @@ void main() {
         final updatedTab = await service.getTabById(tab.id);
         expect(updatedTab!.items.first.lineTotalInCents, 900);
       });
+
+      test('decreases stock as part of adding the item', () async {
+        final tab = await service.createTab(customerName: 'John');
+        final product = createTestProduct();
+
+        await service.addProductToTab(tabId: tab.id, product: product);
+
+        final storedProduct = await (database.select(
+          database.products,
+        )..where((row) => row.id.equals(product.id))).getSingle();
+        expect(storedProduct.stockQuantity, 99);
       });
 
-    group('updateTabItemQuantity', () {
+      test('rolls back the tab insert when stock is unavailable', () async {
+        final tab = await service.createTab(customerName: 'John');
+        final product = createTestProduct();
+
+        await (database.update(database.products)
+              ..where((row) => row.id.equals(product.id)))
+            .write(const ProductsCompanion(stockQuantity: Value(0)));
+
+        await expectLater(
+          service.addProductToTab(tabId: tab.id, product: product),
+          throwsA(isA<InsufficientStockException>()),
+        );
+
+        final updatedTab = await service.getTabById(tab.id);
+        expect(updatedTab!.items, isEmpty);
+      });
+    });
+
+    group('adjustTabItemQuantity', () {
       test('updates item quantity', () async {
         final tab = await service.createTab(customerName: 'John');
         final product = createTestProduct();
@@ -162,40 +211,67 @@ void main() {
 
         final tabItemId = (await service.getTabById(tab.id))!.items.first.id;
 
-        await service.updateTabItemQuantity(
+        await service.adjustTabItemQuantity(tabItemId: tabItemId, delta: 4);
-          tabItemId: tabItemId,
-          quantity: 5,
-        );
 
         final updatedTab = await service.getTabById(tab.id);
         expect(updatedTab!.items.first.quantity, 5);
       });
 
-      test('deletes item when quantity is zero', () async {
+      test('deletes item when the delta removes all quantity', () async {
         final tab = await service.createTab(customerName: 'John');
         final product = createTestProduct();
         await service.addProductToTab(tabId: tab.id, product: product);
 
         final tabItemId = (await service.getTabById(tab.id))!.items.first.id;
 
-        await service.updateTabItemQuantity(tabItemId: tabItemId, quantity: 0);
+        await service.adjustTabItemQuantity(tabItemId: tabItemId, delta: -1);
 
         final updatedTab = await service.getTabById(tab.id);
         expect(updatedTab!.items, isEmpty);
       });
 
-      test('deletes item when quantity is negative', () async {
+      test('restores stock when quantity decreases', () async {
         final tab = await service.createTab(customerName: 'John');
         final product = createTestProduct();
         await service.addProductToTab(tabId: tab.id, product: product);
 
         final tabItemId = (await service.getTabById(tab.id))!.items.first.id;
 
-        await service.updateTabItemQuantity(tabItemId: tabItemId, quantity: -1);
+        await service.adjustTabItemQuantity(tabItemId: tabItemId, delta: -1);
 
         final updatedTab = await service.getTabById(tab.id);
         expect(updatedTab!.items, isEmpty);
+
+        final updatedProduct = await (database.select(
+          database.products,
+        )..where((row) => row.id.equals(product.id))).getSingle();
+        expect(updatedProduct.stockQuantity, 100);
       });
+
+      test(
+        'applies concurrent deltas without losing stock consistency',
+        () async {
+          final tab = await service.createTab(customerName: 'John');
+          final product = createTestProduct();
+          await service.addProductToTab(tabId: tab.id, product: product);
+
+          final tabItemId = (await service.getTabById(tab.id))!.items.first.id;
+
+          final deltas = await Future.wait([
+            service.adjustTabItemQuantity(tabItemId: tabItemId, delta: 1),
+            service.adjustTabItemQuantity(tabItemId: tabItemId, delta: 1),
+          ]);
+
+          expect(deltas, [1, 1]);
+          final updatedTab = await service.getTabById(tab.id);
+          expect(updatedTab!.items.first.quantity, 3);
+
+          final updatedProduct = await (database.select(
+            database.products,
+          )..where((row) => row.id.equals(product.id))).getSingle();
+          expect(updatedProduct.stockQuantity, 97);
+        },
+      );
     });
 
