runtime: fix select pass 3

Fixes #2075

R=rsc, ken, r
CC=golang-dev
https://golang.org/cl/4748045

src/pkg/runtime/chan.c

@@ -1167,12 +1167,15 @@ loop:
 static void
 dequeueg(WaitQ *q, Hchan *c)
 {
-	SudoG **l, *sgp;
-	
-	for(l=&q->first; (sgp=*l) != nil; l=&sgp->link) {
+	SudoG **l, *sgp, *prevsgp;
+
+	prevsgp = nil;
+	for(l=&q->first; (sgp=*l) != nil; l=&sgp->link, prevsgp=sgp) {
 		if(sgp->g == g) {
 			*l = sgp->link;
 			freesg(c, sgp);
+			if(q->last == sgp)
+				q->last = prevsgp;
 			break;
 		}
 	}

test/chan/select6.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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.
+
+// Issue 2075
+// A bug in select corrupts channel queues of failed cases
+// if there are multiple waiters on those channels and the
+// select is the last in the queue. If further waits are made
+// on the channel without draining it first then those waiters
+// will never wake up. In the code below c1 is such a channel.
+
+package main
+
+func main() {
+	c1 := make(chan bool)
+	c2 := make(chan bool)
+	c3 := make(chan bool)
+	go func() { <-c1 }()
+	go func() {
+		select {
+		case <-c1:
+			panic("dummy")
+		case <-c2:
+			c3 <- true
+		}
+		<-c1
+	}()
+	go func() { c2 <- true }()
+	<-c3
+	c1 <- true
+	c1 <- true
+}