test/chan/doubleselect: fix various races

There were duplicate closes and missing closes,
with the result that the program was rarely testing
as much as it seemed to be.  Now it finishes.

R=r
CC=golang-dev
https://golang.org/cl/4008046

test/chan/doubleselect.go

@@ -21,6 +21,8 @@ var iterations *int = flag.Int("n", 100000, "number of iterations")
 func sender(n int, c1, c2, c3, c4 chan<- int) {
 	defer close(c1)
 	defer close(c2)
+	defer close(c3)
+	defer close(c4)
 
 	for i := 0; i < n; i++ {
 		select {
@@ -35,26 +37,18 @@ func sender(n int, c1, c2, c3, c4 chan<- int) {
 // mux receives the values from sender and forwards them onto another channel.
 // It would be simplier to just have sender's four cases all be the same
 // channel, but this doesn't actually trigger the bug.
-func mux(out chan<- int, in <-chan int) {
-	for {
-		v := <-in
-		if closed(in) {
-			close(out)
-			break
-		}
+func mux(out chan<- int, in <-chan int, done chan<- bool) {
+	for v := range in {
 		out <- v
 	}
+	done <- true
 }
 
 // recver gets a steam of values from the four mux's and checks for duplicates.
 func recver(in <-chan int) {
 	seen := make(map[int]bool)
 
-	for {
-		v := <-in
-		if closed(in) {
-			break
-		}
+	for v := range in {
 		if _, ok := seen[v]; ok {
 			println("got duplicate value: ", v)
 			panic("fail")
@@ -70,15 +64,23 @@ func main() {
 	c2 := make(chan int)
 	c3 := make(chan int)
 	c4 := make(chan int)
+	done := make(chan bool)
 	cmux := make(chan int)
 	go sender(*iterations, c1, c2, c3, c4)
-	go mux(cmux, c1)
-	go mux(cmux, c2)
-	go mux(cmux, c3)
-	go mux(cmux, c4)
+	go mux(cmux, c1, done)
+	go mux(cmux, c2, done)
+	go mux(cmux, c3, done)
+	go mux(cmux, c4, done)
+	go func() {
+		<-done
+		<-done
+		<-done
+		<-done
+		close(cmux)
+	}()
 	// We keep the recver because it might catch more bugs in the future.
 	// However, the result of the bug linked to at the top is that we'll
-	// end up panicing with: "throw: bad g->status in ready".
+	// end up panicking with: "throw: bad g->status in ready".
 	recver(cmux)
 	print("PASS\n")
 }