Linux epoll: Don't deregister file descriptor after EPOLLONESHOT is fired

[wulf]

Summary

Linux epoll does not remove descriptor after one-shot event has been triggered.
Set EV_DISPATCH kqueue flag rather then EV_ONESHOT to get the same behavior.

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, events[1];
        int epfd, err, n;

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

        event.events = EPOLLOUT | EPOLLONESHOT;

        err = epoll_ctl(epfd, EPOLL_CTL_ADD, STDOUT_FILENO, &event);
        assert(err == 0);

        n = epoll_wait(epfd, events, 1, -1);
        assert(n > 0);

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

        n = epoll_wait(epfd, events, 1, -1);
        assert(n > 0);

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

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