get_default_random and multiple DLLs (hidden visibility)

[mglisse]

Issue Details

Gudhi contains several python modules, written in C++, using CGAL, and using pybind11 or nanobind (which implies -fvisibility=hidden on linux). To get reproducible results, users want to be able to set the seed of the random generator. However, each DLL ends up with its own copy of the random generator, get_default_random gives a different pointer in each DLL. That makes things complicated. Some options I see are

1. Force a unique (per thread) global generator, with things like

namespace CGAL { Random& get_default_random() __attribute__((visibility("default"))); }

or

namespace CGAL { struct Random; __declspec(dllimport) Random& get_default_random(); }

plus the corresponding dllexport in one DLL.

2. Do something in each module on load (register its generator in some global structure, check if a seed has been set, etc).
3. Merge all those submodules into one mega-module.
4. Tweak the build scripts to remove -fvisibility=hidden (does not help on windows)
5. Modify CGAL (how? ensure that every function that uses get_default_random can also take an argument to override it, like many already do?).

Do you have other ideas? What do you recommend?

Source Code

If you want to reproduce the behavior with a toy example on linux:

i.h

struct A{};
A& i(){ thread_local A a; return a; }

f.cc

#include "i.h"
__attribute__((visibility("default")))
void*f(){return &i();}

g.cc

#include "i.h"
__attribute__((visibility("default")))
void*g(){return &i();}

m.cc

#include <iostream>
void*f();
void*g();
int main(){
  std::cout << f() << ' ' << g() << '\n';
}

g++ f.cc -fPIC -shared -o libf.so -fvisibility=hidden -O2
g++ g.cc -fPIC -shared -o libg.so -fvisibility=hidden -O2
g++ m.cc -L. -Wl,-R. -lf -lg -O2

Without -fvisibility=hidden, I get the same pointer. But with it, I don't (and my understanding is that windows misbehaves this way whatever the options).