mkdtemp doesn't work with chdir and relative paths

[erikcorry]

There's no -at version of mkdtemp like (openat is a version of open with an explicit 'current directory').

Context: Because there are multiple Toit processes in one Unix process, we never use the chdir syscall. Instead, every Toit process has a handle open on its current directory. We don't use the open syscall, which uses the Unix-process-wide current directory, but instead use openat, which takes an open file descriptor (directory descriptor, actually) as the location of relative paths.

Possible fixes

• Use the same technique as Windows: Keeping track of the textual current directory for the process and prepending it.
  • This sort of works, but unlike the rest of current-dir support on Unix it fails if the current directory is renamed at a bad moment.
• Removing support for temporary directories outside of /tmp
  • it's not very useful anyway, I think.
• Introduce a C++-level mkdtemp mutex and fchdir the whole Unix process (all Toit processes) while doing the mkdtemp.
  • this would probably smoke out any bugs where we accidentally still use the Unix-process-wide current directory on Unix.

[floitsch]

We use temporary directories outside of /tmp quite a lot: whenever we need to cache something, we create a temporary directory in the cache. This way we can use an atomic move operation to store the downloaded object in the cache.
If we always used /tmp we could be on a different drive, which would result in a slower and non-atomic copy operation.