Explicitly set pre-vote flag for release 3.4 in robustness test

Signed-off-by: Joshua Zhang <joshuazh@microsoft.com>

tests/framework/e2e/cluster.go

@@ -172,6 +172,9 @@ type EtcdProcessClusterConfig struct {
 	IsPeerAutoTLS      bool
 	CN                 bool
 
+	// Flags to explicitly set even if their values are same as default ones
+	ExplicitFlags map[string]struct{}
+
 	// Removed in v3.6
 
 	Discovery string // v2 discovery
@@ -371,6 +374,19 @@ func WithPeerProxy(enabled bool) EPClusterOption {
 	return func(c *EtcdProcessClusterConfig) { c.PeerProxy = enabled }
 }
 
+func WithPreVote(enabled bool) EPClusterOption {
+	return func(c *EtcdProcessClusterConfig) { c.ServerConfig.PreVote = enabled }
+}
+
+func WithExplicitFlag(flag string) EPClusterOption {
+	return func(c *EtcdProcessClusterConfig) {
+		if c.ExplicitFlags == nil {
+			c.ExplicitFlags = map[string]struct{}{}
+		}
+		c.ExplicitFlags[flag] = struct{}{}
+	}
+}
+
 // NewEtcdProcessCluster launches a new cluster from etcd processes, returning
 // a new EtcdProcessCluster once all nodes are ready to accept client requests.
 func NewEtcdProcessCluster(ctx context.Context, t testing.TB, opts ...EPClusterOption) (*EtcdProcessCluster, error) {
@@ -574,7 +590,9 @@ func (cfg *EtcdProcessClusterConfig) EtcdServerProcessConfig(tb testing.TB, i in
 	overrideValues := values(cfg.ServerConfig)
 	for flag, value := range overrideValues {
 		if defaultValue := defaultValues[flag]; value == "" || value == defaultValue {
-			continue
+			if _, exist := cfg.ExplicitFlags[flag]; !exist {
+				continue
+			}
 		}
 		if flag == "experimental-snapshot-catchup-entries" && !(cfg.Version == CurrentVersion || (cfg.Version == MinorityLastVersion && i <= cfg.ClusterSize/2) || (cfg.Version == QuorumLastVersion && i > cfg.ClusterSize/2)) {
 			continue

tests/robustness/scenarios.go

@@ -79,6 +79,11 @@ func exploratoryScenarios(t *testing.T) []testScenario {
 		e2e.WithCompactionBatchLimit(100),
 		e2e.WithWatchProcessNotifyInterval(100 * time.Millisecond),
 	}
+	if v.LessThan(version.V3_5) {
+		// The default pre-vote configuration prior to version 3.5 is set to false.
+		// We need to explicitly enable prevote to improve availability in robustness test.
+		baseOptions = append(baseOptions, e2e.WithPreVote(true), e2e.WithExplicitFlag("pre-vote"))
+	}
 	scenarios := []testScenario{}
 	for _, tp := range trafficProfiles {
 		name := filepath.Join(tp.Traffic.Name(), tp.Profile.Name, "ClusterOfSize1")