[ANNOUNCEMENT] unison2.48-2.48.4-2 (Warning: possible breakage)
Ken Brown
kbrown@cornell.edu
Mon Aug 31 22:08:34 GMT 2020
On 8/31/2020 4:47 PM, Andrew Schulman via Cygwin-announce wrote:
> The unison2.48 package has been updated in Cygwin. This new release,
> version 2.48.4-2, has been rebuilt with OCaml 4.08.1, which makes it
> compatible for syncing with Ubuntu Focal and the current Debian testing.
>
> *Warning*: This update may be a breaking change, if you use unison2.48 to
> synchronize with hosts that run other Linux distros. This is because two
> Unison executables have to be built with the same version of OCaml in order
> to be compatible. For discussion of this newly-discovered problem and how
> we plan to deal with it in Cygwin, see OCaml Versions, below.
[...]
> == OCaml Versions
>
> There is unfortunately another layer of incompatibility in Unison: Two
> Unison executables are only compatible if they were built with the same
> version of OCaml. If you use, say, Unison 2.48 on two different hosts, but
> one was built with OCaml 4.08.1 and the other with OCaml 4.10.0, you'll get
> the dreaded error message
>
> Fatal error: Fatal error during unmarshaling (input_value: ill-formed
> message), possibly because client and server have been compiled with
> different versions of the OCaml compiler.
>
> This is a bad feature, of OCaml apparently. It means that in order to be
> sure of being able to sync with another host, we could potentially need a
> separate Unison package for every combination of Unison and OCaml.
>
> For now, Unison packages for Cygwin are being built with the OCaml version
> that seems to make them most generally compatible with other Linux distros:
>
> Package OCaml version Compatibility
> ---------- ------------- -------------
> unison2.27 ? ?
> unison2.32 ? ?
> unison2.40 ? ?
> unison2.45 ? ?
> unison2.48 4.08.1 Debian bullseye (testing), Ubuntu Focal
> unison2.49 ? ?
> unison2.51 ? ?
>
> Packages with "?" are older builds, where the OCaml version isn't currently
> known.
>
> If your distro uses a different combination of Unison and OCaml versions
> that you need in order to sync, please let me know here. If it's widely
> used, I might update one of the existing builds, or make a new Cygwin
> package for it.
What a mess! Would you consider embedding the OCaml version in the package name
as suggested by the Debian maintainer
(http://lists.seas.upenn.edu/pipermail/unison-hackers/2020-August/001975.html)?
Thus unison2.48-2.48.4-1 would be replaced by something like
unison2.48+4.04.1... and unison2.48-2.48.4-2 would be replace by something like
unison2.48+4.08.1.... That way people who are happy with unison2.48-2.48.4-1
can keep using it without being pestered by setup to update.
FWIW, here's my situation, which is probably not typical. When you released
unison2.48-2.48.4-1 a few years ago, it was incompatible with the Linux server
that I sync with. So I simply built OCaml and unison myself on that server,
using the same versions that you used, and installed unison in my own ~/bin
directory.
I could do that again, now using OCaml 4.08.1. But I'd much rather just keep my
current arrangement, without having setup try to update unison2.48 every time I
run it.
Ken
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