Cron throws exceptions during a cache purge as localcachedir is purged / get_request_storage_directory should use system temp

1) Add  MDL-66928 -requestdir-purge.php and run it, this touches a file in a request directory and then polls it to make sure it keeps existing. You can open up a couple of these in different tabs 2) Purge the caches php admin/cli/purge_caches.php 3) Confirm all of the progress bars still say 'good' 4) Find your tmp dir on your local box. Note that depending on setup it could be in a magic hidden directory, see: https://www.the-art-of-web.com/php/where-is-tmp/ On my ubuntu box under apache it ends up as: /tmp/systemd-private-9f3b4bc5268d487b93aea4b0be240825-apache2.service-2FvyhK/tmp/ Now confirm you can find for all requestdirs to see the touch files: $ sudo find /tmp/systemd-private-9f3b4bc5268d487b93aea4b0be240825-apache2.service-2FvyhK/tmp/requestdirs /tmp/systemd-private-9f3b4bc5268d487b93aea4b0be240825-apache2.service-2FvyhK/tmp/requestdirs /tmp/systemd-private-9f3b4bc5268d487b93aea4b0be240825-apache2.service-2FvyhK/tmp/requestdirs/fa73f267-cfb4-4a85-b7a8-4c12da5b9c45 /tmp/systemd-private-9f3b4bc5268d487b93aea4b0be240825-apache2.service-2FvyhK/tmp/requestdirs/fa73f267-cfb4-4a85-b7a8-4c12da5b9c45/544a4333-f942-4445-a226-76f135d36ef7 /tmp/systemd-private-9f3b4bc5268d487b93aea4b0be240825-apache2.service-2FvyhK/tmp/requestdirs/fa73f267-cfb4-4a85-b7a8-4c12da5b9c45/544a4333-f942-4445-a226-76f135d36ef7/touch 5) kill of the tabs and confirm that each dir is cleaned    6) Now set an explicit local temp path and repeat all the steps above: $CFG->localtempdir = '/var/lib/sitedata/moodle.local/mytemp';