Backport 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: James Blair <mail@jamesblair.net>

contrib/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
 	}

integration/v3_barrier_test.go

@@ -76,3 +76,42 @@ func testBarrier(t *testing.T, waiters int, chooseClient func() *clientv3.Client
 		}
 	}
 }
+
+func TestBarrierWaitNonexistentKey(t *testing.T) {
+	defer testutil.AfterTest(t)
+	clus := NewClusterV3(t, &ClusterConfig{Size: 1})
+	defer clus.Terminate(t)
+	cli := clus.clients[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:
+		}
+	}
+}