Function registerNotifier

src/linux/cgroups.cpp:1030–1072

In cgroups, there is mechanism which allows to get notifications about changing status of a cgroup. It is based on Linux eventfd. See more information in the kernel documentation ("Notification API"). This function will create an eventfd and write appropriate control file to correlate the eventfd with a type of event so that users can start polling on the eventfd to get notified. It returns the ev

// @return  The eventfd if the operation succeeds.
//          Error if the operation fails.
static Try<int> registerNotifier(
    const string& hierarchy,
    const string& cgroup,
    const string& control,
    const Option<string>& args = None())
{
  int efd = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
  if (efd < 0) {
    return ErrnoError("Failed to create an eventfd");
  }

  // Open the control file.
  string path = path::join(hierarchy, cgroup, control);
  Try<int> cfd = os::open(path, O_RDWR | O_CLOEXEC);
  if (cfd.isError()) {
    os::close(efd);
    return Error("Failed to open '" + path + "': " + cfd.error());
  }

  // Write the event control file (cgroup.event_control).
  ostringstream out;
  out << dec << efd << " " << cfd.get();
  if (args.isSome()) {
    out << " " << args.get();
  }

  Try<Nothing> write = cgroups::write(
      hierarchy,
      cgroup,
      "cgroup.event_control",
      out.str());

  if (write.isError()) {
    os::close(efd);
    os::close(cfd.get());
    return Error(
        "Failed to write control 'cgroup.event_control': " + write.error());
  }

  os::close(cfd.get());

  return efd;
}


// Unregister a notifier.