setup:di:compile's generated cache files inaccessible by the web-server user

[JanisE]

Preconditions

1. OS: Linux Mint 18.2 (based on Ubuntu Xenial).
2. Magento CE 2.2.0 with sample data is installed by Composer.
3. Permissions and ownership set up as instructed on http://devdocs.magento.com/guides/v2.1/install-gde/prereq/file-system-perms.html#perms-private for two-user system (users "magento_user" and "www-data").

Steps to reproduce

1. Run bin/magento setup:di:compile as Magento file system owner ("magento_user").
2. Open the web site.

Expected result

1. Web site opens.

Actual result

1. var/cache/* folders lose their set-gid bits.
2. var/cache/mage-tags/* folders are created with "magento_user"'s primary group and 660 permissions.
3. The web user ("www-data") fails opening the site because it cannot access this content:

1 exception(s):
Exception #0 (Exception): Warning: file_put_contents(/var/www/m2training/public/var/cache//mage-tags/mage---762_COMPILED_CONFIG): failed to open stream: Permission denied in /var/www/m2training/public/vendor/colinmollenhour/cache-backend-file/File.php on line 663

The problem was reported also in this StackExchange question: https://magento.stackexchange.com/q/193727/60128

[magento-engcom-team]

@JanisE, thank you for your report.
We've created internal ticket(s) MAGETWO-83148 to track progress on the issue.

[okorshenko]

Hi @JanisE. Thank you for your report.
The issue has been fixed in #14361 by @xtremeperf in 2.2-develop branch
Related commit(s):

• 9baa83fca8a97fe4c0e02557246bdd6069a50b27

The fix will be available with the upcoming 2.2.5 release.