SIGBUS Accessing SIF on Non-Local Filesystem

[tri-adam]

When a SIF is loaded from a non-local filesystem using LoadContainer or LoadContainerFp, a SIGBUS may occur when an I/O issue occurs in the underlying storage.

The reason for this is that the SIF module memory maps the file when loading with LoadContainer/LoadContainerFp. When combined with non-local storage, mmap is particularly risky. A SIGBUS can be raised any time an underlying I/O operation fails. Since accessing SIF images from non-local storage is a common use case in SingularityCE, we can't ignore this risk.

In theory, the SIGBUS could be caught with a signal handler, and a scheme could be devised to safely recover. In practice though, the github.com/sylabs/sif module is generally imported into applications which may already define a signal handler. Since these are global, there would need to be some scheme devised to coordinate, and in the end this would be quite cumbersome and clunky.

As an alternative, I propose we look at whether we really need to use memory mapping in the first place. On the surface, it wouldn't appear that we're getting much benefit from it.