Figure out how to deal with inaccessible parent directories

[Stebalien]

So, unix really is lovely...

> mkdir -p /foo/bar
> cd /foo/bar
> chmod a-x /foo
> touch first
> touch /foo/bar/second
touch: cannot touch '/foo/bar/second': Permission denied

As written, this library has the same problem as the second case. We use the absolute path when creating temporary files instead of the relative path. I believe this can actually be an issue in practice with some jails.

Fixing this on linux is doable with openat and unlinkat. Fixing this on osx is possible using per-thread CWDs (at the cost of two extra syscalls to set/unset the thread-local CWD).

[tbu-]

According to What non-Linux Unixes support openat? from StackOverflow:

[…], all major BSDs and Apple's OS X seem to support [openat] as of today:

• DragonFly since DragonFly 2.3.
• FreeBSD since FreeBSD 8.0.
• Linux since Linux 2.6.16 (for completeness).
• NetBSD since NetBSD 7.0.
• OpenBSD since OpenBSD 5.0.
• OS X since OS X 10.10.
• Solaris.

[Stebalien]

Ah. The information I was looking at was from 2011. Good to know.

[rbtcollins]

fs_at is a new crate I created as part of fixing the TOCTOU remove_dir_all inherited from the std::fs::remove_dir_all it was based on years ago; it uses NTCreateFile on windows and openat on unix - my intent for it is to be a medium-level crate: no sophisticated features or policy choices, just convenient access to 'at' style syscalls. If its not suitable for solving this bug, I'd like to know why so I can fix it.