fix divide by zero

weed/mq/pub_balancer/allocate.go

@@ -56,7 +56,7 @@ func pickBrokers(brokers cmap.ConcurrentMap[string, *BrokerStats], count int32)
 	return pickedBrokers
 }
 
-func EnsureAssignmentsToActiveBrokers(activeBrokers cmap.ConcurrentMap[string,*BrokerStats], assignments []*mq_pb.BrokerPartitionAssignment) (addedAssignments, updatedAssignments []*mq_pb.BrokerPartitionAssignment) {
+func EnsureAssignmentsToActiveBrokers(activeBrokers cmap.ConcurrentMap[string, *BrokerStats], assignments []*mq_pb.BrokerPartitionAssignment) (addedAssignments, updatedAssignments []*mq_pb.BrokerPartitionAssignment) {
 	for _, assignment := range assignments {
 		if assignment.LeaderBroker == "" {
 			addedAssignments = append(addedAssignments, assignment)
@@ -69,13 +69,17 @@ func EnsureAssignmentsToActiveBrokers(activeBrokers cmap.ConcurrentMap[string,*B
 	}
 
 	// pick the brokers with the least number of partitions
-	pickedBrokers := pickBrokers(activeBrokers, int32(len(addedAssignments)))
-	for i, assignment := range addedAssignments {
-		assignment.LeaderBroker = pickedBrokers[i]
+	if len(addedAssignments) > 0 {
+		pickedBrokers := pickBrokers(activeBrokers, int32(len(addedAssignments)))
+		for i, assignment := range addedAssignments {
+			assignment.LeaderBroker = pickedBrokers[i]
+		}
 	}
-	pickedBrokers = pickBrokers(activeBrokers, int32(len(updatedAssignments)))
-	for i, assignment := range updatedAssignments {
-		assignment.LeaderBroker = pickedBrokers[i]
+	if len(updatedAssignments) == 0 {
+		pickedBrokers := pickBrokers(activeBrokers, int32(len(updatedAssignments)))
+		for i, assignment := range updatedAssignments {
+			assignment.LeaderBroker = pickedBrokers[i]
+		}
 	}
 
 	return