Protect from double call to enable_backtrace()

include/spdlog/logger-inl.h

@@ -120,14 +120,21 @@ SPDLOG_INLINE void logger::set_pattern(std::string pattern, pattern_time_type ti
     set_formatter(std::move(new_formatter));
 }
 
+
+// create new backtrace sink and hand it all sinks
 SPDLOG_INLINE void logger::enable_backtrace(level::level_enum trigger_level, size_t n_messages)
 {
+    if(!backtrace_enabled_)
+    {
+        backtrace_enabled_ = true;
         auto backtrace_sink = std::make_shared<spdlog::sinks::backtrace_sink_mt>(trigger_level, n_messages);
         backtrace_sink->set_sinks(std::move(sinks()));
         sinks().push_back(std::move(backtrace_sink));
         this->set_level(spdlog::level::trace);
+    }
 }
 
+
 // flush functions
 SPDLOG_INLINE void logger::flush()
 {

include/spdlog/logger.h

@@ -347,6 +347,7 @@ protected:
     spdlog::level_t level_{level::info};
     spdlog::level_t flush_level_{level::off};
     err_handler custom_err_handler_{nullptr};
+    bool backtrace_enabled_ {false};
 
     virtual void sink_it_(const details::log_msg &msg);
     virtual void flush_();