Segfault in Gaudi v38r3 when using TPython::Exec

During a Gaudi developers' meeting, it was reported that using TPython::Exec causes a segfault:

I think the easiest way to reproduce the error is with GaudiExamples/TinyExperiment. What I've found to replicate the issue is to add the following lines to Gaudi/GaudiExamples/TinyExperiment/src/CheckerAlg.cpp:

#include "TPython.h" // at the top
TPython::Exec("print(True)");​   // in the operator() function

and then to run the example (GaudiExamples/TinyExperiment/FullExperiment.py).

changed milestone to %v39r0

assigned to @clemenci

I managed to have a look and, as I was expecting, it happens when starting a Gaudi job from Python (at least with gaudirun.py).

I have still to understand what is going wrong.

I do not have all the details, but to me it's a ROOT bug.

I wrote a reproducer similar in the original report (I used the initialize of a service rather than the execute of an algorithm). If I bootstrap the application with Gaudi.exe it works, but using gaudirun.py it segfaults.

If instead of just

TPython::Exec( "print('success')" );

I use

auto gstate = PyGILState_Ensure();
TPython::Exec( "print('success')" );
PyGILState_Release(gstate);

everything works as expected.

The problem comes from the fact that when gaudirun.py starts the main C++ function, it releases the GIL (because we use ctypes.CDLL). When TPython::Exec is invoked the first time it tries to call TPython::Initialize which correctly detects that the Python interpreter is initialized and it proceeds to invoke a Python C API function, but the GIL is not acquired and you get a segmentation fault.

I manage to reproduce the behaviour in just a few lines of code (see reproducer.tar.gz): one library function that invokes TPython::Exec, then I call it from an executable (), via ctypes.CDLL acquiring the GIL (), via ctypes.CDLL without the GIL ().

I'll discuss with ROOT experts about what can be done.

FTR: https://github.com/root-project/root/pull/16427

I'm closing this as there is nothing to do in Gaudi.

closed