runtime: dequeue the correct SudoG

select {
       case <- c:
       case <- c:
}

In this case, c.recvq lists two SudoGs which have the same G.
So we can't use the G as the key to dequeue the correct SudoG,
as that key is ambiguous.  Dequeueing the wrong SudoG ends up
freeing a SudoG that is still in c.recvq.

The fix is to use the actual SudoG pointer as the key.

LGTM=dvyukov
R=rsc, bradfitz, dvyukov, khr
CC=austin, golang-codereviews
https://golang.org/cl/159040043

src/runtime/chan_test.go

@@ -482,6 +482,35 @@ func TestShrinkStackDuringBlockedSend(t *testing.T) {
 	<-done
 }
 
+func TestSelectDuplicateChannel(t *testing.T) {
+	// This test makes sure we can queue a G on
+	// the same channel multiple times.
+	c := make(chan int)
+	d := make(chan int)
+	e := make(chan int)
+
+	// goroutine A
+	go func() {
+		select {
+		case <-c:
+		case <-c:
+		case <-d:
+		}
+		e <- 9
+	}()
+	time.Sleep(time.Millisecond) // make sure goroutine A gets qeueued first on c
+
+	// goroutine B
+	go func() {
+		<-c
+	}()
+	time.Sleep(time.Millisecond) // make sure goroutine B gets queued on c before continuing
+
+	d <- 7 // wake up A, it dequeues itself from c.  This operation used to corrupt c.recvq.
+	<-e    // A tells us it's done
+	c <- 8 // wake up B.  This operation used to fail because c.recvq was corrupted (it tries to wake up an already running G instead of B)
+}
+
 func BenchmarkChanNonblocking(b *testing.B) {
 	myc := make(chan int)
 	b.RunParallel(func(pb *testing.PB) {

src/runtime/select.go

@@ -398,9 +398,9 @@ loop:
 		} else {
 			c = k._chan
 			if k.kind == _CaseSend {
-				c.sendq.dequeueg(gp)
+				c.sendq.dequeueSudoG(sglist)
 			} else {
-				c.recvq.dequeueg(gp)
+				c.recvq.dequeueSudoG(sglist)
 			}
 		}
 		sgnext = sglist.waitlink
@@ -628,7 +628,7 @@ func reflect_rselect(cases []runtimeSelect) (chosen int, recvOK bool) {
 	return
 }
 
-func (q *waitq) dequeueg(gp *g) {
+func (q *waitq) dequeueSudoG(s *sudog) {
 	var prevsgp *sudog
 	l := &q.first
 	for {
@@ -636,7 +636,7 @@ func (q *waitq) dequeueg(gp *g) {
 		if sgp == nil {
 			return
 		}
-		if sgp.g == gp {
+		if sgp == s {
 			*l = sgp.next
 			if q.last == sgp {
 				q.last = prevsgp