FileSystem.runAtomicAction seems like a neat way for a filesystem to support
real transactions, if it has that ability--i.e. if within an atomic block the
client creates a new FileObject, writes contents to it, and sets attributes,
then the filesystem could store up this info and only actually write it when the
atomic block finishes; if there is a problem it would rollback the block.

But runAtomicAction is final, and associated methods such as beginAtomicAction
and finishAtomicAction are package-private (anyway you would need probably
rollbackAtomicAction or something like this). So it is not possible AFAIK.