Identify all elements belonging to previous and current labels thanks to cleartool find -exec cleartool desc command.

Then for each element, attach these versions or default one "/main/0" (when a file was added or removed).

Add only versions not matching to the changeset.

When one label is provided, set a temporary label on current directory before proceeding to label comparison. This label is removed when a error happen eg. current directory contains checked out elements.

tested done within a clearcase dynamic view and a snapshot view from a linux machine
rbt diff generation between one label and current => OK

rbt diff generation between two labels => OK
rbt post on a reviewboard server 2.0.0 => OK

rbt post on a reviewboard server 1.7.6 with this patch using rbtools release-0.6.2 + 5defd0c + 30b0194

error message when root dir is checked out => OK

error message when any element under root dir is checked out => OK
Label comparison failed because:

ClearCase backend cannot set label when some elements are checked out:

<element xxx>