Unison incompatible between ubuntu 20.04 and 22.04

[Hadmut Danisch]

The same problem as every other year:

With every new LTS release unison becomes incompatible with the new and the last Ubuntu release. Even if not incompatible in a technical way, unison compares the version string and denies to communicate with a different version.

So it is (without dirty workarounds such as running an old unison in a docker image) not possible to synchronize files between a 20.04 and a 22.04 machine.

Would be nice, to either have the old version available in 22.04, or the new version available in 20.04.

regards

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: unison 2.48.4-4ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-109.123-generic 5.4.178
Uname: Linux 5.4.0-109-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.23
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: LXQt
Date: Sat Apr 30 13:06:45 2022
InstallationDate: Installed on 2020-06-12 (686 days ago)
InstallationMedia: Lubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: unison
UpgradeStatus: No upgrade log present (probably fresh install)

[christos pontikis (pontikis)]

Same problem here.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Andrew Piskorski (atp)]

But the "unison-all" package in 20.04 fixed this! It let 20.04 talk with the Unison from earlier versions of Ubuntu. Why is the unison-all package missing from 22.04?

Related bug about unison-all missing from 22.04:
https://bugs.launchpad.net/ubuntu/+source/meta-unison/+bug/1989815

[Andrew Piskorski (atp)]

In older versions of Ubuntu, I believe multiple "unison-VERSION" packages were provided, and the "unison-all" metapackage simply installed all of them for you. In both Ubuntu 20.04.5 and 22.04.1, those version-specific Unison packages seem to be long gone, and only ONE version of Unison is provided. Thus the unison-all package was already effectively useless on 20.04.5, but at some point in the past, it was very useful indeed!

Why were the version-specific Unison binary packages discontinued? They were very useful and user-friendly.

Ubuntu 20.04.5 LTS provides "/usr/bin/unison-2.48.4" from its "unison" package. I tried copying the 20.04.5 /usr/bin/unison-2.48.4 into /usr/local/bin/ on my 22.04.1 machine. Then I specified "servercmd = unison-2.48.4" in my Profile file. This does NOT work, as Unison reports:

Fatal error
Server: Archive file seems damaged (input_value: ill-formed message): throw away archives on both machines and try again

On Ubuntu 20.04.5 (focal), simply copying the old /usr/bin/unison-* binaries from older versions of Ubuntu often worked fine, they would run on 20.04 and work for syncing with older versions of Ubuntu. That apparently no longer works with 22.04.1 (jammy) vs. 20.04.5 (focal).

[Andrew Piskorski (atp)]

Older discussions of essentially the same problem:
https://bugs.launchpad.net/ubuntu/+source/unison/+bug/1568459
https://askubuntu.com/questions/1234767/can-i-install-unison-for-ubuntu-18-04-lts-on-ubuntu-20-04-lts
https://bugs.launchpad.net/bugs/1875475

Apparently the unison-all package has been broken since at least Ubuntu 18.04.

[Andrew Piskorski (atp)]

Upstream provides binary builds for many different platforms! Here:
https://github.com/bcpierce00/unison/releases

Currently the latest Unison there is 2.52.1, and the latest OCaml compiler is 4.14.0, so I installed "unison-v2.52.1+ocaml-4.14.0+x86_64.linux.tar.gz" (2022-05-08) into /usr/local/bin/ on both my Ubuntu 20.04.5 (focal) and 22.04.1 (jammy) machines. That works!

So, since the Unison author provides binary builds that seem to work everywhere, shouldn't the Ubuntu packaging problem reduce to simply providing those exact same binaries for EACH version of Ubuntu, either in its main repository or in backports?