integration: ensure leader is up in waitLeader() and clusterMustProgress()

The issue is caused by leader loss even after waitLeader() returns
which can happen if the test machine is flaky which triggers a leader loss
or the killed node is the leader since waitLeader() only scans followers in
TestRestartMember() and they can have the same older leader.
In those cases, clusterMustProgress() proceeds with no leader which triggers
the no leader error.

To get around that, use linearizable get in waitLeader() to ensure leader is up
and retries on kapi.create() in clusterMustProgress() to ensure it proceeds with
a leader.

FIX #7258

integration/cluster.go

@@ -175,8 +175,12 @@ func (c *cluster) URL(i int) string {
 
 // URLs returns a list of all active client URLs in the cluster
 func (c *cluster) URLs() []string {
+	return getMembersURLs(c.Members)
+}
+
+func getMembersURLs(members []*member) []string {
 	urls := make([]string, 0)
-	for _, m := range c.Members {
+	for _, m := range members {
 		select {
 		case <-m.s.StopNotify():
 			continue
@@ -343,6 +347,18 @@ func (c *cluster) waitLeader(t *testing.T, membs []*member) int {
 	for _, m := range membs {
 		possibleLead[uint64(m.s.ID())] = true
 	}
+	cc := MustNewHTTPClient(t, getMembersURLs(membs), nil)
+	kapi := client.NewKeysAPI(cc)
+
+	// ensure leader is up via linearizable get
+	for {
+		ctx, cancel := context.WithTimeout(context.Background(), 10*tickDuration)
+		_, err := kapi.Get(ctx, "0", &client.GetOptions{Quorum: true})
+		cancel()
+		if err == nil || strings.Contains(err.Error(), "Key not found") {
+			break
+		}
+	}
 
 	for lead == 0 || !possibleLead[lead] {
 		lead = 0

integration/cluster_test.go

@@ -447,13 +447,24 @@ func TestRejectUnhealthyRemove(t *testing.T) {
 func clusterMustProgress(t *testing.T, membs []*member) {
 	cc := MustNewHTTPClient(t, []string{membs[0].URL()}, nil)
 	kapi := client.NewKeysAPI(cc)
-	ctx, cancel := context.WithTimeout(context.Background(), requestTimeout)
 	key := fmt.Sprintf("foo%d", rand.Int())
-	resp, err := kapi.Create(ctx, "/"+key, "bar")
+	var (
+		err  error
+		resp *client.Response
+	)
+	// retry in case of leader loss induced by slow CI
+	for i := 0; i < 3; i++ {
+		ctx, cancel := context.WithTimeout(context.Background(), requestTimeout)
+		resp, err = kapi.Create(ctx, "/"+key, "bar")
+		cancel()
+		if err == nil {
+			break
+		}
+		t.Logf("failed to create key on %q (%v)", membs[0].URL(), err)
+	}
 	if err != nil {
 		t.Fatalf("create on %s error: %v", membs[0].URL(), err)
 	}
-	cancel()
 
 	for i, m := range membs {
 		u := m.URL()