use random number as pid on linux

Reason: Flatpak might use same pid for several instances.

src/libslic3r/utils.cpp

@@ -12,6 +12,7 @@
 #include <ctime>
 #include <cstdarg>
 #include <stdio.h>
+#include <random>
 
 #include "Platform.hpp"
 #include "Time.hpp"
@@ -941,8 +942,25 @@ unsigned get_current_pid()
 {
 #ifdef WIN32
     return GetCurrentProcessId();
-#else
+#elif __APPLE__
     return ::getpid();
+#else
+    // On flatpak getpid() might return same number for each concurent instances.
+    static std::atomic<unsigned> instance_uuid{0};
+    if (instance_uuid == 0) {
+        unsigned generated_value;
+        {
+            // Use a thread-local random engine
+            thread_local std::random_device rd;
+            thread_local std::mt19937 generator(rd());
+            std::uniform_int_distribution<unsigned> distribution;
+            generated_value = distribution(generator);
+        }
+        unsigned expected = 0;
+        // Atomically initialize the instance_uuid if it has not been set
+        instance_uuid.compare_exchange_strong(expected, generated_value);
+    }
+    return instance_uuid.load();
 #endif
 }