clientv3: remove v3.WithFirstKey() in Barrier.Wait()

fix the unexpected blocking when using Barrier.Wait(), e.g.
NewBarrier(client, "a").Wait() will block if key "a" is not existed but "a0" is existed, but it should return immediately.

Signed-off-by: zhangwenkang <zwenkang@vmware.com>

client/v3/experimental/recipes/barrier.go

@@ -49,7 +49,7 @@ func (b *Barrier) Release() error {
 // Wait blocks on the barrier key until it is deleted. If there is no key, Wait
 // assumes Release has already been called and returns immediately.
 func (b *Barrier) Wait() error {
-	resp, err := b.client.Get(b.ctx, b.key, v3.WithFirstKey()...)
+	resp, err := b.client.Get(b.ctx, b.key)
 	if err != nil {
 		return err
 	}

tests/integration/clientv3/experimental/recipes/v3_barrier_test.go

@@ -83,3 +83,42 @@ func testBarrier(t *testing.T, waiters int, chooseClient func() *clientv3.Client
 		}
 	}
 }
+
+func TestBarrierWaitNonexistentKey(t *testing.T) {
+	integration2.BeforeTest(t)
+	clus := integration2.NewCluster(t, &integration2.ClusterConfig{Size: 1})
+	defer clus.Terminate(t)
+	cli := clus.Client(0)
+
+	if _, err := cli.Put(cli.Ctx(), "test-barrier-0", ""); err != nil {
+		t.Errorf("could not put test-barrier0, err:%v", err)
+	}
+
+	donec := make(chan struct{})
+	stopc := make(chan struct{})
+	defer close(stopc)
+
+	waiters := 5
+	for i := 0; i < waiters; i++ {
+		go func() {
+			br := recipe.NewBarrier(cli, "test-barrier")
+			if err := br.Wait(); err != nil {
+				t.Errorf("could not wait on barrier (%v)", err)
+			}
+			select {
+			case donec <- struct{}{}:
+			case <-stopc:
+			}
+		}()
+	}
+
+	// all waiters should return immediately if waiting on a nonexistent key "test-barrier" even if key "test-barrier-0" exists
+	timerC := time.After(time.Duration(waiters*100) * time.Millisecond)
+	for i := 0; i < waiters; i++ {
+		select {
+		case <-timerC:
+			t.Fatal("barrier timed out")
+		case <-donec:
+		}
+	}
+}