moor_ffi: Properly implement close()

moor_ffi/lib/src/impl/database.dart

@@ -61,6 +61,8 @@ class Database {
   /// an error occurs while closing the database, an exception will be thrown.
   /// The allocated memory will be freed either way.
   void close() {
+    if (_isClosed) return;
+
     // close all prepared statements first
     _isClosed = true;
     for (final stmt in _preparedStmt) {

moor_ffi/lib/src/vm_database.dart

@@ -91,6 +91,11 @@ class _VmDelegate extends DatabaseDelegate {
 
     return Future.value(QueryResult(result.columnNames, result.rows));
   }
+
+  @override
+  Future<void> close() async {
+    _db.close();
+  }
 }
 
 class _VmVersionDelegate extends DynamicVersionDelegate {

moor_ffi/test/database/database_test.dart

@@ -0,0 +1,21 @@
+import 'package:moor_ffi/database.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("database can't be used after close", () {
+    final db = Database.memory();
+    db.execute('SELECT 1');
+
+    db.close();
+
+    expect(() => db.execute('SELECT 1'), throwsA(anything));
+  });
+
+  test('closing multiple times works', () {
+    final db = Database.memory();
+    db.execute('SELECT 1');
+
+    db.close();
+    db.close(); // shouldn't throw or crash
+  });
+}