fix(storage): the legacy profile migration puts querya.db in last and whole (#1766)

lib/core/storage/app_data_root.dart

@@ -194,19 +194,36 @@ abstract final class AppDataRoot {
     return const [];
   }
 
+  /// Copies [from] into [to] so that `querya_desktop/querya.db` appears last
+  /// and whole: the migration is skipped once that file exists, so an
+  /// interrupted copy must never leave it behind. Its `-wal` / `-shm` files
+  /// are copied before it, and the database goes in under a temporary name
+  /// and is renamed into place. [from] is not touched.
   static Future<void> _copyDirectory(Directory from, Directory to) async {
     if (!await to.exists()) {
       await to.create(recursive: true);
     }
+    final dbRelative = p.join('querya_desktop', 'querya.db');
+    File? legacyDb;
     await for (final entity in from.list(recursive: true, followLinks: false)) {
       final relative = p.relative(entity.path, from: from.path);
       final destPath = p.join(to.path, relative);
       if (entity is Directory) {
         await Directory(destPath).create(recursive: true);
       } else if (entity is File) {
+        if (relative == dbRelative) {
+          legacyDb = entity;
+          continue;
+        }
         await File(destPath).parent.create(recursive: true);
         await entity.copy(destPath);
       }
     }
+    if (legacyDb == null) return;
+    final destDb = p.join(to.path, dbRelative);
+    await Directory(p.dirname(destDb)).create(recursive: true);
+    final staged = '$destDb.migrating';
+    await legacyDb.copy(staged);
+    await File(staged).rename(destDb);
   }
 }

test/core/storage/app_data_root_test.dart

@@ -159,5 +159,60 @@ void main() {
       );
       expect(again, isFalse);
     });
+
+    test('a copy that fails midway leaves no querya.db and is retried (#1766)',
+        () async {
+      final legacy = Directory(p.join(tempDir.path, 'legacy_support'));
+      final next = Directory(p.join(tempDir.path, 'new_support'));
+      await Directory(p.join(legacy.path, 'querya_desktop'))
+          .create(recursive: true);
+      await File(p.join(legacy.path, 'querya_desktop', 'querya.db'))
+          .writeAsString('legacy-db');
+      await File(p.join(legacy.path, 'querya_desktop', 'querya.db-wal'))
+          .writeAsString('legacy-wal');
+      await Directory(p.join(legacy.path, 'themes')).create(recursive: true);
+      await File(p.join(legacy.path, 'themes', 'a.json')).writeAsString('{}');
+      // A directory where a file has to go makes that copy fail.
+      await Directory(p.join(next.path, 'themes', 'a.json'))
+          .create(recursive: true);
+
+      await expectLater(
+        AppDataRoot.migrateLegacySupportIfNeeded(
+          newSupport: next,
+          legacyCandidates: [legacy],
+        ),
+        throwsA(isA<FileSystemException>()),
+      );
+      expect(
+        await File(p.join(next.path, 'querya_desktop', 'querya.db')).exists(),
+        isFalse,
+      );
+
+      await Directory(p.join(next.path, 'themes', 'a.json')).delete();
+      final retried = await AppDataRoot.migrateLegacySupportIfNeeded(
+        newSupport: next,
+        legacyCandidates: [legacy],
+      );
+      expect(retried, isTrue);
+      expect(
+        await File(p.join(next.path, 'querya_desktop', 'querya.db'))
+            .readAsString(),
+        'legacy-db',
+      );
+      expect(
+        await File(p.join(next.path, 'querya_desktop', 'querya.db-wal'))
+            .readAsString(),
+        'legacy-wal',
+      );
+      expect(
+        await File(p.join(next.path, 'querya_desktop', 'querya.db.migrating'))
+            .exists(),
+        isFalse,
+      );
+      expect(
+        await File(p.join(legacy.path, 'querya_desktop', 'querya.db')).exists(),
+        isTrue,
+      );
+    });
   });
 }