flush before rotating

include/spdlog/sinks/rotating_file_sink-inl.h

@@ -76,11 +76,15 @@ SPDLOG_INLINE void rotating_file_sink<Mutex>::sink_it_(const details::log_msg &m
     // rotate if the new estimated file size exceeds max size.
     // rotate only if the real size > 0 to better deal with full disk (see issue #2261).
     // we only check the real size when new_size > max_size_ because it is relatively expensive.
-    if (new_size > max_size_ && file_helper_.size() > 0)
+    if (new_size > max_size_)
+    {
+        file_helper_.flush();
+        if(file_helper_.size() > 0)
         {
             rotate_();
             new_size = formatted.size();
         }
+    }
     file_helper_.write(formatted);
     current_size_ = new_size;
 }