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. == Unison Versions in Cygwin Unison is packaged for Cygwin as several independent packages, each providing a version of Unison that is incompatible with the others. You have to run compatible versions of Unison on the client and server, or Unison will issue an error message and quit. Two versions of Unison are compatible only if the first two numbers in their version strings are the same. For example, all versions 2.48.* are compatible with each other, and incompatible with versions 2.51.*. By installing one or more of the packages listed above, you can run whichever version you need in order to synchronize with your server. The unison* packages install a convenience symlink from /usr/bin/unison to one of the versioned unison executables, e.g. unison-2.48.exe. alternatives(8) is used to manage the symlink. If the symlink is being managed in "auto" mode, then it will automatically track the highest numbered version of unison that you have installed. You can override this by either changing the symlink manually with ln(1), or, preferably, by using alternatives(8): alternatives --display unison shows you the installed versions of unison, their priorities, which executable the symlink points to, and whether the symlink is being managed in "auto" or "manual" mode. alternatives --config unison presents a menu from which you can choose which version you want to use as the default. This puts the symlink in manual mode. alternatives --auto unison puts the symlink in automatic mode, where it will always point to the highest numbered version of unison that's installed on your host. alternatives --set unison /usr/bin/unison-$V.exe forces the symlink to point to unison-$V.exe, and puts it in manual mode. Please see alternatives(8) for more details. == 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. Future versions of Unison may get around the OCaml version incompatibility. For more discussion of the problem, see http://lists.seas.upenn.edu/pipermail/unison-hackers/2020-August/001972.html. Andrew E. Schulman -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
