Sets locales before any thread start participating in the GCode processing pipeline.

Locales should be set once per any participating threads in tbb::parallel_pipeline.
It should fix the issue with appearing comma instead of the decimal point in generated Gcode.

src/libslic3r/GCode.cpp

@@ -35,6 +35,8 @@
 #include "SVG.hpp"
 
 #include <tbb/parallel_for.h>
+#include <tbb/task_scheduler_observer.h>
+#include <tbb/enumerable_thread_specific.h>
 
 // Intel redesigned some TBB interface considerably when merging TBB with their oneAPI set of libraries, see GH #7332.
 // We are using quite an old TBB 2017 U7. Before we update our build servers, let's use the old API, which is deprecated in up to date TBB.
@@ -1537,6 +1539,32 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
     print.throw_if_canceled();
 }
 
+// For unknown reasons and in sporadic cases when GCode export is processing, some participating thread
+// in tbb::parallel_pipeline has not set locales to "C", probably because this thread is newly spawned.
+// So in this class method on_scheduler_entry is called for every thread before it starts participating
+// in tbb::parallel_pipeline to ensure that locales are set correctly
+
+// For tbb::parallel_pipeline, it seems that on_scheduler_entry is called for every layer and every filter.
+// We ensure using thread-local storage that locales will be set to "C" just once for any participating thread.
+class TBBLocalesSetter : public tbb::task_scheduler_observer
+{
+public:
+    TBBLocalesSetter() { this->observe(true); }
+    ~TBBLocalesSetter() override = default;
+
+    void on_scheduler_entry(bool is_worker) override
+    {
+        if (bool &is_locales_sets = m_is_locales_sets.local(); !is_locales_sets) {
+            // Set locales of the worker thread to "C".
+            set_c_locales();
+            is_locales_sets = true;
+        }
+    }
+
+private:
+    tbb::enumerable_thread_specific<bool, tbb::cache_aligned_allocator<bool>, tbb::ets_key_usage_type::ets_key_per_instance> m_is_locales_sets;
+};
+
 // Process all layers of all objects (non-sequential mode) with a parallel pipeline:
 // Generate G-code, run the filters (vase mode, cooling buffer), run the G-code analyser
 // and export G-code into file.
@@ -1580,6 +1608,10 @@ void GCode::process_layers(
         [&output_stream](std::string s) { output_stream.write(s); }
     );
 
+    // It registers a handler that sets locales to "C" before any TBB thread starts participating in tbb::parallel_pipeline.
+    // Handler is unregistered when the destructor is called.
+    TBBLocalesSetter locales_setter;
+
     // The pipeline elements are joined using const references, thus no copying is performed.
     output_stream.find_replace_supress();
     if (m_spiral_vase && m_find_replace)
@@ -1633,6 +1665,10 @@ void GCode::process_layers(
         [&output_stream](std::string s) { output_stream.write(s); }
     );
 
+    // It registers a handler that sets locales to "C" before any TBB thread starts participating in tbb::parallel_pipeline.
+    // Handler is unregistered when the destructor is called.
+    TBBLocalesSetter locales_setter;
+
     // The pipeline elements are joined using const references, thus no copying is performed.
     output_stream.find_replace_supress();
     if (m_spiral_vase && m_find_replace)