Bug#752208: [unison-gtk] Doesn't synchronize windows fat subfolders

["Conchúr Navid" <conchur@web.de>]

Package: unison-gtk
Version: 2.40.65-2
Severity: grave
Tags: wheezy

Here we are using USB sticks to store important data which has to be transported to places where we don't have internet access. They are regularly backed up and updated using unison. This worked great in squeeze. Now we lost some USB sticks due to hardware failure. This wouldn't be problematic when the wheezy unison would actually work. We noticed that all data saved by Windows machines weren't backed up correctly when using unison the next time on a Debian Wheezy machine. This is a type of data loss caused by a malfunction of this software (therefore grave).

This problem is caused by a non-feature of 2.40.x of unison called "Experimental update detection optimization: do not read the contents of unchanged directories" which is enabled by default and cannot be disabled. It was introduced in r374 and refined in r375.

But here is the problem with this non-feature (aka bug): Not all systems update the folder time when the content is changed. This for example happens when you use Windows to add a new file to a subfolder. When unison already synchronized the folder once then it  will just look at the subfolder time, notice that it doesn't changed and not look at the files in it. So the new files will not be synchronized at all.

This caused the data loss here and thus this non-feature should be disabled again or at least possible to disable it (including disabling it by default when the fat option is enabled). Maybe even disable it by default to reduce the possible data loss when other systems don't update the folder time when its content is changed.