Nested variable not evaluated before transfer to child pipeline when passed with another name

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Summary

Nested variables (e.g. MY_VAR='${CI_PROJECT_NAME}-${CI_PIPELINE_IID}' are not evaluated before being sent to a child pipeline when passed with another name (e.g. TRIGGER_VAR='${MY_VAR}')

Steps to reproduce

# parent project
---
build1:
  stage: build
  variables:
    VERSION: "${CI_PROJECT_NAME}-${CI_PIPELINE_IID}"
  script:
    - echo "VERSION='${VERSION}'"

deploy1:
  stage: deploy
  variables:
    VERSION: "${CI_PROJECT_NAME}-${CI_PIPELINE_IID}"
    APP_VERSION: ${VERSION}
  trigger:
    strategy: depend
    project: child-project

# child project
---
deploy1:
  stage: deploy
  script:
    - echo "APP_VERSION='${APP_VERSION}'"

Example Project

• https://gitlab.com/zigarn/test-trigger-var-parent
• https://gitlab.com/zigarn/test-trigger-var-child

What is the current bug behavior?

Output on parent project (as expected):

$ echo "VERSION='${VERSION}'"
VERSION='parent-project-2'

Output on child project:

$ echo "APP_VERSION='${APP_VERSION}'"
APP_VERSION='child-project-3'

When looking at the pipeline variables through the API, we can see that the value for APP_VERSION is the composed string, not the value of VERSION:

// curl -s -H 'PRIVATE-TOKEN: ...' "https://gitlab.com/api/v4/projects/<project_id>/pipelines/<pipeline_id>/variables"
[
  {
    "variable_type": "env_var",
    "key": "APP_VERSION",
    "value": "${CI_PROJECT_NAME}-${CI_PIPELINE_IID}"
  },
  {
    "variable_type": "env_var",
    "key": "VERSION",
    "value": "parent-project-2"
  }
]

What is the expected correct behavior?

Output on child project:

$ echo "APP_VERSION='${APP_VERSION}'"
APP_VERSION='parent-project-2'

// curl -s -H 'PRIVATE-TOKEN: ...' "https://gitlab.com/api/v4/projects/<project_id>/pipelines/<pipeline_id>/variables"
[
  {
    "variable_type": "env_var",
    "key": "APP_VERSION",
    "value": "parent-project-2"
  },
  {
    "variable_type": "env_var",
    "key": "VERSION",
    "value": "parent-project-2"
  }
]

Relevant logs and/or screenshots

N/A

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

N/A

Possible fixes

Looks like the equivalent of !48627 (merged) for triggers.

A fix would be to use sort_and_expand_all(keep_undefined: true) in https://gitlab.com/gitlab-org/gitlab/-/blob/02f5614e3791768ceb29f5f846dd76be9225a350/app/models/ci/bridge.rb#L204