runtime: correct memory leak in select

  * adds pass 3 to dequeue from channels eagerly

various other cleanup/churn:
  * use switch on cas->send in each pass to
    factor out common code.
  * longer goto labels, commented at target
  * be more agressive about can't happen:
    throw instead of print + cope.
  * use "select" instead of "selectgo" in errors
  * use printf for debug prints when possible

R=ken2, ken3
CC=golang-dev, r
https://golang.org/cl/875041

src/pkg/runtime/chan.c

@@ -79,6 +79,7 @@ struct	Select
 	Scase*	scase[1];		// one per case
 };
 
+static	void	dequeueg(WaitQ*, Hchan*);
 static	SudoG*	dequeue(WaitQ*, Hchan*);
 static	void	enqueue(WaitQ*, SudoG*);
 static	SudoG*	allocsg(Hchan*);
@@ -126,19 +127,9 @@ makechan(Type *elem, uint32 hint)
 		c->dataqsiz = hint;
 	}
 
-	if(debug) {
-		prints("makechan: chan=");
-		·printpointer(c);
-		prints("; elemsize=");
-		·printint(elem->size);
-		prints("; elemalg=");
-		·printint(elem->alg);
-		prints("; elemalign=");
-		·printint(elem->align);
-		prints("; dataqsiz=");
-		·printint(c->dataqsiz);
-		prints("\n");
-	}
+	if(debug)
+		printf("makechan: chan=%p; elemsize=%D; elemalg=%d; elemalign=%d; dataqsiz=%d\n",
+			c, (int64)elem->size, elem->alg, elem->align, c->dataqsiz);
 
 	return c;
 }
@@ -175,7 +166,12 @@ incerr(Hchan* c)
  * occur. if pres is not nil,
  * then the protocol will not
  * sleep but return if it could
- * not complete
+ * not complete.
+ *
+ * sleep can wake up with g->param == nil
+ * when a channel involved in the sleep has
+ * been closed.  it is easiest to loop and re-run
+ * the operation; we'll see that it's now closed.
  */
 void
 chansend(Hchan *c, byte *ep, bool *pres)
@@ -187,9 +183,7 @@ chansend(Hchan *c, byte *ep, bool *pres)
 		gosched();
 
 	if(debug) {
-		prints("chansend: chan=");
-		·printpointer(c);
-		prints("; elem=");
+		printf("chansend: chan=%p; elem=", c);
 		c->elemalg->print(c->elemsize, ep);
 		prints("\n");
 	}
@@ -292,11 +286,8 @@ chanrecv(Hchan* c, byte *ep, bool* pres)
 	if(gcwaiting)
 		gosched();
 
-	if(debug) {
-		prints("chanrecv: chan=");
-		·printpointer(c);
-		prints("\n");
-	}
+	if(debug)
+		printf("chanrecv: chan=%p\n", c);
 
 	lock(c);
 loop:
@@ -471,13 +462,8 @@ void
 	sel->tcase = size;
 	sel->ncase = 0;
 	*selp = sel;
-	if(debug) {
-		prints("newselect s=");
-		·printpointer(sel);
-		prints(" size=");
-		·printint(size);
-		prints("\n");
-	}
+	if(debug)
+		printf("newselect s=%p size=%d\n", sel, size);
 }
 
 // selectsend(sel *byte, hchan *chan any, elem any) (selected bool);
@@ -511,19 +497,9 @@ void
 	ae = (byte*)&sel + eo;
 	c->elemalg->copy(c->elemsize, cas->u.elem, ae);
 
-	if(debug) {
-		prints("selectsend s=");
-		·printpointer(sel);
-		prints(" pc=");
-		·printpointer(cas->pc);
-		prints(" chan=");
-		·printpointer(cas->chan);
-		prints(" so=");
-		·printint(cas->so);
-		prints(" send=");
-		·printint(cas->send);
-		prints("\n");
-	}
+	if(debug)
+		printf("selectsend s=%p pc=%p chan=%p so=%d send=%d\n",
+			sel, cas->pc, cas->chan, cas->so, cas->send);
 }
 
