Linux epoll: Register events with zero event mask.

[wulf]

Summary

Such events are legal and should be interpreted as EPOLLERR | EPOLLHUP.
Register a disabled kqueue event in that case as we do not support EPOLLHUP yet.

Required for Linux Steam client.

PR/240590

Test Plan

/* Sample testcase */

#include <sys/epoll.h>
#include <assert.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
        struct epoll_event event;
        int epfd;

        epfd = epoll_create1(EPOLL_CLOEXEC);
        assert(epfd);

        event.events = 0;

        if (epoll_ctl(epfd, EPOLL_CTL_ADD, STDIN_FILENO, &event) != 0) {
                perror("EPOLL_CTL_ADD failed");
                exit(EXIT_FAILURE);
        }

        if (epoll_ctl(epfd, EPOLL_CTL_MOD, STDIN_FILENO, &event) != 0) {
                perror("EPOLL_CTL_MOD failed");
                exit(EXIT_FAILURE);
        }

        if (epoll_ctl(epfd, EPOLL_CTL_DEL, STDIN_FILENO, &event) != 0) {
                perror("EPOLL_CTL_DEL failed");
                exit(EXIT_FAILURE);
        }

        printf("ok\n");
        return (EXIT_SUCCESS);
}