[sagarm]

That hash would be expensive to maintain, and the end result would still be racy since the file could be modified after the hash was read .

[actionfromafar]

In the current POSIX paradigm yes, it would be expensive. But if the hash was defined as the hash of fixed blocks, it wouldn't be expensive. The raciness depends, a lot, on the semantics we would define. (In the context of a build system, it's no different than that the file could get a new mtime after we read the mtime.)