GODRIVER-1549 Select all servers when an empty tag set is given

x/mongo/driver/description/selector_test.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/stretchr/testify/require"
+	"go.mongodb.org/mongo-driver/internal/testutil/assert"
 	"go.mongodb.org/mongo-driver/mongo/readpref"
 	"go.mongodb.org/mongo-driver/tag"
 	"go.mongodb.org/mongo-driver/x/mongo/driver/address"
@@ -506,6 +507,43 @@ func TestSelector_Secondary_with_tags(t *testing.T) {
 	require.Equal([]Server{readPrefTestSecondary2}, result)
 }
 
+func TestSelector_Secondary_with_empty_tag_set(t *testing.T) {
+	t.Parallel()
+
+	primaryNoTags := Server{
+		Addr:        address.Address("localhost:27017"),
+		Kind:        RSPrimary,
+		WireVersion: &VersionRange{Min: 0, Max: 5},
+	}
+	firstSecondaryNoTags := Server{
+		Addr:        address.Address("localhost:27018"),
+		Kind:        RSSecondary,
+		WireVersion: &VersionRange{Min: 0, Max: 5},
+	}
+	secondSecondaryNoTags := Server{
+		Addr:        address.Address("localhost:27019"),
+		Kind:        RSSecondary,
+		WireVersion: &VersionRange{Min: 0, Max: 5},
+	}
+	topologyNoTags := Topology{
+		Kind:    ReplicaSetWithPrimary,
+		Servers: []Server{primaryNoTags, firstSecondaryNoTags, secondSecondaryNoTags},
+	}
+
+	nonMatchingSet := tag.Set{
+		{Name: "foo", Value: "bar"},
+	}
+	emptyTagSet := tag.Set{}
+	rp := readpref.Secondary(
+		readpref.WithTagSets(nonMatchingSet, emptyTagSet),
+	)
+
+	result, err := ReadPrefSelector(rp).SelectServer(topologyNoTags, topologyNoTags.Servers)
+	assert.Nil(t, err, "SelectServer error: %v", err)
+	expectedResult := []Server{firstSecondaryNoTags, secondSecondaryNoTags}
+	assert.Equal(t, expectedResult, result, "expected result %v, got %v", expectedResult, result)
+}
+
 func TestSelector_Secondary_with_tags_that_do_not_match(t *testing.T) {
 	t.Parallel()
 

x/mongo/driver/description/server_selector.go

@@ -223,8 +223,15 @@ func selectByTagSet(candidates []Server, tagSets []tag.Set) []Server {
 	}
 
 	for _, ts := range tagSets {
+		// If this tag set is empty, we can take a fast path because the empty list is a subset of all tag sets, so
+		// all candidate servers will be selected.
+		if len(ts) == 0 {
+			return candidates
+		}
+
 		var results []Server
 		for _, s := range candidates {
+			// ts is non-empty, so only servers with a non-empty set of tags need to be checked.
 			if len(s.Tags) > 0 && s.Tags.ContainsAll(ts) {
 				results = append(results, s)
 			}