From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <0fee984c7b0eb63ea4f0ff13c7945c2c@coraid.com>
From: erik quanstrom <quanstro@coraid.com>
Date: Mon, 18 Dec 2006 15:00:51 -0500
To: rsc@swtch.com, 9fans@cse.psu.edu
Subject: Re: [9fans] wait hang
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On Mon Dec 18 14:27:31 EST 2006, rsc@swtch.com wrote:
> I'm not really convinced this part of rc ever worked right to
> begin with, so this is no big loss.

you're right.  this feature of rc doesn't work as expected.  and your example 
illustrates this well.  for example if we try 20 wait records instead of 128:

	; sleep 1000000 &
	; pids=()
	; for(i in `{seq 20}){
		  echo foo >/dev/null &
		  pids=($pids $apid)
		}
	; for(p in $pids){
			echo $p
		  	wait $p
		}
	4627
	[hang]

since 20 is much less than 128, this can't be the kernel.  i think the
problem is in the discarded wait messages here: rc/plan9.c/^Waitfor

int
Waitfor(int pid, int)
{
	thread *p;
	Waitmsg *w;
	char errbuf[ERRMAX];

	while((w = wait()) != nil){
		if(w->pid==pid){
			setstatus(w->msg);
			free(w);
			return 0;
		}
		for(p = runq->ret;p;p = p->ret)
			if(p->pid==w->pid){
				p->pid=-1;
				strcpy(p->status, w->msg);
			}
		free(w);
	}

	errstr(errbuf, sizeof errbuf);
	if(strcmp(errbuf, "interrupted")==0) return -1;
	return 0;
}

byron's rc solves this problem by maintaining it's own waitlist in $apids.

	; for(i in 1 2 3 4) { echo fu>/dev/null&}
	31427
	31428
	31429
	31430
	; echo $apids
	31427 31428 31429 31430
	; echo $apids
	31427 31428 31429 31430
	; wait $apids(1)
	; echo $apids
	31428 31429 31430
	; wait $apids(3)
	; echo $apids
	
	; wait 31428
	;

i would think the right way to go may be to either remove
"wait $pid" from rc's vocabulary or do something like byron's rc.

- erik