Add a loop construct to .gitlab-ci.yml

Problem to solve

Currently, if I want to generate a list of jobs based on some variable, I have no way of doing so. In GitLab's own .gitlab-ci.yml, we resort to repetition around a YAML template, e.g.:

rspec-pg 0 30: *rspec-metadata-pg
rspec-pg 1 30: *rspec-metadata-pg
rspec-pg 2 30: *rspec-metadata-pg
rspec-pg 3 30: *rspec-metadata-pg
rspec-pg 4 30: *rspec-metadata-pg
rspec-pg 5 30: *rspec-metadata-pg
rspec-pg 6 30: *rspec-metadata-pg
rspec-pg 7 30: *rspec-metadata-pg
rspec-pg 8 30: *rspec-metadata-pg
rspec-pg 9 30: *rspec-metadata-pg
rspec-pg 10 30: *rspec-metadata-pg
rspec-pg 11 30: *rspec-metadata-pg
rspec-pg 12 30: *rspec-metadata-pg
rspec-pg 13 30: *rspec-metadata-pg
rspec-pg 14 30: *rspec-metadata-pg
rspec-pg 15 30: *rspec-metadata-pg
rspec-pg 16 30: *rspec-metadata-pg
rspec-pg 17 30: *rspec-metadata-pg
rspec-pg 18 30: *rspec-metadata-pg
rspec-pg 19 30: *rspec-metadata-pg
rspec-pg 20 30: *rspec-metadata-pg
rspec-pg 21 30: *rspec-metadata-pg
rspec-pg 22 30: *rspec-metadata-pg
rspec-pg 23 30: *rspec-metadata-pg
rspec-pg 24 30: *rspec-metadata-pg
rspec-pg 25 30: *rspec-metadata-pg
rspec-pg 26 30: *rspec-metadata-pg
rspec-pg 27 30: *rspec-metadata-pg
rspec-pg 28 30: *rspec-metadata-pg
rspec-pg 29 30: *rspec-metadata-pg

rspec-mysql 0 30: *rspec-metadata-mysql
rspec-mysql 1 30: *rspec-metadata-mysql
rspec-mysql 2 30: *rspec-metadata-mysql
rspec-mysql 3 30: *rspec-metadata-mysql
rspec-mysql 4 30: *rspec-metadata-mysql
rspec-mysql 5 30: *rspec-metadata-mysql
rspec-mysql 6 30: *rspec-metadata-mysql
rspec-mysql 7 30: *rspec-metadata-mysql
rspec-mysql 8 30: *rspec-metadata-mysql
rspec-mysql 9 30: *rspec-metadata-mysql
rspec-mysql 10 30: *rspec-metadata-mysql
rspec-mysql 11 30: *rspec-metadata-mysql
rspec-mysql 12 30: *rspec-metadata-mysql
rspec-mysql 13 30: *rspec-metadata-mysql
rspec-mysql 14 30: *rspec-metadata-mysql
rspec-mysql 15 30: *rspec-metadata-mysql
rspec-mysql 16 30: *rspec-metadata-mysql
rspec-mysql 17 30: *rspec-metadata-mysql
rspec-mysql 18 30: *rspec-metadata-mysql
rspec-mysql 19 30: *rspec-metadata-mysql
rspec-mysql 20 30: *rspec-metadata-mysql
rspec-mysql 21 30: *rspec-metadata-mysql
rspec-mysql 22 30: *rspec-metadata-mysql
rspec-mysql 23 30: *rspec-metadata-mysql
rspec-mysql 24 30: *rspec-metadata-mysql
rspec-mysql 25 30: *rspec-metadata-mysql
rspec-mysql 26 30: *rspec-metadata-mysql
rspec-mysql 27 30: *rspec-metadata-mysql
rspec-mysql 28 30: *rspec-metadata-mysql
rspec-mysql 29 30: *rspec-metadata-mysql

rspec-pg-rails5 0 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 1 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 2 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 3 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 4 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 5 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 6 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 7 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 8 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 9 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 10 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 11 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 12 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 13 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 14 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 15 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 16 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 17 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 18 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 19 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 20 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 21 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 22 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 23 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 24 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 25 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 26 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 27 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 28 30: *rspec-metadata-pg-rails5
rspec-pg-rails5 29 30: *rspec-metadata-pg-rails5

rspec-mysql-rails5 0 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 1 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 2 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 3 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 4 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 5 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 6 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 7 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 8 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 9 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 10 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 11 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 12 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 13 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 14 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 15 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 16 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 17 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 18 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 19 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 20 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 21 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 22 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 23 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 24 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 25 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 26 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 27 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 28 30: *rspec-metadata-mysql-rails5
rspec-mysql-rails5 29 30: *rspec-metadata-mysql-rails5

Further details

The gitlab-ci.yml file can be thought of as a declarative programming language. In that context, loops are a natural construct to have. Missing them means that we have to make more use of advanced YAML-specific features like templating, and even then, we still have a huge amount of boilerplate repetition. This makes the file harder to read, and harder to write, than it needs to be.

Proposal

We could introduce a looping construct. Ansible has several of these in its own yaml format, so it's not a novel idea.

What does success look like, and how can we measure that?

For instance, we could restate the above as:

rspec-pg %{n} 30:
  <<: *rspec-metadata-pg
  with_range:
    start: 0
    end: 29
    name: n

rspec-mysql %{n} 30:
  <<: *rspec-metadata-mysql
  with_range:
    start: 0
    end: 29
    name: n

rspec-mysql-rails5 %{n} 30:
  <<: *rspec-metadata-mysql-rails5
  with_range:
    start: 0
    end: 29
    name: n

I suspect (but haven't checked) that this would make the rspec-metadata-x templates unnecessary. We'd just put the loop on the jobs that are currently templates, simplifying again.

We could also support nested loops, although I feel like that's significantly more niche than 1-dimensional ones.

Ansible has a range of with_x directives, and also supports jinja2 templating, and variables at a higher level than we do. I think we can just import the loops without having to import jinja2 or variables, and still get considerable value.

Links / references

https://docs.ansible.com/ansible/latest/user_guide/playbooks_loops.html

/cc @bikebilly @ayufan @DylanGriffith