From 6641d85f0550ca60433fc8e0d13a3dafdbb664d6 Mon Sep 17 00:00:00 2001
From: Krzysztof kuhy Rudnicki <krzysztofrudnicki0@gmail.com>
Date: Mon, 9 Nov 2026 15:24:01 +0100
Subject: [PATCH] crdt_sync_dart 0.12.1: serialize FileLogPersistence writes

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 crdt_sync_dart/lib/src/io_store.dart          | 20 +++++++++++---
 crdt_sync_dart/pubspec.yaml                   |  2 +-
 .../test/default_client_and_io_test.dart      | 27 +++++++++++++++++++
 3 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/crdt_sync_dart/lib/src/io_store.dart b/crdt_sync_dart/lib/src/io_store.dart
index 89795db..6ef9acc 100644
--- a/crdt_sync_dart/lib/src/io_store.dart
+++ b/crdt_sync_dart/lib/src/io_store.dart
@@ -7,8 +7,11 @@ import 'store.dart';
 /// Writes atomically -- to a per-process temp file, then `rename`d over the
 /// real path -- so a concurrent reader (e.g. a background sync isolate writing
 /// while the foreground app also writes) never observes a half-written file.
-/// The pid in the temp name keeps two writers from colliding. This generalizes
-/// diet_guard's hand-rolled `LogStorageService` write scheme.
+/// The pid in the temp name keeps two processes from colliding; within one
+/// instance, writes are serialized in issue order (last issued wins), so
+/// overlapping calls (e.g. per-keystroke saves) never share the temp file
+/// mid-flight. This generalizes diet_guard's hand-rolled `LogStorageService`
+/// write scheme.
 ///
 /// Lives behind the `package:crdt_sync/crdt_sync_io.dart` entrypoint, not the
 /// main barrel, because it imports `dart:io` and so is unavailable on web --
@@ -20,6 +23,10 @@ class FileLogPersistence implements LogPersistence {
 
   final File _file;
 
+  /// Completes when the most recently issued write has settled. Never
+  /// rejects, so one failed write cannot block the ones queued after it.
+  Future<void> _tail = Future<void>.value();
+
   @override
   Future<String?> read() async {
     if (!_file.existsSync()) return null;
@@ -31,7 +38,14 @@ class FileLogPersistence implements LogPersistence {
   }
 
   @override
-  Future<void> write(String text) async {
+  Future<void> write(String text) {
+    final result = _tail.then((_) => _writeNow(text));
+    // The caller still sees its own error via [result]; the chain ignores it.
+    _tail = result.then((_) {}, onError: (Object _) {});
+    return result;
+  }
+
+  Future<void> _writeNow(String text) async {
     await _file.parent.create(recursive: true);
     final tmp = File('${_file.path}.$pid.tmp');
     await tmp.writeAsString(text);
diff --git a/crdt_sync_dart/pubspec.yaml b/crdt_sync_dart/pubspec.yaml
index b76c2a9..6abcb6a 100644
--- a/crdt_sync_dart/pubspec.yaml
+++ b/crdt_sync_dart/pubspec.yaml
@@ -2,7 +2,7 @@ name: crdt_sync
 description: >-
   Shared CRDT merge scheme + GitHub-Contents-API sync transport for
   cross-device apps. Pure Dart, no Flutter dependency.
-version: 0.12.0
+version: 0.12.1
 publish_to: 'none'
 
 environment:
diff --git a/crdt_sync_dart/test/default_client_and_io_test.dart b/crdt_sync_dart/test/default_client_and_io_test.dart
index 74d898e..2bbddaf 100644
--- a/crdt_sync_dart/test/default_client_and_io_test.dart
+++ b/crdt_sync_dart/test/default_client_and_io_test.dart
@@ -128,5 +128,32 @@ void main() {
       await store.write('{"hello":"world"}');
       expect(await store.read(), '{"hello":"world"}');
     });
+
+    test('50 overlapping writes all succeed and the last issued wins',
+        () async {
+      // Regression: every call shared one `<file>.<pid>.tmp`, so the first
+      // rename moved it away and a later rename threw PathNotFoundException.
+      final store = FileLogPersistence(File('${dir.path}/log.json'));
+      await Future.wait([
+        for (var i = 0; i < 50; i++) store.write('{"n":$i}'),
+      ]);
+      expect(await store.read(), '{"n":49}');
+      expect(dir.listSync().map((e) => e.uri.pathSegments.last),
+          ['log.json']);
+    });
+
+    test('a failed write rejects its caller but does not block later writes',
+        () async {
+      final target = File('${dir.path}/log.json');
+      // A directory at the target path makes the rename fail.
+      Directory(target.path).createSync();
+      final store = FileLogPersistence(target);
+      await expectLater(
+          store.write('{"n":0}'), throwsA(isA<FileSystemException>()));
+
+      Directory(target.path).deleteSync();
+      await store.write('{"n":1}');
+      expect(await store.read(), '{"n":1}');
+    });
   });
 }