On Sun, Sep 16, 2012 at 05:10:53PM +0200, Alexander Neumann wrote:
> I am a happy unison user for more than seven year (using Debian Linux).
> Yesterday I gave Archlinux a try and discovered that I seem to be unable to
> sync directories between my new Archlinux workstation and a server running
> Debian Linux. Even the latest version (2.40.65) running at both ends does
> not solve my problem. Copying and running the binary from Debian on my
> Archlinux machine works, so I think the identification of the machines
> via host names works correctly.

> Fatal error: Internal error: New archives are not identical.
> Retaining original archives. Please run Unison again to bring them up to date.

I suspect Archlinux has already switched to OCaml 4. There is an
incompatibility between OCaml 3 and OCaml 4 which results in this
error message. (Unison ends up using two distinct hash functions to
compare the archives and thus wrongly believes that the archives are
not identical.) The workaround is to use a Unison binary compiled with
the same version of OCaml on both machines.

Thanks for your report!

-- Jerome

On Sep 17, 2012, at 3:29 AM, Alexander Neumann wrote:

> Hi,
>
> * Jerome Vouillon <Jerome.Vouillon@...> wrote:
>> On Sun, Sep 16, 2012 at 05:10:53PM +0200, Alexander Neumann wrote:
>>> Fatal error: Internal error: New archives are not identical.
>>> Retaining original archives. Please run Unison again to bring them up to date.
>> I suspect Archlinux has already switched to OCaml 4.
>
> That is correct.
>
>> There is an incompatibility between OCaml 3 and OCaml 4 which results in
>> this error message. (Unison ends up using two distinct hash functions to
>> compare the archives and thus wrongly believes that the archives are not
>> identical.) The workaround is to use a Unison binary compiled with the
>> same version of OCaml on both machines.
>
> Uh, that's really bad, as it breaks syncing with (appearently) the same
> version of unison in an ugly way without any obvious causes.
>
> Are there any plans for mitigating this? For example, a good mitigation seems
> to be releasing a new version of unison which uses the same (old) hash
> function. The hash seems to be used only for identifying archives and this
> way seems to be not security relevant. This way installations using the old
> OCaml3 can still be synced. As OCaml4 gets wider use in the next months,
> maybe even add a FAQ entry (at least I invested several hours researching
> this problem ;)
>
> Thanks for your reply,
> - Alex
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>

> [...]
> I'm going to wait a bit before making it the official stable copy, but if
> you want to try it out now, you can grab it like this:
> svn co https://webdav.seas.upenn.edu/svn/unison/branches/2.40

> I ran into a similar problem with macports (ocaml 4) and Ubuntu (ocaml 3).
> but I found the sync itself to work, just the archives were never
> updated, because the hash function used for comparing the archive files
> was different on both ends.

> did you see other problems as well?

> I think the rsync algorithm is broken when using ocaml 4.

On Tue, Sep 18, 2012 at 06:14:34PM +0200, Alexander Neumann wrote:
> * Hartmut "hase" Semken <hase@...> wrote:
> > I ran into a similar problem with macports (ocaml 4) and Ubuntu (ocaml 3).
> > but I found the sync itself to work, just the archives were never
> > updated, because the hash function used for comparing the archive files
> > was different on both ends.
>
> Interesting. I observed that unison exits right after checking for
> changed/new/deleted files locally and on the server, before any sync is
> performed.

That's the same issue. The Mac OS graphic UI will only save the
archive after synchronization. The text UI will also save the
archive just after update detection, and thus fails sooner.

> > I think the rsync algorithm is broken when using ocaml 4.

Indeed, it is broken on Mac OS for files larger than 175K. This is due
to a bug in OCaml 4. (The bug is also in MacPorts since March.)

-- Jerome

>
>>> I think the rsync algorithm is broken when using ocaml 4.
> Indeed, it is broken on Mac OS for files larger than 175K. This is due
> to a bug in OCaml 4. (The bug is also in MacPorts since March.)