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Whether the dir creation is done right-to-left or
left-to-right is less important. If the expected usage
pattern is that most of the directories already exist, then
right-to-left may be faster, otherwise left-to-right is. One
advantage with the former is its slightly simpler code (no
need to check for ENOENT).

>current 2.4 code does not return an error if the directory
exists,
>the patch must not change that behavior.

You mean the contrary? From what I can see of the 2.4 code,
it throws an error if the directory exists. This is almost
never what you want, so I strongly doubt fixing that
misfeature in 2.5 would break anything. I'm happy with the
suggested patch for 2.5 in #1239890.