GODRIVER-2205 Retain empty tag sets when building a read preference document. (#787)

Author: matthewdale

x/mongo/driver/operation.go

@@ -1233,9 +1233,6 @@ func (op Operation) createReadPref(serverKind description.ServerKind, topologyKi
 
 	sets := make([]bsoncore.Document, 0, len(rp.TagSets()))
 	for _, ts := range rp.TagSets() {
-		if len(ts) == 0 {
-			continue
-		}
 		i, set := bsoncore.AppendDocumentStart(nil)
 		for _, t := range ts {
 			set = bsoncore.AppendStringElement(set, t.Name, t.Value)

x/mongo/driver/operation_test.go

@@ -16,6 +16,7 @@ import (
 	"go.mongodb.org/mongo-driver/mongo/readconcern"
 	"go.mongodb.org/mongo-driver/mongo/readpref"
 	"go.mongodb.org/mongo-driver/mongo/writeconcern"
+	"go.mongodb.org/mongo-driver/tag"
 	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
 	"go.mongodb.org/mongo-driver/x/mongo/driver/session"
 	"go.mongodb.org/mongo-driver/x/mongo/driver/uuid"
@@ -397,6 +398,25 @@ func TestOperation(t *testing.T) {
 				readpref.SecondaryPreferred(readpref.WithTags("disk", "ssd", "use", "reporting")),
 				description.RSSecondary, description.ReplicaSet, false, rpWithTags,
 			},
+			// GODRIVER-2205: Ensure empty tag sets are written as an empty document in the read
+			// preference document. Empty tag sets match any server and are used as a fallback when
+			// no other tag sets match any servers.
+			{
+				"secondaryPreferred/withTags/emptyTagSet",
+				readpref.SecondaryPreferred(readpref.WithTagSets(
+					tag.Set{{Name: "disk", Value: "ssd"}},
+					tag.Set{})),
+				description.RSSecondary,
+				description.ReplicaSet,
+				false,
+				bsoncore.NewDocumentBuilder().
+					AppendString("mode", "secondaryPreferred").
+					AppendArray("tags", bsoncore.NewArrayBuilder().
+						AppendDocument(bsoncore.NewDocumentBuilder().AppendString("disk", "ssd").Build()).
+						AppendDocument(bsoncore.NewDocumentBuilder().Build()).
+						Build()).
+					Build(),
+			},
 			{
 				"secondaryPreferred/withMaxStaleness",
 				readpref.SecondaryPreferred(readpref.WithMaxStaleness(25 * time.Second)),