Fix TSAN race: add mutex protection for file_stream access

The flush() method was accessing file_stream without synchronization
while the writer thread was also accessing it. This adds a file_mutex
to protect all file_stream operations in both flush() and writerLoop().

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

Author: facontidavide

src/loggers/bt_file_logger_v2.cpp

@@ -19,6 +19,7 @@ int64_t ToUsec(Duration ts)
 struct detail::FileLogger2Private
 {
   std::ofstream file_stream;
+  std::mutex file_mutex;  // Protects file_stream access from multiple threads
 
   Duration first_timestamp = {};
 
@@ -132,6 +133,7 @@ void FileLogger2::callback(Duration timestamp, const TreeNode& node,
 
 void FileLogger2::flush()
 {
+  const std::scoped_lock lock(_p->file_mutex);
   _p->file_stream.flush();
 }
 
@@ -159,18 +161,21 @@ void FileLogger2::writerLoop()
       // simple way to pop all the transitions from _p->transitions_queue into transitions
       std::swap(transitions, _p->transitions_queue);
     }
-    while(!transitions.empty())
     {
-      const auto trans = transitions.front();
-      std::array<char, 9> write_buffer{};
-      std::memcpy(write_buffer.data(), &trans.timestamp_usec, 6);
-      std::memcpy(write_buffer.data() + 6, &trans.node_uid, 2);
-      std::memcpy(write_buffer.data() + 8, &trans.status, 1);
-
-      _p->file_stream.write(write_buffer.data(), 9);
-      transitions.pop_front();
+      const std::scoped_lock file_lock(_p->file_mutex);
+      while(!transitions.empty())
+      {
+        const auto trans = transitions.front();
+        std::array<char, 9> write_buffer{};
+        std::memcpy(write_buffer.data(), &trans.timestamp_usec, 6);
+        std::memcpy(write_buffer.data() + 6, &trans.node_uid, 2);
+        std::memcpy(write_buffer.data() + 8, &trans.status, 1);
+
+        _p->file_stream.write(write_buffer.data(), 9);
+        transitions.pop_front();
+      }
+      _p->file_stream.flush();
     }
-    _p->file_stream.flush();
   }
 }