 // selectrecv(sel *byte, hchan *chan any, elem *any) (selected bool);
@@ -553,19 +529,9 @@ void
 	cas->send = 0;
 	cas->u.elemp = *(byte**)((byte*)&sel + eo);
 
-	if(debug) {
-		prints("selectrecv s=");
-		·printpointer(sel);
-		prints(" pc=");
-		·printpointer(cas->pc);
-		prints(" chan=");
-		·printpointer(cas->chan);
-		prints(" so=");
-		·printint(cas->so);
-		prints(" send=");
-		·printint(cas->send);
-		prints("\n");
-	}
+	if(debug)
+		printf("selectrecv s=%p pc=%p chan=%p so=%d send=%d\n",
+			sel, cas->pc, cas->chan, cas->so, cas->send);
 }
 
 
@@ -590,17 +556,9 @@ void
 	cas->send = 2;
 	cas->u.elemp = nil;
 
-	if(debug) {
-		prints("selectdefault s=");
-		·printpointer(sel);
-		prints(" pc=");
-		·printpointer(cas->pc);
-		prints(" so=");
-		·printint(cas->so);
-		prints(" send=");
-		·printint(cas->send);
-		prints("\n");
-	}
+	if(debug)
+		printf("selectdefault s=%p pc=%p so=%d send=%d\n",
+			sel, cas->pc, cas->so, cas->send);
 }
 
 static void
@@ -657,15 +615,12 @@ void
 	if(gcwaiting)
 		gosched();
 
-	if(debug) {
-		prints("selectgo: sel=");
-		·printpointer(sel);
-		prints("\n");
-	}
+	if(debug)
+		printf("select: sel=%p\n", sel);
 
 	if(sel->ncase < 2) {
 		if(sel->ncase < 1)
-			throw("selectgo: no cases");
+			throw("select: no cases");
 		// make special case of one.
 	}
 
@@ -674,9 +629,8 @@ void
 		p = fastrand1();
 		if(gcd(p, sel->ncase) == 1)
 			break;
-		if(i > 1000) {
-			throw("selectgo: failed to select prime");
-		}
+		if(i > 1000)
+			throw("select: failed to select prime");
 	}
 
 	// select an initial offset
@@ -700,43 +654,40 @@ loop:
 	dfl = nil;
 	for(i=0; i<sel->ncase; i++) {
 		cas = sel->scase[o];
-
-		if(cas->send == 2) {	// default
-			dfl = cas;
-			goto next1;
-		}
-
 		c = cas->chan;
-		if(c->dataqsiz > 0) {
-			if(cas->send) {
-				if(c->closed & Wclosed)
-					goto sclose;
-				if(c->qcount < c->dataqsiz)
-					goto asyns;
-				goto next1;
+
+		switch(cas->send) {
+		case 0:	// recv
+			if(c->dataqsiz > 0) {
+				if(c->qcount > 0)
+					goto asyncrecv;
+			} else {
+				sg = dequeue(&c->sendq, c);
+				if(sg != nil)
+					goto syncrecv;
 			}
-			if(c->qcount > 0)
-				goto asynr;
 			if(c->closed & Wclosed)
 				goto rclose;
-			goto next1;
-		}
+			break;
 
-		if(cas->send) {
+		case 1:	// send
 			if(c->closed & Wclosed)
 				goto sclose;
-			sg = dequeue(&c->recvq, c);
-			if(sg != nil)
-				goto gots;
-			goto next1;
+			if(c->dataqsiz > 0) {
+				if(c->qcount < c->dataqsiz)
+					goto asyncsend;
+			} else {
+				sg = dequeue(&c->recvq, c);
+				if(sg != nil)
+					goto syncsend;
+			}
+			break;
+
+		case 2:	// default
+			dfl = cas;
+			break;
 		}
-		sg = dequeue(&c->sendq, c);
-		if(sg != nil)
-			goto gotr;
-		if(c->closed & Wclosed)
-			goto rclose;
 
