make lifecycle signal thread safe

staging/src/k8s.io/apiserver/pkg/server/lifecycle_signals.go

@@ -16,6 +16,10 @@ limitations under the License.
 
 package server
 
+import (
+	"sync"
+)
+
 /*
 We make an attempt here to identify the events that take place during
 lifecycle of the apiserver.
@@ -143,22 +147,21 @@ func newLifecycleSignals() lifecycleSignals {
 func newNamedChannelWrapper(name string) lifecycleSignal {
 	return &namedChannelWrapper{
 		name: name,
+		once: sync.Once{},
 		ch:   make(chan struct{}),
 	}
 }
 
 type namedChannelWrapper struct {
 	name string
+	once sync.Once
 	ch   chan struct{}
 }
 
 func (e *namedChannelWrapper) Signal() {
-	select {
-	case <-e.ch:
-		// already closed, don't close again.
-	default:
+	e.once.Do(func() {
 		close(e.ch)
-	}
+	})
 }
 
 func (e *namedChannelWrapper) Signaled() <-chan struct{} {