Report empty address list instead of error. (#11603)

This is what I originally had and I was under the impression that
ReportError had the same behavior but from reading the code it seems
like the default balancer (pickfirst) handles these two calls
differently.

Author: vadimberezniker

server/util/kuberesolver/kuberesolver.go

@@ -436,7 +436,13 @@ func (b *kubeResolverBuilder) Build(target resolver.Target, cc resolver.ClientCo
 
 	cancel, err := b.manager.WatchPodIP(endpoint, func(addr string, watchErr error) {
 		if watchErr != nil {
-			r.cc.ReportError(watchErr)
+			// Report an empty address list. At least for the "pickfirst"
+			// balancer it causes the existing connection to be closed
+			// whereas calling ReportError causes it to keep using the old
+			// address list. In the case where a pod is removed/restarted,
+			// we wanted to close the connection since we know the IP is
+			// no longer valid.
+			_ = r.cc.UpdateState(resolver.State{})
 			return
 		}
 		r.updateState(addr)

server/util/kuberesolver/kuberesolver_test.go

@@ -237,12 +237,12 @@ func TestResolveNonExistentPod(t *testing.T) {
 	require.NoError(t, err)
 	defer r.Close()
 
-	// Should get an error reported since pod doesn't exist.
+	// Should get an empty address list since pod doesn't exist.
 	select {
-	case err := <-cc.errors:
-		require.Contains(t, err.Error(), "not found")
+	case state := <-cc.states:
+		require.Empty(t, state.Addresses)
 	case <-time.After(5 * time.Second):
-		require.FailNow(t, "timed out waiting for error")
+		require.FailNow(t, "timed out waiting for state update")
 	}
 }