readAll

fn readAll(
    ev: *Evented,
    cancel_region: *CancelRegion,
    fd: fd_t,
    buffer: []u8,
) (File.Reader.Error || error{EndOfStream})!void {
    var index: usize = 0;
    while (buffer.len - index != 0) {
        const len = try ev.preadv(cancel_region, fd, &.{
            .{ .base = buffer[index..].ptr, .len = buffer.len - index },
        }, null);
        if (len == 0) return error.EndOfStream;
        index += len;
    }
}