In cluster mode prefer node with slots in 2 masters case

internal/app/master.go

@@ -54,6 +54,22 @@ func (app *App) getMasterHost(shardState map[string]*HostState) (string, error)
 		}
 	}
 	if len(masters) > 1 {
+		if app.mode == modeCluster {
+			mastersWithSlots := make([]string, 0)
+			for _, master := range masters {
+				node := app.shard.Get(master)
+				hasSlots, err := node.HasClusterSlots(app.ctx)
+				if err != nil {
+					return "", fmt.Errorf("unable to check slots on %s", master)
+				}
+				if hasSlots {
+					mastersWithSlots = append(mastersWithSlots, master)
+				}
+			}
+			if len(mastersWithSlots) == 1 {
+				return mastersWithSlots[0], nil
+			}
+		}
 		return "", fmt.Errorf("got more than 1 master: %s", masters)
 	}
 	if len(masters) == 0 {

internal/redis/node.go

@@ -542,3 +542,23 @@ func (n *Node) ClusterMeet(ctx context.Context, addr string, port, clusterBusPor
 	cmd := n.conn.Do(ctx, n.config.Renames.Cluster, n.config.Renames.ClusterMeet, addr, strconv.Itoa(port), strconv.Itoa(clusterBusPort))
 	return cmd.Err()
 }
+
+// HasClusterSlots checks if node has any slot assigned
+func (n *Node) HasClusterSlots(ctx context.Context) (bool, error) {
+	cmd := n.conn.ClusterNodes(ctx)
+	err := cmd.Err()
+	if err != nil {
+		return false, err
+	}
+	lines := strings.Split(cmd.Val(), "\n")
+	for _, line := range lines {
+		splitted := strings.Split(line, " ")
+		if len(splitted) < 3 {
+			continue
+		}
+		if strings.Contains(splitted[2], "myself") {
+			return len(splitted) > 8, nil
+		}
+	}
+	return false, nil
+}

tests/features/05_cluster_replication_fix.feature

@@ -171,6 +171,10 @@ Feature: Cluster mode broken replication fix
             rm -f /etc/redis/cluster.conf
         """
         And I run command on host "redis3"
+        """
+            sed -i -e 's/offline yes/offline no/' /etc/redis/redis.conf
+        """
+        And I run command on host "redis3"
         """
             supervisorctl signal KILL redis
         """
@@ -184,3 +188,78 @@ Feature: Cluster mode broken replication fix
         """
             ["redis1","redis2","redis3"]
         """
+
+    Scenario: Cluster splitbrain is fixed in favor of node with slots
+        Given clustered shard is up and running
+        Then redis host "redis1" should be master
+        And redis host "redis2" should become replica of "redis1" within "15" seconds
+        And replication on redis host "redis2" should run fine within "15" seconds
+        And redis host "redis3" should become replica of "redis1" within "15" seconds
+        And replication on redis host "redis3" should run fine within "15" seconds
+        And zookeeper node "/test/active_nodes" should match json_exactly within "30" seconds
+        """
+            ["redis1","redis2","redis3"]
+        """
+        When I run command on host "redis1"
+        """
+            supervisorctl signal STOP rdsync
+        """
+        And I run command on host "redis2"
+        """
+            supervisorctl signal STOP rdsync
+        """
+        And I run command on host "redis3"
+        """
+            supervisorctl signal STOP rdsync
+        """
+        And I run command on host "redis3"
+        """
+            rm -f /etc/redis/cluster.conf
+        """
+        And I run command on host "redis3"
+        """
+            sed -i -e 's/offline yes/offline no/' /etc/redis/redis.conf
+        """
+        And I run command on host "redis3"
+        """
+            supervisorctl signal KILL redis
+        """
+        And I run command on host "redis3"
+        """
+            supervisorctl start redis
+        """
+        Then redis host "redis3" should become available within "60" seconds
+        When I run command on redis host "redis1"
+        """
+            SET very-important-key foo
+        """
+        And I set zookeeper node "/test/master" to
+        """
+            "redis3"
+        """
+        And I run command on host "redis1"
+        """
+            supervisorctl signal CONT rdsync
+        """
+        And I run command on host "redis2"
+        """
+            supervisorctl signal CONT rdsync
+        """
+        And I run command on host "redis3"
+        """
+            supervisorctl signal CONT rdsync
+        """
+        Then redis host "redis3" should become replica of "redis1" within "60" seconds
+        And replication on redis host "redis3" should run fine within "15" seconds
+        And zookeeper node "/test/active_nodes" should match json_exactly within "30" seconds
+        """
+            ["redis1","redis2","redis3"]
+        """
+        When I run command on redis host "redis1"
+        """
+            GET very-important-key
+        """
+        Then redis cmd result should match regexp
+        """
+            .*foo.*
+        """