Robust out-of-space handling

[DemiMarie]

In a production server, space exhaustion on a thin pool is a system misconfiguration. However, servers are not the only place Stratis could be used. Stratis can also be used on desktops, and there robust handling of ENOSPC is critical.

In this context, “robust” means:

• Committed (fsync’d) data is not lost.
• On-disc and in-memory data structures are not corrupted.
• All data remains readable.
• The user can recover without having to expand the storage pool, such as by deleting files or performing fstrim on filesystems.
• Once sufficient space is freed up, the user can begin using the system again.