refactor/generalize native-mode code for passing FUSE mounts by file descriptor?

[preminger]

Type of issue

Technical debt

Description of issue

Evaluate the prospects for refactoring and/or generalizing the native-mode code that passes FUSE mounts by file descriptor from host to container.

In particular, in the following source files:

• internal/pkg/runtime/engine/singularity/container_linux.go
• internal/pkg/runtime/engine/singularity/prepare_linux.go
• internal/pkg/runtime/engine/singularity/process_linux.go

[dtrudg]

Because we won't be exploring FUSE mount by fd for setuid squashfs/extfs mount, this isn't important at this point.

It may be worth taking up when fuse2fs supports FUSE3, with fd mounts, and setuid FUSE can be revisited.