protect multi-job jit compilation from race conditions

Phys/FunctorCore/src/Components/Factory.cpp

@@ -16,6 +16,7 @@
 #include <chrono>
 #include <cstdio>
 #include <dlfcn.h>
+#include <filesystem>
 #include <fstream>
 #include <set>
 #include <vector>
@@ -164,10 +165,17 @@ struct FunctorFactory : public extends<Service, Functors::IFactory> {
         out.close();
       }
 
+      // In a multi job scenario we want to avoid many jobs directly writing to
+      // the same file and potentially causing problems. Thus we first create a
+      // uniquely named library and then later rename it. The rename is atomic
+      // in the sense that a dlopen call will either load the already existing
+      // file or the newly renamed one. But it can't fail due to e.g. the file
+      // not having been fully overwritten or similar weird things.
+      auto const lib_tmp_full_path = m_jit_lib_dir + "FunctorJitLib_tmp_" + std::to_string( getpid() );
       // functor_jitter is a shell script generated by cmake to invoke the
       // correct compiler with the correct flags see:
       // Phys/FunctorCore/CMakeLists.txt
-      auto cmd = "functor_jitter " + cpp_filename + " " + lib_full_path;
+      auto cmd = "functor_jitter " + cpp_filename + " " + lib_tmp_full_path;
 
       if ( msgLevel( MSG::VERBOSE ) ) { verbose() << "Command that will be executed:\n" << cmd << endmsg; }
 
@@ -178,6 +186,11 @@ struct FunctorFactory : public extends<Service, Functors::IFactory> {
       info() << "Compilation of functor library took " << total_time.count() << " seconds" << endmsg;
       if ( return_code != 0 ) { throw exception( "Non zero return code!\n" ); }
 
+      if ( msgLevel( MSG::VERBOSE ) ) {
+        verbose() << "Rename " << lib_tmp_full_path << " to " << lib_full_path << endmsg;
+      }
+      std::filesystem::rename( lib_tmp_full_path, lib_full_path );
+
       m_lib_handle = LibHandle{dlopen( lib_full_path.c_str(), RTLD_LOCAL | RTLD_LAZY ), lib_closer};
     }