test_python_callback2.cc

// Author: Pranav Sharma
#include <pybind11/pybind11.h>
#include <pybind11/functional.h>
#include <thread>
#include <iostream>
#include <chrono>
#include <mutex>
#include <queue>

#define STRINGIFY(x) #x
#define MACRO_STRINGIFY(x) STRINGIFY(x)

namespace py = pybind11;

#include <pybind11/pybind11.h>

using PyCallback = std::function<void()>;
using CppCallback = std::function<void(void*)>;

struct ThreadPool {
  std::mutex m;
  std::queue<std::function<void(void)>> q;
  std::thread t;

  void Schedule(std::function<void(void)> f) {
    std::lock_guard<std::mutex> l(m);
    q.push(f);
  }
  
  ~ThreadPool() {
    t.join();
  }
  
  ThreadPool() {
    t = std::thread([this](){
      while (true) {
	std::function<void(void)> f;      
	{
	  std::lock_guard<std::mutex> l(m);
	  if (!q.empty()) {
	    f = q.front();
	    q.pop();
	  }
	}
	if (f) {
	  f();
	}
	std::this_thread::sleep_for(std::chrono::milliseconds(100));
      }
    });
  }
};
      
ThreadPool* tp = nullptr;

struct UserData {
  PyCallback py_callback;
};

void Callback(void* user_data) {
  std::cout << "Inside CPP callback\n";
	       UserData* user_data_holder = reinterpret_cast<UserData*>(user_data);
	       PyGILState_STATE gstate;
	       gstate = PyGILState_Ensure();  
	       user_data_holder->py_callback();
	       PyGILState_Release(gstate);
}
  
  void RunAsync(CppCallback cpp_callback, void* user_data) {
    tp->Schedule([=]()->void
    {
      std::cout << "Inside thread..sleeping\n";
      std::this_thread::sleep_for(std::chrono::milliseconds(10));
      std::cout << "Inside thread...done sleeping, calling cpp callback\n";
      if (cpp_callback) {
	cpp_callback(user_data);
      }
    });
  }

PYBIND11_MODULE(test_python_callback, m) {
  m.def("init", [](){
    tp = new ThreadPool();
  });
  m.def("run_async", [](PyCallback py_callback)->void{
    auto user_data_holder = new UserData();
    user_data_holder->py_callback = py_callback;
    {
      //py::gil_scoped_release release;
      RunAsync(Callback, user_data_holder);
    }
  });
  
#ifdef VERSION_INFO
  m.attr("__version__") = MACRO_STRINGIFY(VERSION_INFO);
#else
  m.attr("__version__") = "dev";
#endif
}