Fix topo.ClusterMembers to get hidden members ...

for the sharded cluster

Previously the bug was present, and hidden memebers were not present in
the resulting array of the RS members.

Author: boris-ilijic

pbm/topo/cluster.go

@@ -2,6 +2,7 @@ package topo
 
 import (
 	"context"
+	"fmt"
 	"strings"
 
 	"go.mongodb.org/mongo-driver/bson"
@@ -153,18 +154,35 @@ func getShardMapImpl(ctx context.Context, m *mongo.Client) (map[ReplsetName]Shar
 	// if shard name is not set, mongodb will provide unique name for it
 	// (e.g. the replset name of the shard)
 	// for configsvr, key name is "config"
-	var shardMap struct{ Map map[string]string }
+	// hosts field is used to discover hidden members, which are not present in map field
+	var shardMap struct {
+		Map   map[string]string
+		Hosts map[string]string
+	}
 	if err := res.Decode(&shardMap); err != nil {
 		return nil, errors.Wrap(err, "decode")
 	}
 
+	// Example of the hosts field from command output:
+	// hosts: {
+	//   'rs103:27017': 'rs1',
+	//   'rs101:27017': 'rs1',
+	//   'cfg02:27017': 'config',
+	//   ...
+	// }
+	// hostsByShard will contain even hidden RS members
+	hostsByShard := map[string][]string{}
+	for host, shardID := range shardMap.Hosts {
+		hostsByShard[shardID] = append(hostsByShard[shardID], host)
+	}
+
 	shards := make(map[string]Shard, len(shardMap.Map))
 	for id, host := range shardMap.Map {
 		rs, _, _ := strings.Cut(host, "/")
 		shards[rs] = Shard{
 			ID:   id,
 			RS:   rs,
-			Host: host,
+			Host: fmt.Sprintf("%s/%s", rs, strings.Join(hostsByShard[id], ",")),
 		}
 	}