GlideinWMS

Sometimes CVMFS is not available and Singularity neither.

The Glidein should be able to connect to CVMFS and start Singularity from there.
There are some possibility, from Dave:

1. mountrepo/umountrepo only
• requires fusermount; mounts in user space
• map /cvmfs in container with singularity –bind (not run from cvmfs because of path)
2. cvmfsexec on RHEL 7.6 or 7.7
• requires fusermount and additionally unprivileged user namespaces enabled
• maps /cvmfs without singularity, can run singularity under it
• unmounts repos on exit, but not with kill -9
3. cvmfsexec on RHEL >= 7.8
• no fusermount needed, and cleans up mounts even with kill -9
• still needs unprivileged user namespaces enabled; that is default on RHEL 8
4. singcvmfs on any system with singularity >= 3.4.0
• drop-in replacement for singularity; set environment variable with cvmfs repos to mount
• requires container image to already be present (not read from cvmfs)
• requires setuid-root singularity except when RHEL >= 7.8 and and singularity >= 3.6.0

• package building (at reconfig/upgreade)
• proxy search and setup (WPAD)