Default:tags does not inherit to jobs

Summary

Tagging jobs allows for granular control over which jobs are executed on which runners. In issue #16043 (closed), a suggestion was raised to allow for tags to be set globally, or configured as default options (which the resulting MR eventually settled on).

When using tags as a default parameter, the value does not inherit to jobs that do not explicitly specify the tag to use. Is this working as intended? If so, the documentation should be updated to clarify that the default tags will not be inherited by jobs.

The workaround is to explicitly define tags for each job, or to use job templates as was mentioned in the original issue that lead to the merge request.

This functionality was added in !20747 (merged).

Premium customer facing this problem: ticket 142554 (internal).

Steps to reproduce

Have a runner configured to only accept tagged jobs with docker-dind as the tag.

A sample CI:

default:
  tags: ['docker-dind']

job1:
  tags: ['docker-dind']
  script: echo "explicit tags"
  
job2:
  script: echo "inherit tags"

job1 runs as expected; job2 holds indefinitely waiting for a runner. It is marked as "stuck" in the UI.

Example Project

I have a quick demo of project on GitLab.com using a specified runner, configured to only accept tagged jobs: https://gitlab.com/kevenhughes/default-tags

specific runner NqziYn6g, available tags are cluster and kubernetes.

CI has two jobs, one which explicitly defines a tag, and one which relies on default to inherit a tag.

The resulting pipeline, wherein job1 succeeds but job2 sits on hold indefinitely due to being untagged.

https://gitlab.com/kevenhughes/default-tags/pipelines/106902940/builds

What is the current bug behavior?

CI config default:tags is not inherit by jobs which don't otherwise specify a list of tags. If this isn't a bug, but is working correctly by design, the documentation should be clarified.

What is the expected correct behavior?

As a default config, it should be applied to jobs without an explicit definition of tags.

Possible fixes

My original thought was that tags is not set to inherit: true in lib/gitlab/ci/config/entry/default.rb