On some Linux distributions tmpfiles.d cleans files and
directories under /tmp if they haven't been accessed for
10 days.
This can cause problems for ML as ML is currently the only
component that uses the temp directory more than a few
seconds after startup. If you didn't open an ML job for
10 days and then tried to open one then the temp directory
would have been deleted.
This commit prevents the problem occurring in the case of
Elasticsearch being managed by systemd, as systemd private
temp directories are not subject to periodic cleanup (by
default).
Additionally there are now some docs to warn people about
the risk and suggest a manual mitigation for .tar.gz users.
2608012422