fix(loggers): add explicit sync between writer thread and callbacks

Add writer_ready atomic flag to ensure the writer thread is fully
initialized before callbacks can fire. This provides a clear
happens-before relationship that TSAN can verify.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

include/behaviortree_cpp/loggers/bt_sqlite_logger.h

@@ -100,6 +100,7 @@ class SqliteLogger : public StatusChangeLogger
 
   std::thread writer_thread_;
   std::atomic_bool loop_ = true;
+  std::atomic_bool writer_ready_ = false;
 
   ExtraCallback extra_func_;
 

src/loggers/bt_file_logger_v2.cpp

@@ -37,6 +37,7 @@ struct FileLogger2::Pimpl
 
   std::thread writer_thread;
   std::atomic_bool loop = true;
+  std::atomic_bool writer_ready = false;  // Signals writer thread is in wait loop
 };
 
 FileLogger2::FileLogger2(const BT::Tree& tree, std::filesystem::path const& filepath)
@@ -82,6 +83,12 @@ FileLogger2::FileLogger2(const BT::Tree& tree, std::filesystem::path const& file
   _p->file_stream.write(write_buffer.data(), 8);
 
   _p->writer_thread = std::thread(&FileLogger2::writerLoop, this);
+
+  // Wait for writer thread to be ready before subscribing to callbacks
+  while(!_p->writer_ready.load(std::memory_order_acquire))
+  {
+    std::this_thread::yield();
+  }
   subscribeToTreeChanges(tree.rootNode());
 }
 
@@ -118,6 +125,9 @@ void FileLogger2::writerLoop()
   // local buffer in this thread
   std::deque<Transition> transitions;
 
+  // Signal that writer is ready to receive callbacks
+  _p->writer_ready.store(true, std::memory_order_release);
+
   while(_p->loop)
   {
     transitions.clear();

src/loggers/bt_sqlite_logger.cpp

@@ -129,6 +129,12 @@ SqliteLogger::SqliteLogger(const Tree& tree, std::filesystem::path const& filepa
   }
 
   writer_thread_ = std::thread(&SqliteLogger::writerLoop, this);
+
+  // Wait for writer thread to be ready before subscribing to callbacks
+  while(!writer_ready_.load(std::memory_order_acquire))
+  {
+    std::this_thread::yield();
+  }
   subscribeToTreeChanges(tree.rootNode());
 }
 
@@ -205,6 +211,9 @@ void SqliteLogger::writerLoop()
 {
   std::deque<Transition> transitions;
 
+  // Signal that writer is ready to receive callbacks
+  writer_ready_.store(true, std::memory_order_release);
+
   while(loop_)
   {
     transitions.clear();