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 depend

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.)