config.MkdirAll does not return an error if the requested directory already exists but does not have the requested permissions

[brianaydemir]

Pretty much what the title says.

Pelican can be configured to read/write data from many locations on the filesystem. Some of these locations are passed on to other applications, such as XRootD. Especially for these other applications, it is not sufficient to check that the directory merely exists. The directory must exist with the correct permissions — which Pelican knows, but does not check for if the directory already exists.

This issue came to light where OSDF caches running v7.11.8 crashed on start up, but not on earlier versions. To make a long story short, the caches crashed on start-up because an upgraded version of XRootD performed additional sanity checks on its configuration. Pelican is responsible for providing XRootD it's configuration in the first place, so Pelican should be performing those same, or at least substantively similar, sanity checks.