sync: add Once test with panic

Tests behavior of Once when initialization function panics.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6554047

src/pkg/sync/once_test.go

@@ -17,8 +17,11 @@ func (o *one) Increment() {
 	*o++
 }
 
-func run(once *Once, o *one, c chan bool) {
+func run(t *testing.T, once *Once, o *one, c chan bool) {
 	once.Do(func() { o.Increment() })
+	if v := *o; v != 1 {
+		t.Errorf("once failed inside run: %d is not 1", v)
+	}
 	c <- true
 }
 
@@ -28,16 +31,36 @@ func TestOnce(t *testing.T) {
 	c := make(chan bool)
 	const N = 10
 	for i := 0; i < N; i++ {
-		go run(once, o, c)
+		go run(t, once, o, c)
 	}
 	for i := 0; i < N; i++ {
 		<-c
 	}
 	if *o != 1 {
-		t.Errorf("once failed: %d is not 1", *o)
+		t.Errorf("once failed outside run: %d is not 1", *o)
 	}
 }
 
+func TestOncePanic(t *testing.T) {
+	once := new(Once)
+	for i := 0; i < 2; i++ {
+		func() {
+			defer func() {
+				if recover() == nil {
+					t.Fatalf("Once.Do() has not panic'ed")
+				}
+			}()
+			once.Do(func() {
+				panic("failed")
+			})
+		}()
+	}
+	once.Do(func() {})
+	once.Do(func() {
+		t.Fatalf("Once called twice")
+	})
+}
+
 func BenchmarkOnce(b *testing.B) {
 	const CallsPerSched = 1000
 	procs := runtime.GOMAXPROCS(-1)