Parallel download via a .torrent URL and a HTTP URL (with --follow-torrent) is not possible

[intelfx]

Suppose I want to download a file that is available both via BitTorrent and on a regular HTTP mirror, taking advantage of multiple connections:

• https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso
• https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso.torrent

If I first download the .torrent file and then feed it into aria2c together with the HTTP mirror, the download will happen in parallel:

$ aria2c --follow-torrent=false -q 'https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso.torrent'

$ aria2c -T ubuntu-24.04-desktop-amd64.iso.torrent 'https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso'

05/16 04:00:00 [NOTICE] Downloading 1 item(s)

05/16 04:00:00 [NOTICE] IPv4 DHT: listening on UDP port 6952

05/16 04:00:00 [NOTICE] IPv4 BitTorrent: listening on TCP port 6932

05/16 04:00:00 [NOTICE] IPv6 BitTorrent: listening on TCP port 6932
[#82e553 2.8MiB/5.6GiB(0%) CN:2 SD:0 DL:661KiB ETA:2h30m28s]

---

However, if I try to directly start a download against the .torrent URL and a regular HTTP URL using --follow-torrent, then aria2c seemingly loses the ability to use multiple connections:

$ aria2c --follow-torrent=true 'https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso.torrent' 'https://releases.ubuntu.com/24.04/ubuntu-24.04-desktop-amd64.iso'

05/16 04:05:55 [NOTICE] Downloading 1 item(s)
[#5899be 64KiB/455KiB(14%) CN:1 DL:154KiB ETA:2s]
05/16 04:05:56 [NOTICE] Download complete: /home/intelfx/tmp/big/tmp-2024-05-16T03:44:01+02:00/ubuntu-24.04-desktop-amd64.iso.torrent

05/16 04:05:56 [NOTICE] IPv4 DHT: listening on UDP port 6893

05/16 04:05:56 [NOTICE] IPv4 BitTorrent: listening on TCP port 6933

05/16 04:05:56 [NOTICE] IPv6 BitTorrent: listening on TCP port 6933
[#060d31 256KiB/5.6GiB(0%) CN:1 SD:0 DL:193KiB ETA:8h34m41s]

It can also be corroborated via lsof that in the latter case, aria2c does not keep an open connection to the HTTP mirror.