requestCancel

    fn requestCancel(fiber: *Fiber, ev: *Evented) void {
        const cancel_status = @atomicRmw(
            Fiber.CancelStatus,
            &fiber.cancel_status,
            .Or,
            .{ .requested = true, .awaiting = @fromBackingInt(@intCast(0)) },
            .acquire,
        );
        assert(!cancel_status.requested);
        switch (cancel_status.awaiting) {
            .nothing => {},
            .group => {
                // The awaiter received a cancelation request while awaiting a group,
                // so propagate the cancelation to the group.
                if (fiber.status.awaiting_group.cancel(ev, null)) {
                    fiber.status = .{ .queue_next = null };
                    _ = ev.schedule(.current(), .{ .head = fiber, .tail = fiber });
                }
            },
            _ => |awaiting| {
                const awaiting_io_uring_fd = awaiting.toIoUringFd();
                const thread: *Thread = .current();
                thread.enqueue().* = if (thread.io_uring.fd == awaiting_io_uring_fd) .{
                    .opcode = .ASYNC_CANCEL,
                    .flags = linux.IOSQE_CQE_SKIP_SUCCESS,
                    .ioprio = 0,
                    .fd = 0,
                    .off = 0,
                    .addr = @intFromPtr(fiber),
                    .len = 0,
                    .rw_flags = 0,
                    .user_data = @backingInt(Completion.Userdata.wakeup),
                    .buf_index = 0,
                    .personality = 0,
                    .splice_fd_in = 0,
                    .addr3 = 0,
                    .resv = 0,
                } else .{
                    .opcode = .MSG_RING,
                    .flags = linux.IOSQE_CQE_SKIP_SUCCESS,
                    .ioprio = 0,
                    .fd = awaiting_io_uring_fd,
                    .off = @intFromPtr(fiber) | 0b01,
                    .addr = @backingInt(linux.IORING_MSG_RING_COMMAND.DATA),
                    .len = 0,
                    .rw_flags = 0,
                    .user_data = @backingInt(Completion.Userdata.cleanup),
                    .buf_index = 0,
                    .personality = 0,
                    .splice_fd_in = 0,
                    .addr3 = 0,
                    .resv = 0,
                };
            },
        }
    }