eups distrib does not allow to build a distrib against a stack in a non-writeable place

[johannct]

My use case is the LSST stack distributed through CVMFS, and I want to eups distrib lsst_sims. This seems like a natural idea to base one's work on a derived set of packages like sims built against a "production" version of the stack, installed in a non writeable directory (whether cvmfs or otherwise).
But https://github.com/RobertLuptonTheGood/eups/blob/master/python/eups/Eups.py#L2134 raises exception if it can't tag the package in the stack repository. See also https://community.lsst.org/t/why-is-eups-distrib-needing-write-access-to-a-product-stack-and-database/497.

In my use case, I have EUSP_PATH=/home/cohen/lsst/dm/stack/lsst_sims:/cvmfs/lsst.in2p3.fr/software/linux-x86_64/lsst-v11.0 , so at any rate eups could tag it in the local directory if not in the remote non writeable one. There may be a rationale behind, that I do not understand, but installing a local sims distrib for development or use without having to first install the whole stack in a local directory seems like a reasonable option when the stack will become stable and start to be widely distributed in production mode. CVMFS precisely paves the road for such site-wide production distribution.