Callback from Python to C++ with stl container

[joakim-hove]

Hello;

this is a simplified version of the code I have. The system as a whole is a C++ application, which has a plugin system with Python, and the plugins can call back into C++ functionality:

void callback(const std::string& msg, const std::vector<std::string>& arg_list) {
      printf("callback invoked with msg: %s and %ld arguments \n",msg.c_str(), arg_list.size());
}

py::cpp_function py_callback() {
     std::function<void(const std::string&, const std::vector<std::string>)>& cpp_func = callback;
     return py::cpp_function( cpp_func );
}


void run_python_plugin { 
     auto embedded = py::module::import("my_embedded");
     auto module = py::module::import("plugin.py");
     auto run_function = module.attr("run");
     
     run_function( 100, py_callback() );
}

The module "my_embedded" looks like:

#include <pybind11/pybind11.h>
#include <pybind11/stl_bind.h>

PYBIND11_MAKE_OPAQUE(std::vector<std::string>);

The python plugin looks like:

def run(arg1, callback):
      print(f"Running python - arg={arg1}")
      callback("Calling callback", ["ARG1", "arg2", "...])

when I run this I get a segmentation fault and the gdb backtrace looks like:

Thread 1 "test_msim_ACTIO" received signal SIGSEGV, Segmentation fault.
0x00007ffff534e3a0 in _PyErr_Fetch () from /usr/lib/x86_64-linux-gnu/libpython3.9.so.1.0
(gdb) bt
#0  0x00007ffff534e3a0 in _PyErr_Fetch () from /usr/lib/x86_64-linux-gnu/libpython3.9.so.1.0
#1  0x00007ffff6d378ff in pybind11::error_scope::error_scope (this=0x7fffffffda00) at /home/hove/work/OPM/opm-common/python/pybind11/include/pybind11/detail/common.h:715
#2  0x00007ffff6d448f6 in pybind11::error_already_set::~error_already_set (this=0x5555559aa940, __in_chrg=<optimized out>) at /home/hove/work/OPM/opm-common/python/pybind11/include/pybind11/pybind11.h:2002
....

The problem seems to with the std::vector<std::string> argument - if I replace that with a scalar argument as for instance another string or an integer this works as desired.

[joakim-hove]

Hmmm - seems that if I make a symbol args = ["ARG1", "arg2", "..."] in Python it works:

def run(arg1, callback):
      print(f"Running python - arg={arg1}")
      args = ["ARG1", "arg2", "..."]
      callback("Calling callback", args)