-	next1:
 		o += p;
 		if(o >= sel->ncase)
 			o -= sel->ncase;
@@ -752,52 +703,34 @@ loop:
 	for(i=0; i<sel->ncase; i++) {
 		cas = sel->scase[o];
 		c = cas->chan;
+		sg = allocsg(c);
+		sg->offset = o;
 
-		if(c->dataqsiz > 0) {
-			if(cas->send) {
-				if(c->qcount < c->dataqsiz) {
-					prints("selectgo: pass 2 async send\n");
-					goto asyns;
-				}
-				sg = allocsg(c);
-				sg->offset = o;
-				enqueue(&c->sendq, sg);
-				goto next2;
-			}
-			if(c->qcount > 0) {
-				prints("selectgo: pass 2 async recv\n");
-				goto asynr;
+		switch(cas->send) {
+		case 0:	// recv
+			if(c->dataqsiz > 0) {
+				if(c->qcount > 0)
+					throw("select: pass 2 async recv");
+			} else {
+				if(dequeue(&c->sendq, c))
+					throw("select: pass 2 sync recv");
 			}
-			sg = allocsg(c);
-			sg->offset = o;
 			enqueue(&c->recvq, sg);
-			goto next2;
-		}
-
-		if(cas->send) {
-			sg = dequeue(&c->recvq, c);
-			if(sg != nil) {
-				prints("selectgo: pass 2 sync send\n");
-				g->selgen++;
-				goto gots;
+			break;
+		
+		case 1:	// send
+			if(c->dataqsiz > 0) {
+				if(c->qcount < c->dataqsiz)
+					throw("select: pass 2 async send");
+			} else {
+				if(dequeue(&c->recvq, c))
+					throw("select: pass 2 sync send");
+				c->elemalg->copy(c->elemsize, sg->elem, cas->u.elem);
 			}
-			sg = allocsg(c);
-			sg->offset = o;
-			c->elemalg->copy(c->elemsize, sg->elem, cas->u.elem);
 			enqueue(&c->sendq, sg);
-			goto next2;
-		}
-		sg = dequeue(&c->sendq, c);
-		if(sg != nil) {
-			prints("selectgo: pass 2 sync recv\n");
-			g->selgen++;
-			goto gotr;
+			break;
 		}
-		sg = allocsg(c);
-		sg->offset = o;
-		enqueue(&c->recvq, sg);
 
-	next2:
 		o += p;
 		if(o >= sel->ncase)
 			o -= sel->ncase;
@@ -810,6 +743,24 @@ loop:
 
 	sellock(sel);
 	sg = g->param;
+
+	// pass 3 - dequeue from unsuccessful chans
+	// otherwise they stack up on quiet channels
+	for(i=0; i<sel->ncase; i++) {
+		if(sg == nil || o != sg->offset) {
+			cas = sel->scase[o];
+			c = cas->chan;
+			if(cas->send)
+				dequeueg(&c->sendq, c);
+			else
+				dequeueg(&c->recvq, c);
+		}
+		
+		o += p;
+		if(o >= sel->ncase)
+			o -= sel->ncase;
+	}
+
 	if(sg == nil)
 		goto loop;
 
@@ -822,19 +773,9 @@ loop:
 		goto loop;
 	}
 
-	if(debug) {
-		prints("wait-return: sel=");
-		·printpointer(sel);
-		prints(" c=");
-		·printpointer(c);
-		prints(" cas=");
-		·printpointer(cas);
-		prints(" send=");
-		·printint(cas->send);
-		prints(" o=");
-		·printint(o);
-		prints("\n");
-	}
+	if(debug)
+		printf("wait-return: sel=%p c=%p cas=%p send=%d o=%d\n",
+			sel, c, cas, cas->send, o);
 
 	if(!cas->send) {
 		if(cas->u.elemp != nil)
@@ -844,7 +785,8 @@ loop:
 	freesg(c, sg);
 	goto retc;
 
