Comment 5 for bug 47055

For reference, this (from partman-base/parted_server.c) is the libparted code that a partition editor can use to grow the extended partition if necessary, assuming a PedDisk *disk:

        PedPartition *extended;
        extended = ped_disk_extended_partition(disk);
        ped_disk_maximize_partition(disk, extended,
                                    ped_constraint_any(disk->dev));

Not rocket science. :-)