runtime: fix pseudo-randomness on some selects

Fixes #2152.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/4888044

src/pkg/runtime/chan.c

@@ -904,7 +904,8 @@ loop:
 
 	// pass 2 - enqueue on all chans
 	for(i=0; i<sel->ncase; i++) {
-		cas = &sel->scase[i];
+		o = sel->pollorder[i];
+		cas = &sel->scase[o];
 		c = cas->chan;
 		sg = &cas->sg;
 		sg->g = g;

src/pkg/runtime/chan_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	"runtime"
+	"sync"
 	"sync/atomic"
 	"testing"
 )
@@ -26,6 +27,38 @@ func TestChanSendInterface(t *testing.T) {
 	}
 }
 
+func TestPseudoRandomSend(t *testing.T) {
+	n := 100
+	c := make(chan int)
+	l := make([]int, n)
+	var m sync.Mutex
+	m.Lock()
+	go func() {
+		for i := 0; i < n; i++ {
+			runtime.Gosched()
+			l[i] = <-c
+		}
+		m.Unlock()
+	}()
+	for i := 0; i < n; i++ {
+		select {
+		case c <- 0:
+		case c <- 1:
+		}
+	}
+	m.Lock() // wait
+	n0 := 0
+	n1 := 0
+	for _, i := range l {
+		n0 += (i + 1) % 2
+		n1 += i
+		if n0 > n/10 && n1 > n/10 {
+			return
+		}
+	}
+	t.Errorf("Want pseudo random, got %d zeros and %d ones", n0, n1)
+}
+
 func BenchmarkSelectUncontended(b *testing.B) {
 	const CallsPerSched = 1000
 	procs := runtime.GOMAXPROCS(-1)