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

Yes, it was a bug in the implementation of NACK evts.

The following program illustrates a problem, where a NACK becomes
ready even though its event was previously selected by `sync':

 #lang racket
 (thread
  (lambda ()
    (sync (nack-guard-evt 
           (lambda (nack)
             (thread (lambda () 
                       (sync nack) 
                       (error "wrong!")))
             always-evt)))))
 (sync (system-idle-evt))

The memory leak is a reflection of this NACK bug; a thread that is
blocked on a NACK and should be GCed along with the NACK was sticking
around --- because the thread actually can run when some other thread
(the one representing the connection in the original example)
terminates.

I've pushed a repair.

Thanks, Danny and Jay, for narrowing down the problem!

At Mon, 22 Apr 2013 14:31:52 -0600, Danny Yoo wrote:
> Ok, good!  Matthew Flatt is investigating.
> 
> It looks like something like:
> 
>     
> https://github.com/dyoo/racket/commit/6547d18c006e7390c3cbce84a93dd9d9c926341b
> 
> can clear the memory leak, but mflatt says that we shouldn't have to
> do this, that the VM should automatically kill the monitoring thread
> once it detects the nack is inaccessible.  This is totally out of my
> domain, but thankfully he's on it now.  :)
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