[Rd] tempdir() may be deleted during long-running R session

[frederik at ofb.net]

> > It might still be useful if R could sometimes detect when automated
> > cleanup happened and warn the user. Perhaps a simple way could be to
> > always create an empty file inside session directory, like
> > ".tmp_cleaner_trap". R would never touch this file, but check its
> > existence time-to-time. If it gets deleted, R would issue a warning and
> > ask the user to check tmp cleaner configuration. The idea is that this
> > file will be the oldest one in the session directory, so would get
> > cleaned up first.

It may kill two birds with one stone if we put the PID into such a
file. Then it could be used to automate the cleanup of /tmp/RtmpXXXX
directories left around from crashed R sessions (since we're about to
tell Systemd not to touch them).

I'm not sure what is meant by "R would ... check its existence
time-to-time". It doesn't sound to me like an important enough task to
go into the main event loop of the interpreter. Maybe a warning/error
in 'tempdir()' to tell us that either the directory has been deleted
or that the PID file no longer exists.

Thank you,

Frederick