C++ libhiredis with libev and custom event loop

Question

My application uses libhiredis with libev backend. I need to send Redis async commands and process the resulting Redis async callback. However, unlike the simple example from here I cannot use the default event loop. The following code approximates the example with a custom event loop. However, when compiled with only the redisLibevAttach() induced libev io watcher, the event loop thread terminates immediately. You can see this by running

g++ -g -std=c++11 -Wall -Wextra -Werror hiredis_ev.cpp -o hiredis_ev -lpthread -lhiredis -lev && gdb ./hiredis_ev

where GDB happily prints that a new thread is created and almost immediately terminates. This is further confirmed by running info thread in GDB which does not show my_ev_loop. However, if I change the code to add any other libev watcher, like a timer, then everything is good. You can see this by running

g++ -g -DTIMER -std=c++11 -Wall -Wextra -Werror hiredis_ev.cpp -o hiredis_ev -lpthread -lhiredis -lev &&  ./hiredis_ev

I should not need a dummy libev timer to keep the event loop running. What am I missing?

#include <iostream>
#include <thread>
#include <hiredis/hiredis.h>
#include <hiredis/async.h>
#include <hiredis/adapters/libev.h>

static struct ev_loop *loop = nullptr;
static void redis_async_cb(redisAsyncContext *, void *, void *)
{
  std::cout << "Redis async callback" << std::endl;
  fflush(nullptr);
}

#ifdef TIMER
  static ev_timer timer_w;
  static void ev_timer_cb(EV_P_ ev_timer *, int)
  {
    std::cout << "EV timer callback" << std::endl;
    fflush(nullptr);
  }
#endif

int main()
{
  loop = ev_loop_new(EVFLAG_AUTO);

#ifdef TIMER
  ev_timer_init(&timer_w, ev_timer_cb, 0, 0.1);
  ev_timer_start(loop, &timer_w);
#endif

  redisAsyncContext* async_context = redisAsyncConnect("localhost", 6379);
  if (nullptr == async_context)
  {
    throw std::runtime_error("No redis async context");
  }

  redisLibevAttach(loop, async_context);
  std::thread ev_thread(ev_run, loop, 0); 
  pthread_setname_np(ev_thread.native_handle(), "my_ev_loop");
  ev_thread.detach();

  // Give the event loop time to start
  while (!ev_iteration(loop))
  {
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
  }

  // Send a SUBSCRIBE message which should generate an async callback
  if (REDIS_OK != redisAsyncCommand(async_context, redis_async_cb, nullptr, "SUBSCRIBE foo"))
  {
    throw std::runtime_error("Could not issue redis async command");
  }
  std::cout << "Waiting for async callback" << std::endl;
  fflush(nullptr);
    fflush(nullptr);

  // Wait forever (use CTRL-C to terminate)
  while (true)
  {
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
  }

  return 0;
}

Answer 1

I found out that hiredis community has their own GitHub instance where I can ask questions. Since I had't yet received the answer here, I asked there. The answer can be found at https://github.com/redis/hiredis/issues/801#issuecomment-626400959