Perhaps something like this? This should provide the following
guarantees for zf_mkdir -p:

- If it succeeds, the directory must have existed at some point during
the execution of the function (either created by zf_mkdir itself or by
some other concurrent process).
- If it fails, there must have been a point in time during the
execution of the function where the target directory or one of its
parents didn't exist and it was impossible to create it.

`zf_mkdir -p foo` It should work as expected in the face of concurrent
`mkdir foo && rmdir foo` or `touch foo && rm foo`.

I confess that I haven't tested it.

Roman.