handle single broker case

weed/mq/pub_balancer/allocate.go

@@ -126,14 +126,30 @@ func EnsureAssignmentsToActiveBrokers(activeBrokers cmap.ConcurrentMap[string, *
 				i++
 				hasChanges = true
 			}
+
+			hasEmptyFollowers := false
 			j := 0
 			for ; j<len(assignment.FollowerBrokers); j++ {
 				if assignment.FollowerBrokers[j] == "" {
-					assignment.FollowerBrokers[j] = pickedBrokers[i]
-					i++
 					hasChanges = true
+					if i < len(pickedBrokers) {
+						assignment.FollowerBrokers[j] = pickedBrokers[i]
+						i++
+					} else {
+						hasEmptyFollowers = true
+					}
+				}
+			}
+			if hasEmptyFollowers {
+				var followerBrokers []string
+				for _, follower := range assignment.FollowerBrokers {
+					if follower != "" {
+						followerBrokers = append(followerBrokers, follower)
+					}
 				}
+				assignment.FollowerBrokers = followerBrokers
 			}
+
 			if i < len(pickedBrokers) {
 				assignment.FollowerBrokers = append(assignment.FollowerBrokers, pickedBrokers[i:]...)
 				hasChanges = true

weed/mq/pub_balancer/allocate_test.go

@@ -83,6 +83,8 @@ func TestEnsureAssignmentsToActiveBrokersX(t *testing.T) {
 	lowActiveBrokers := cmap.New[*BrokerStats]()
 	lowActiveBrokers.SetIfAbsent("localhost:1", &BrokerStats{})
 	lowActiveBrokers.SetIfAbsent("localhost:2", &BrokerStats{})
+	singleActiveBroker := cmap.New[*BrokerStats]()
+	singleActiveBroker.SetIfAbsent("localhost:1", &BrokerStats{})
 	tests := []struct {
 		name                   string
 		args                   args
@@ -207,6 +209,23 @@ func TestEnsureAssignmentsToActiveBrokersX(t *testing.T) {
 			},
 			hasChanges: false,
 		},
+		{
+			name: "test single active broker",
+			args: args{
+				activeBrokers: singleActiveBroker,
+				followerCount: 3,
+				assignments:   []*mq_pb.BrokerPartitionAssignment{
+					{
+						LeaderBroker: "localhost:1",
+						Partition: &mq_pb.Partition{},
+						FollowerBrokers: []string{
+							"localhost:2",
+						},
+					},
+				},
+			},
+			hasChanges: true,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {