Multiple sections in "extends" these days (Gitlab)

Question

I need to do extends of more than one section in .gitlab-ci.yml.

I have three+ "prerequisite" sections, but I don't want to use them all every time in subsequent "production" stages, so I can't combine them into one big prerequisite section.

Specifically I would like to do something like:

.prereq1:
  before_script:
    apt-get update ...
    # many other steps

.prereq2:      
  before_script:
    apt-get install package2 ...
    # many other steps

.prereq3:      
  before_script:
    apt-get install package3 ...
    # many other steps

usable_stage_A:
  stage: usable_stage_A
  only:
    # this will only be ran in pipeline schedule A
    variables:
      - "$JOB_A"
  extends:
   .prereq1
   .prereq2 

usable_stage_B:
  stage: usable_stage_B
  # this will only be ran in pipeline schedule B
  only:
    variables:
      - "$JOB_B"
  extends:
   .prereq1
   .prereq3

How can I do that?

Answer 1

extends: won't work well in this case because arrays can't be combined through this method.

However, you can use !reference to compose the script sections.

.install_scripts:
  prereq1:
    - apt install -y package1
  prereq2:
    - apt install -y package2
  # etc

jobA:
  before_script:
    - !reference [.install_scripts, prereq1]
    - !reference [.install_scripts, prereq2]
    - echo 'more before script for jobA'
  # ...

---

It is possible to provide an array of values to extends: to extend from multiple keys. However, only hashes are merged. Arrays or scalar values are simply overridden. Hence, because before_script: only accepts arrays or strings, it cannot be merged this way.

Reference: yaml merge details:

You can use extends to merge hashes but not arrays. The algorithm used for merge is “closest scope wins”. When there are duplicate keys, GitLab performs a reverse deep merge based on the keys. Keys from the last member always override anything defined on other levels.