Fixed a race condition in close. The operation may not be dequeued by the
second loop and the closing process may block forever in rendez.

Notes:
Thu Jul 15 13:45:31 EDT 2010 geoff
    We ultimately installed a later channel.c from Gorka.


Reference: /n/sources/patch/applied/race-close
Date: Tue May 11 17:59:14 CES 2010
Signed-off-by: paurea@lsub.org
Reviewed-by: geoff

--- /sys/src/libthread/channel.c	Tue May 11 17:59:01 2010
+++ /sys/src/libthread/channel.c	Tue May 11 17:58:58 2010
@@ -225,6 +225,8 @@
 {
 	Alt *a;
 	int i, s, some;
+	char stags[16];
+	char *q = stags;
 
 	s = _procsplhi();	/* note handlers; see :/^alt */
 	lock(&chanlock);
@@ -246,7 +248,7 @@
 	do{
 		some = 0;
 		for(i = 0; i < c->nentry; i++){
-			if((a = c->qentry[i]) == nil || *a->tag != nil)
+			if(q[i] != 0 || (a = c->qentry[i]) == nil || *a->tag != nil)
 				continue;
 			if(a->op != CHANSND && (a->op != CHANRCV || c->n != 0))
 				continue;
@@ -255,6 +257,11 @@
 			_procsplx(s);
 			while(_threadrendezvous(a->tag, Closed) == Intred)
 				;
+			if(i >= nelem(stags)){		/* should happen seldom */
+				q = mallocz(c->nentry, 1);
+				memmove(q, stags, nelem(stags));
+			}
+			q[i] = 1;
 			s = _procsplhi();
 			lock(&chanlock);
 			some++;
@@ -266,6 +273,8 @@
 		_chanfree(c);
 	unlock(&chanlock);
 	_procsplx(s);
+	if(q != stags)
+		free(q);
 	return 0;
 }