Fix re-registration of internal events after fork

[ceggers-arri]

If adding an (internal) descriptor to epoll fails, mark it for not being re-registered after fork (this would throw an exception).

In my case, during notify_fork() the io_uring service (re)registers with the epoll reactor. As the eventfd descriptor is already registered, EPOLL_CTL_ADD fails with EEXIST (which is ignored here). During re-registration after fork, epoll (again) refuses the 2nd registration of the epoll descriptor and the child crashes because of an exception:

Call stack during fork_prepare:

void io_uring_service::notify_fork(
    asio::execution_context::fork_event fork_ev)
{
  switch (fork_ev)
  {
  case asio::execution_context::fork_prepare:
    {
...
      // Restart and eventfd operation.
      register_with_reactor();   // <-- trouble starts here...
    }
    break;

...
}

void io_uring_service::register_with_reactor()
{
#if !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
  // ---> Return code is ignored here <---
  reactor_.register_internal_descriptor(reactor::read_op,
      event_fd_, reactor_data_, new event_fd_read_op(this));
#endif // !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
}

int epoll_reactor::register_internal_descriptor(
    int op_type, socket_type descriptor,
    epoll_reactor::per_descriptor_data& descriptor_data, reactor_op* op)
{
  ...

  epoll_event ev = { 0, { 0 } };
  ev.events = EPOLLIN | EPOLLERR | EPOLLHUP | EPOLLPRI | EPOLLET;
  descriptor_data->registered_events_ = ev.events;
  ev.data.ptr = descriptor_data;
  int result = epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, descriptor, &ev);
  if (result != 0)
    return errno;   // <--- descriptor_data doesn't reflect that epoll registration failed

  return 0;
}

Call stack during fork_child:

void epoll_reactor::notify_fork(
    asio::execution_context::fork_event fork_ev)
{
  if (fork_ev == asio::execution_context::fork_child)
  {
...
    // Re-register all descriptors with epoll.
    mutex::scoped_lock descriptors_lock(registered_descriptors_mutex_);
    for (descriptor_state* state = registered_descriptors_.first();
        state != 0; state = state->next_)
    {
      if (state->registered_events_ != 0)
      {
        ev.events = state->registered_events_;
        ev.data.ptr = state;
        int result = epoll_ctl(epoll_fd_,
            EPOLL_CTL_ADD, state->descriptor_, &ev);
        if (result != 0)
        {
          asio::error_code ec(errno,
              asio::error::get_system_category());
          asio::detail::throw_error(ec, "epoll re-registration");    // <--- bang!
        }
      }
    }
  }
}

Reproducer (should dump the output of the top command to a file):

#include <boost/asio.hpp>
#include <boost/process/v2.hpp>

int main(void)
{
	boost::system::error_code ec;
	boost::asio::io_context ioContext;
	boost::asio::stream_file outputFile(ioContext, "top.txt",
		boost::asio::stream_file::create | boost::asio::stream_file::write_only);

	boost::process::v2::process proc(ioContext,
		"/usr/bin/top", {"-H", "-b", "-n", "1"},
		boost::process::v2::process_stdio{{/*stdin default*/}, outputFile, {/*stderr default*/}});
	proc.wait();
}