fix(cluster): remove duplicates when combining Host and KnownHosts

Ref #3935 (comment)

pkg/scyllaclient/client_scylla.go

@@ -238,10 +238,8 @@ func (c *Client) CheckHostsChanged(ctx context.Context) (bool, error) {
 	if err != nil {
 		return false, err
 	}
-	if len(cur) != len(c.config.Hosts) {
-		return true, err
-	}
-	return !strset.New(c.config.Hosts...).Has(cur...), nil
+	s := strset.New(c.config.Hosts...)
+	return s.Size() != len(cur) || !s.Has(cur...), nil
 }
 
 // hosts returns a list of all hosts in a cluster.

pkg/service/cluster/model.go

@@ -6,6 +6,7 @@ import (
 	"crypto/tls"
 
 	"github.com/pkg/errors"
+	"github.com/scylladb/go-set/strset"
 	"github.com/scylladb/scylla-manager/v3/pkg/service"
 	"github.com/scylladb/scylla-manager/v3/pkg/util/uuid"
 	"go.uber.org/multierr"
@@ -71,6 +72,16 @@ func (c *Cluster) Validate() error {
 	return service.ErrValidate(errors.Wrap(errs, "invalid cluster"))
 }
 
+// AllHosts returns KnownHosts with Host at the front and without duplicates.
+func (c *Cluster) AllHosts() []string {
+	hostSet := strset.New(c.KnownHosts...)
+	if c.Host == "" {
+		return hostSet.List()
+	}
+	hostSet.Remove(c.Host)
+	return append([]string{c.Host}, hostSet.List()...)
+}
+
 // Filter filters Clusters.
 type Filter struct {
 	Name string

pkg/service/cluster/service.go

@@ -160,11 +160,7 @@ func (s *Service) clientConfig(c *Cluster) scyllaclient.Config {
 		config.Port = fmt.Sprint(c.Port)
 	}
 	config.AuthToken = c.AuthToken
-	config.Hosts = nil
-	if c.Host != "" {
-		config.Hosts = []string{c.Host}
-	}
-	config.Hosts = append(config.Hosts, c.KnownHosts...)
+	config.Hosts = c.AllHosts()
 	return config
 }
 
@@ -467,9 +463,7 @@ func (s *Service) validateHostsConnectivity(ctx context.Context, c *Cluster) err
 	if err := s.loadKnownHosts(c); err != nil && !errors.Is(err, gocql.ErrNotFound) {
 		return errors.Wrap(err, "load known hosts")
 	}
-	if c.Host != "" {
-		c.KnownHosts = append([]string{c.Host}, c.KnownHosts...)
-	}
+	c.KnownHosts = c.AllHosts()
 
 	knownHosts, err := s.filterKnownHosts(ctx, c)
 	if err != nil {