clientv3: check if KV.Client is closed

For https://github.com/coreos/etcd/issues/5495.

clientv3/integration/kv_test.go

@@ -279,6 +279,42 @@ func TestKVRange(t *testing.T) {
 	}
 }
 
+func TestKVGetErrConnClosed(t *testing.T) {
+	defer testutil.AfterTest(t)
+
+	clus := integration.NewClusterV3(t, &integration.ClusterConfig{Size: 1})
+	defer clus.Terminate(t)
+
+	cli := clus.Client(0)
+	kv := clientv3.NewKV(cli)
+
+	closed, donec := make(chan struct{}), make(chan struct{})
+	go func() {
+		select {
+		case <-time.After(3 * time.Second):
+			t.Fatal("cli.Close took too long")
+		case <-closed:
+		}
+
+		if _, err := kv.Get(context.TODO(), "foo"); err != rpctypes.ErrConnClosed {
+			t.Fatalf("expected %v, got %v", rpctypes.ErrConnClosed, err)
+		}
+		close(donec)
+	}()
+
+	if err := cli.Close(); err != nil {
+		t.Fatal(err)
+	}
+	clus.TakeClient(0)
+	close(closed)
+
+	select {
+	case <-time.After(3 * time.Second):
+		t.Fatal("kv.Get took too long")
+	case <-donec:
+	}
+}
+
 func TestKVDeleteRange(t *testing.T) {
 	defer testutil.AfterTest(t)