change channel read to clear

data just read from the channel.
this will make it easier to
recognize when to garbage
collect and finalize.

R=rsc
CC=golang-dev
https://golang.org/cl/882043

src/pkg/runtime/chan.c

@@ -300,6 +300,7 @@ loop:
 	sg = dequeue(&c->sendq, c);
 	if(sg != nil) {
 		c->elemalg->copy(c->elemsize, ep, sg->elem);
+		c->elemalg->copy(c->elemsize, sg->elem, nil);
 
 		gp = sg->g;
 		gp->param = sg;
@@ -331,6 +332,7 @@ loop:
 		goto loop;
 
 	c->elemalg->copy(c->elemsize, ep, sg->elem);
+	c->elemalg->copy(c->elemsize, sg->elem, nil);
 	freesg(c, sg);
 	unlock(c);
 	return;
@@ -356,6 +358,7 @@ asynch:
 		goto asynch;
 	}
 	c->elemalg->copy(c->elemsize, ep, c->recvdataq->elem);
+	c->elemalg->copy(c->elemsize, c->recvdataq->elem, nil);
 	c->recvdataq = c->recvdataq->link;
 	c->qcount--;
 	sg = dequeue(&c->sendq, c);
@@ -778,8 +781,10 @@ loop:
 			sel, c, cas, cas->send, o);
 
 	if(!cas->send) {
-		if(cas->u.elemp != nil)
+		if(cas->u.elemp != nil) {
 			c->elemalg->copy(c->elemsize, cas->u.elemp, sg->elem);
+			c->elemalg->copy(c->elemsize, sg->elem, nil);
+		}
 	}
 
 	freesg(c, sg);
@@ -787,8 +792,10 @@ loop:
 
 asyncrecv:
 	// can receive from buffer
-	if(cas->u.elemp != nil)
+	if(cas->u.elemp != nil) {
 		c->elemalg->copy(c->elemsize, cas->u.elemp, c->recvdataq->elem);
+		c->elemalg->copy(c->elemsize, c->recvdataq->elem, nil);
+	}
 	c->recvdataq = c->recvdataq->link;
 	c->qcount--;
 	sg = dequeue(&c->sendq, c);
@@ -817,8 +824,10 @@ syncrecv:
 	// can receive from sleeping sender (sg)
 	if(debug)
 		printf("syncrecv: sel=%p c=%p o=%d\n", sel, c, o);
-	if(cas->u.elemp != nil)
+	if(cas->u.elemp != nil) {
 		c->elemalg->copy(c->elemsize, cas->u.elemp, sg->elem);
+		c->elemalg->copy(c->elemsize, sg->elem, nil);
+	}
 	gp = sg->g;
 	gp->param = sg;
 	ready(gp);