[racket] Getting unusual behavior from racket/sandbox: not truly capping memory usage?!

Tracked it.  There's something funky going on with poll-or-spawn, right here:

    https://github.com/plt/racket/blob/master/collects/racket/port.rkt#L1122-L1125

I don't quite understand what's going on here yet, but the highlighted
thread is always blocking on nack, and never gets killed.  It's this
thread that's accumulating in the loop of:

    #lang racket
    (define ip (open-input-string "hello world"))
    (define evt (peek-bytes-evt 1 0 #f ip))
    (let loop ()
      (sync (handle-evt evt
                        (lambda (res)
                          (loop)))))