Fix TSAN: move mutex init to base class constructor

The mutex initialization must happen in FileLogger2PImplBase constructor,
BEFORE StatusChangeLogger's constructor runs. StatusChangeLogger subscribes
to callbacks during construction, which may fire and access the mutex.
Having the mutex "used" by the main thread before any other thread accesses
it establishes the happens-before relationship that TSAN requires.

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

Author: facontidavide

src/loggers/bt_file_logger_v2.cpp

@@ -36,7 +36,12 @@ struct detail::FileLogger2Private
 // Base class constructor - initializes _p before StatusChangeLogger
 detail::FileLogger2PImplBase::FileLogger2PImplBase()
   : _p(std::make_unique<FileLogger2Private>())
-{}
+{
+  // Acquire mutex once to establish happens-before relationship for TSAN.
+  // This must happen before StatusChangeLogger's constructor subscribes to
+  // callbacks that may access the mutex from other threads.
+  std::scoped_lock lock(_p->queue_mutex);
+}
 
 // Destructor must be defined where FileLogger2Private is complete
 detail::FileLogger2PImplBase::~FileLogger2PImplBase() = default;
@@ -87,13 +92,6 @@ FileLogger2::FileLogger2(const BT::Tree& tree, std::filesystem::path const& file
   flatbuffers::WriteScalar(write_buffer.data(), timestamp_usec);
   _p->file_stream.write(write_buffer.data(), 8);
 
-  // Acquire mutex once before starting writer thread to establish
-  // happens-before relationship for TSAN (mutex created in this thread,
-  // first accessed in writer thread)
-  {
-    std::scoped_lock lock(_p->queue_mutex);
-  }
-
   _p->writer_thread = std::thread(&FileLogger2::writerLoop, this);
 
   // Wait for writer thread to signal it's ready