create_dir_all is not thread safe/races with itself.

[gczzdc]

the relevant section of create_dir_all (https://github.com/manuel-woelker/rust-vfs/blob/master/src/path.rs#L228):

if !self.fs.fs.exists(directory)? {
  self.fs.fs.create_dir(directory)?;
}

Suppose I have a Vec<(&str, &vfspath)>, and I want to concurrently write the strings to the paths, which may or may not exist. Naively, one approach is to run create_dir_all on the paths' parents and then write out to each path. But doing that can race because thread A creates a common ancestor path between the check and the create call from thread B.

Sort of similar to rust-lang/rust#33707