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Subject:	Re: [ANNOUNCEMENT] unison2.48-2.48.4-2 (Warning: possible breakage)
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Date:	Mon, 31 Aug 2020 18:08:34 -0400
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From:	Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Ken Brown <kbrown AT cornell DOT edu>
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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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