Implementing a test whether the current thread is the main (UI) thread

and using it on AppConfig::save() to assert if save is called
from a worker thread.
The old assert was using thread names, which did not always work
on Windows.
Fixes #7839 #9178 #9370 #9420

src/PrusaSlicer.cpp

@@ -72,6 +72,8 @@ int CLI::run(int argc, char **argv)
 {
     // Mark the main thread for the debugger and for runtime checks.
     set_current_thread_name("slic3r_main");
+    // Save the thread ID of the main thread.
+    save_main_thread_id();
 
 #ifdef __WXGTK__
     // On Linux, wxGTK has no support for Wayland, and the app crashes on

src/libslic3r/AppConfig.cpp

@@ -417,12 +417,8 @@ std::string AppConfig::load()
 
 void AppConfig::save()
 {
-    {
-        // Returns "undefined" if the thread naming functionality is not supported by the operating system.
-        std::optional<std::string> current_thread_name = get_current_thread_name();
-        if (current_thread_name && *current_thread_name != "slic3r_main")
-            throw CriticalException("Calling AppConfig::save() from a worker thread!");
-    }
+    if (! is_main_thread_active())
+        throw CriticalException("Calling AppConfig::save() from a worker thread!");
 
     // The config is first written to a file with a PID suffix and then moved
     // to avoid race conditions with multiple instances of Slic3r

src/libslic3r/Thread.cpp

@@ -191,6 +191,26 @@ std::optional<std::string> get_current_thread_name()
 
 #endif // _WIN32
 
+// To be called at the start of the application to save the current thread ID as the main (UI) thread ID.
+static boost::thread::id g_main_thread_id;
+
+void save_main_thread_id()
+{
+	g_main_thread_id = boost::this_thread::get_id();
+}
+
+// Retrieve the cached main (UI) thread ID.
+boost::thread::id get_main_thread_id()
+{
+	return g_main_thread_id;
+}
+
+// Checks whether the main (UI) thread is active.
+bool is_main_thread_active()
+{
+	return get_main_thread_id() == boost::this_thread::get_id();
+}
+
 // Spawn (n - 1) worker threads on Intel TBB thread pool and name them by an index and a system thread ID.
 // Also it sets locale of the worker threads to "C" for the G-code generator to produce "." as a decimal separator.
 void name_tbb_thread_pool_threads_set_locale()

src/libslic3r/Thread.hpp

@@ -29,6 +29,13 @@ inline bool set_thread_name(boost::thread &thread, const std::string &thread_nam
 bool set_current_thread_name(const char *thread_name);
 inline bool set_current_thread_name(const std::string &thread_name) { return set_current_thread_name(thread_name.c_str()); }
 
+// To be called at the start of the application to save the current thread ID as the main (UI) thread ID.
+void save_main_thread_id();
+// Retrieve the cached main (UI) thread ID.
+boost::thread::id get_main_thread_id();
+// Checks whether the main (UI) thread is active.
+bool is_main_thread_active();
+
 // OSX specific: Set Quality of Service to "user initiated", so that the threads will be scheduled to high performance
 // cores if available.
 void set_current_thread_qos();