-asynr:
+asyncrecv:
+	// can receive from buffer
 	if(cas->u.elemp != nil)
 		c->elemalg->copy(c->elemsize, cas->u.elemp, c->recvdataq->elem);
 	c->recvdataq = c->recvdataq->link;
@@ -857,7 +799,8 @@ asynr:
 	}
 	goto retc;
 
-asyns:
+asyncsend:
+	// can send to buffer
 	if(cas->u.elem != nil)
 		c->elemalg->copy(c->elemsize, c->senddataq->elem, cas->u.elem);
 	c->senddataq = c->senddataq->link;
@@ -870,17 +813,10 @@ asyns:
 	}
 	goto retc;
 
-gotr:
-	// recv path to wakeup the sender (sg)
-	if(debug) {
-		prints("gotr: sel=");
-		·printpointer(sel);
-		prints(" c=");
-		·printpointer(c);
-		prints(" o=");
-		·printint(o);
-		prints("\n");
-	}
+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)
 		c->elemalg->copy(c->elemsize, cas->u.elemp, sg->elem);
 	gp = sg->g;
@@ -889,23 +825,17 @@ gotr:
 	goto retc;
 
 rclose:
+	// read at end of closed channel
 	if(cas->u.elemp != nil)
 		c->elemalg->copy(c->elemsize, cas->u.elemp, nil);
 	c->closed |= Rclosed;
 	incerr(c);
 	goto retc;
 
-gots:
-	// send path to wakeup the receiver (sg)
-	if(debug) {
-		prints("gots: sel=");
-		·printpointer(sel);
-		prints(" c=");
-		·printpointer(c);
-		prints(" o=");
-		·printint(o);
-		prints("\n");
-	}
+syncsend:
+	// can send to sleeping receiver (sg)
+	if(debug)
+		printf("syncsend: sel=%p c=%p o=%d\n", sel, c, o);
 	if(c->closed & Wclosed)
 		goto sclose;
 	c->elemalg->copy(c->elemsize, sg->elem, cas->u.elem);
@@ -915,12 +845,14 @@ gots:
 	goto retc;
 
 sclose:
+	// send on closed channel
 	incerr(c);
 	goto retc;
 
 retc:
 	selunlock(sel);
 
+	// return to pc corresponding to chosen case
 	·setcallerpc(&sel, cas->pc);
 	as = (byte*)&sel + cas->so;
 	freesel(sel);
@@ -1020,6 +952,20 @@ loop:
 	return sgp;
 }
 
+static void
+dequeueg(WaitQ *q, Hchan *c)
+{
+	SudoG **l, *sgp;
+	
+	for(l=&q->first; (sgp=*l) != nil; l=&sgp->link) {
+		if(sgp->g == g) {
+			*l = sgp->link;
+			freesg(c, sgp);
+			break;
+		}
+	}
+}
+
 static void
 enqueue(WaitQ *q, SudoG *sgp)
 {

test/chan/select2.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "runtime"
+
+func sender(c chan int, n int) {
+	for i := 0; i < n; i++ {
+		c <- 1
+	}
+}
+
+func receiver(c, dummy chan int, n int) {
+	for i := 0; i < n; i++ {
+		select {
+		case <-c:
+			// nothing
+		case <-dummy:
+			panic("dummy")
+		}
+	}
+}
+
+func main() {
+	runtime.MemProfileRate = 0
+
+	c := make(chan int)
+	dummy := make(chan int)
+
+	// warm up
+	go sender(c, 100000)
+	receiver(c, dummy, 100000)
+	runtime.GC()
+	runtime.MemStats.Alloc = 0
+
+	// second time shouldn't increase footprint by much
+	go sender(c, 100000)
+	receiver(c, dummy, 100000)
+	runtime.GC()
+
+	if runtime.MemStats.Alloc > 1e5 {
+		println("BUG: too much memory for 100,000 selects:", runtime.MemStats.Alloc)
+	}
+}