[$] Composefs for integrity protection and data sharing

A read-only filesystem that will transparently share file data between disparate
directory trees, while also providing integrity verification for the data
and the
directory metadata, was recently was developed
by Alexander Larsson (who posted it) and Giuseppe Scrivano for use by (or "libostree" as it
is now known) root directories, but there are likely others who want the
abilities
it provides. So far, there has been little response, either with feedback or
complaints, but it is a small patch set (around 2K lines of code) and
generally self-contained since it is a filesystem, so it would not be a
surprise to see it appear in some upcoming kernel.