Do not sort before sending to the listener.

reader.go

@@ -311,9 +311,6 @@ func (r *Reader) run(cg *ConsumerGroup) {
 						for _, partition := range partitions {
 							assignedPartitions = append(assignedPartitions, partition.ID)
 						}
-						sort.Slice(assignedPartitions, func(i, j int) bool {
-							return assignedPartitions[i] < assignedPartitions[j]
-						})
 						assignments = append(assignments, GroupMemberTopic{
 							Topic:      topic,
 							Partitions: assignedPartitions,

reader_test.go

@@ -10,6 +10,7 @@ import (
 	"net"
 	"os"
 	"reflect"
+	"sort"
 	"strconv"
 	"sync"
 	"testing"
@@ -891,7 +892,7 @@ func TestReaderConsumerGroup(t *testing.T) {
 	}
 }
 
-func TestAssignmentListener(t *testing.T) {
+func TestPartitionAssignmentListener(t *testing.T) {
 	// It appears that some of the tests depend on all these tests being
 	// run concurrently to pass... this is brittle and should be fixed
 	// at some point.
@@ -917,6 +918,12 @@ func TestAssignmentListener(t *testing.T) {
 		AssignmentListener: func(partitions []GroupMemberTopic) {
 			lock.Lock()
 			defer lock.Unlock()
+			// we sort the received partitions for easier comparison
+			for _, partition := range partitions {
+				sort.Slice(partition.Partitions, func(i, j int) bool {
+					return partition.Partitions[i] < partition.Partitions[j]
+				})
+			}
 			assignments = append(assignments, partitions)
 		},
 	})