     group('closeTab', () {
@@ -257,7 +333,9 @@ void main() {
 
       test('returns closed tabs sorted by closedAt descending', () async {
         final now = DateTime.now();
-        await database.into(database.closedTabs).insert(
+        await database
+            .into(database.closedTabs)
+            .insert(
               ClosedTabsCompanion.insert(
                 id: 'closed-first',
                 originalTabId: 'original-1',
@@ -265,7 +343,9 @@ void main() {
                 closedAt: now.subtract(const Duration(hours: 1)),
               ),
             );
-        await database.into(database.closedTabs).insert(
+        await database
+            .into(database.closedTabs)
+            .insert(
               ClosedTabsCompanion.insert(
                 id: 'closed-second',
                 originalTabId: 'original-2',

test/product_service_test.dart

@@ -26,7 +26,9 @@ void main() {
       });
 
       test('returns only active products', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: '1',
                 name: 'Active Product',
@@ -36,7 +38,9 @@ void main() {
                 priceInCents: 500,
               ),
             );
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: '2',
                 name: 'Inactive Product',
@@ -55,7 +59,9 @@ void main() {
       });
 
       test('returns products sorted by name', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: '1',
                 name: 'Zebra',
@@ -65,7 +71,9 @@ void main() {
                 priceInCents: 500,
               ),
             );
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: '2',
                 name: 'Apple',
@@ -85,7 +93,9 @@ void main() {
 
     group('getProductById', () {
       test('returns product when exists', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: 'prod-123',
                 name: 'Test Product',
@@ -128,7 +138,9 @@ void main() {
 
     group('updateProduct', () {
       test('updates product fields', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: 'update-test',
                 name: 'Original',
@@ -160,7 +172,9 @@ void main() {
 
     group('deleteProduct', () {
       test('removes product from database', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: 'delete-test',
                 name: 'To Delete',
@@ -180,7 +194,9 @@ void main() {
 
     group('decreaseStock', () {
       test('decreases stock quantity', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: 'stock-test',
                 name: 'Stock Test',
@@ -205,7 +221,9 @@ void main() {
       });
 
       test('throws when not enough stock', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: 'low-stock',
                 name: 'Low Stock',
@@ -225,7 +243,9 @@ void main() {
 
     group('increaseStock', () {
       test('increases stock quantity', () async {
-        await database.into(database.products).insert(
+        await database
+            .into(database.products)
+            .insert(
               ProductsCompanion.insert(
                 id: 'increase-test',
                 name: 'Increase Test',
@@ -248,6 +268,56 @@ void main() {
           throwsA(isA<Exception>()),
         );
       });
+
+      test('rejects non-positive stock adjustments', () async {
+        await database
+            .into(database.products)
+            .insert(
+              ProductsCompanion.insert(
+                id: 'invalid-adjustment',
+                name: 'Invalid Adjustment',
+                category: 'Drinks',
+                stockQuantity: 10,
+                lowStockThreshold: 2,
+                priceInCents: 500,
+              ),
+            );
+
+        expect(
+          () => service.decreaseStock('invalid-adjustment', 0),
+          throwsA(isA<ArgumentError>()),
+        );
+        expect(
+          () => service.increaseStock('invalid-adjustment', -1),
+          throwsA(isA<ArgumentError>()),
+        );
+      });
+
+      test('does not allow concurrent decrements below zero', () async {
+        await database
+            .into(database.products)
+            .insert(
+              ProductsCompanion.insert(
+                id: 'concurrent-stock',
+                name: 'Concurrent Stock',
+                category: 'Drinks',
+                stockQuantity: 1,
+                lowStockThreshold: 0,
+                priceInCents: 500,
+              ),
+            );
+
+        await expectLater(
+          Future.wait([
+            service.decreaseStock('concurrent-stock', 1),
+            service.decreaseStock('concurrent-stock', 1),
+          ]),
+          throwsA(isA<InsufficientStockException>()),
+        );
+
+        final product = await service.getProductById('concurrent-stock');
+        expect(product!.stockQuantity, 0);
+      });
     });
   });
 }