Avoid references to race data filename

include/spdlog/sinks/daily_file_sink.h

@@ -70,7 +70,7 @@ public:
         }
     }
 
-    const filename_t &filename()
+    filename_t filename()
     {
         std::lock_guard<Mutex> lock(base_sink<Mutex>::mutex_);
         return file_helper_.filename();

include/spdlog/sinks/rotating_file_sink-inl.h

@@ -54,7 +54,7 @@ SPDLOG_INLINE filename_t rotating_file_sink<Mutex>::calc_filename(const filename
 }
 
 template<typename Mutex>
-SPDLOG_INLINE const filename_t &rotating_file_sink<Mutex>::filename()
+SPDLOG_INLINE filename_t rotating_file_sink<Mutex>::filename()
 {
     std::lock_guard<Mutex> lock(base_sink<Mutex>::mutex_);
     return file_helper_.filename();

include/spdlog/sinks/rotating_file_sink.h

@@ -24,7 +24,7 @@ class rotating_file_sink final : public base_sink<Mutex>
 public:
     rotating_file_sink(filename_t base_filename, std::size_t max_size, std::size_t max_files, bool rotate_on_open = false);
     static filename_t calc_filename(const filename_t &filename, std::size_t index);
-    const filename_t &filename();
+    filename_t filename();
 
 protected:
     void sink_it_(const details::log_msg &msg) override;