apiserver: fix Cacher.Stop() race

staging/src/k8s.io/apiserver/pkg/storage/cacher.go

@@ -649,13 +649,15 @@ func (c *Cacher) isStopped() bool {
 }
 
 func (c *Cacher) Stop() {
-	// TODO : Do not check for isStopped (and return) when PR
-	// https://github.com/kubernetes/kubernetes/pull/50690
-	// merges as that shuts down storage properly
+	// avoid stopping twice (note: cachers are shared with subresources)
 	if c.isStopped() {
 		return
 	}
 	c.stopLock.Lock()
+	if c.stopped {
+		// avoid that it was locked meanwhile as isStopped only read-locks
+		return
+	}
 	c.stopped = true
 	c.stopLock.Unlock()
 	close(c.stopCh)