Bug#802919: unison: synchronization incompatibility when built with Ocaml versions pre/post-4.02

[Conrad Hughes <debbugs@xrad.org>]

Hi Ralf,

Ralf> The problem is that any software used to build packages in a
Ralf> distribution (sid, in this case) must also be in the distribution.
Ralf> Hence, providing builds of unison produced with different ocaml
Ralf> versions implies having packages for different ocaml versions
Ralf> in sid.

That's kindof what I thought, but if I understand correctly, unison in
stretch must have been built with a different version of ocaml than the
ocaml that's actually in stretch, which confusingly contradicts this?
Stretch's unison is only compatible with other pre-ocaml-4.02 unisons,
which suggests it was built with ocaml-4.01 — but stretch's ocaml is
4.02.3-9?  This would seem to be confirmed by the fact that I had to
downgrade ocaml to 4.01 on one of my (non-Debian, more recently updated)
servers when building unison there, so that unison on my jessie and
stretch machines could talk to unison on that server.

Sorry if I'm being slow or missing the point here.

If I've gotten that right, then the current version of unison on stretch
is built with the "wrong" ocaml (i.e. not stretch's ocaml).  The idea
that it should be built with stretch's ocaml would then argue that
technically unison on stretch needs to be updated to an ocaml-4.02
build.  When that update rolls out, unison on stretch will become
incompatible with everything that it currently works with (except
itself, of course).  So a warning release note would seem appropriate
when that "upgrade" happens..?

It would be nice if we could persuade upstream to at least give a more
helpful error message when the incompatibility arises.

Conrad