File.mkdir_p: Fix TOCTOU issue & handle errors from parent mkdir

[lanodan]

Fix TOCTOU, meaning :file.make_dir returning {:error, :eexists} is
now tested without having to trigger a race-condition.
Also change dir?() returning false from returning :eexist to :enotdir

And handling errors from parent directory creation allows to avoid
non-sensical errors like File.mkdir_p returning :enoent instead of :eacces

[josevalim]

Thank you for the PR, can you please expand which errors/issues are you trying to avoid?

[lanodan]

For TOCTOU, the first dir?() is effectively useless and hid false status from the second dir?() not being handled correctly as it would return {:error, :eexists} instead of the more proper {:error, :enotdir}, which happens when parallel calls to mkdir_p are done with shared parent directories which needs to be created.

And one problem which Pleroma has been hitting few times is one of the parent directories having wrong permissions,
currently Elixir just ignores the error and so ends up with "No such file or directory" errors (which makes no sense for mkdir_p) instead of "Permission denied".

[josevalim]

And one problem which Pleroma has been hitting few times is one of the parent directories having wrong permissions,
currently Elixir just ignores the error and so ends up with "No such file or directory" errors (which makes no sense for mkdir_p) instead of "Permission denied".

Fantastic, can we please try to add a test that reproduces this then? Thank you!

[josevalim]

💚 💙 💜 💛 ❤️