passenger: passenger-config fails with "too long unix socket path" on Ubuntu 18.04 LTS

Issue report

When running Passenger on hosts upgraded from Ubuntu 16.04 LTS to 18.04 LTS, passenger-config fails with

Traceback (most recent call last):
	9: from /usr/bin/passenger-config:37:in `<main>'
	8: from /usr/lib/ruby/vendor_ruby/phusion_passenger/config/main.rb:79:in `run!'
	7: from /usr/lib/ruby/vendor_ruby/phusion_passenger/config/restart_app_command.rb:46:in `run'
	6: from /usr/lib/ruby/vendor_ruby/phusion_passenger/config/restart_app_command.rb:144:in `select_app_group_name'
	5: from /usr/lib/ruby/vendor_ruby/phusion_passenger/config/restart_app_command.rb:174:in `select_app_group_name_interactively'
	4: from /usr/lib/ruby/vendor_ruby/phusion_passenger/config/restart_app_command.rb:164:in `query_group_names'
	3: from /usr/lib/ruby/vendor_ruby/phusion_passenger/config/restart_app_command.rb:255:in `query_pool_xml'
	2: from /usr/lib/ruby/vendor_ruby/phusion_passenger/admin_tools/instance.rb:95:in `http_request'
	1: from /usr/lib/ruby/vendor_ruby/phusion_passenger/admin_tools/instance.rb:95:in `new'
/usr/lib/ruby/vendor_ruby/phusion_passenger/admin_tools/instance.rb:95:in `initialize': too long unix socket path (116bytes given but 108bytes max) (ArgumentError)

Question 1: What is the problem?

• What is the expected behavior?

Running passenger-config is possible.

• What is the actual behavior?

passenger-config fails with the above stack trace. Debugging shows that the UNIX socket path Passenger tries to use is something like

/tmp/systemd-private-673f3b327552487c9d5cc4d934decaf-apache2.service-GfIBRc/tmp/passenger.UqAQsAS/agents.s/core_api

which is over the system limit.

• How can we reproduce it?

I have not tried on a vanilla 18.04, but there seems to be additional evidence here, with no solution:

https://stackoverflow.com/questions/70012839/phusion-passenger-status-crashing-with-too-long-socket-name

Your answer:

Question 2: Passenger version and integration mode:

Your answer:

Ubuntu libapache2-mod-passenger 1:6.0.12-1~bionic1

Question 3: OS or Linux distro, platform (including version):

Your answer:

Ubuntu 18.04.6 LTS

Question 4: Passenger installation method:

Your answer:

4.15.0-162-generic #170-Ubuntu SMP Mon Oct 18 11:38:05 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

[] RubyGems + Gemfile [ ] RubyGems, no Gemfile [x] Phusion APT repo [ ] Phusion YUM repo [ ] OS X Homebrew [ ] source tarball [ ] Other, please specify:

Question 5: Your app’s programming language (including any version managers) and framework (including versions):

Your answer:

System Ruby is ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]

Question 6: Are you using a PaaS and/or containerization? If so which one?

Your answer:

No, plain VM

Question 7: Anything else about your setup that we should know?

Your answer:

As mentioned before, the system was upgraded from Ubuntu 16.04 LTS.