Draft: Prevent file variable content expansion in downstream pipeline

What does this MR do and why?

Part 2 of #390252 (closed)

Prevent file variable expansion in downstream pipeline.

Before this change, when a trigger job interpolates a file type variable to be used in a downstream pipeline, it would expand the file variable to show the content of the file. This change fixes it such that the content of file variables are not interpolated into a new variable.

This MR depends on the implementation of expand_file_refs: option in ::ExpandVariables.expand, implemented in this MR.

Screenshots or screen recordings

Given the following file variables set in the group, project and pipeline: GROUP_FILE_VARIABLE, PROJECT_FILE_VARIABLE, PIPELINE_FILE_VARIABLE.

Without interpolating file variable:

trigger_child:
  trigger:
    include:
      - local: child-pipeline.yml

trigger_cross_project:
  trigger:
    project: test-group-vars/downstream-project

Without interpolation, there is no change in behaviour. The downstream pipeline will or will not have access to the file variable, depending on the scope (group, project, pipeline). If it has access to the file variable, the file variable will be loaded to the file system and the environment variable set to the file path.

Pipeline	Before	After
Child pipeline
Cross project

With interpolating file variable:

trigger_child_with_interpolation:
  variables:
    GROUP_FILE_VARIABLE: "$GROUP_FILE_VARIABLE"
    PROJECT_FILE_VARIABLE: "$PROJECT_FILE_VARIABLE"
    PIPELINE_FILE_VARIABLE: "$PIPELINE_FILE_VARIABLE"
  trigger:
    include:
      - local: child-pipeline.yml

trigger_cross_project_with_interpolation:
  variables:
    GROUP_FILE_VARIABLE: "$GROUP_FILE_VARIABLE"
    PROJECT_FILE_VARIABLE: "$PROJECT_FILE_VARIABLE"
    PIPELINE_FILE_VARIABLE: "$PIPELINE_FILE_VARIABLE"
  trigger:
    project: test-group-vars/downstream-project

With interpolation, there is a change in behaviour. Previously, the downstream pipeline will have the variables interpolated to the content of the files. After the change, the downstream pipeline will get the variables set in the trigger job's variables without interpolation.

Pipeline	Before	After
Child pipeline
Cross project

How to set up and validate locally

1. Create a group test-group.
2. Create 2 projects test-group/main-project and test-group/downstream-project
3. In test-group, create a File variable GROUP_FILE_VARIABLE.
4. In test-group/main-project, create a File variable PROJECT_FILE_VARIABLE.
5. In test-group/main-project, create the following CI YMLs for the parent pipeline and child pipeline.
6. In test-group/downstream-project, create the following CI yml
7. Run pipeline manually on test-group/main-project, specifying a File variabel PIPELINE_FILE_VARIABLE.
8. Observe the results of the echo jobs
9. Enable feature flag :ci_prevent_file_var_expansion_downstream_pipeline
10. Repeat steps to run pipeline manually
11. Observe the results of the echo jobs

main-project CI YMLs

# .gitlab-ci.yml
echo:
  script:
    - echo "GROUP_FILE_VARIABLE $GROUP_FILE_VARIABLE"
    - echo "PROJECT_FILE_VARIABLE $PROJECT_FILE_VARIABLE"
    - echo "PIPELINE_FILE_VARIABLE $PIPELINE_FILE_VARIABLE"

trigger_child:
  trigger:
    include:
      - local: child-pipeline.yml

trigger_child_with_interpolation:
  variables:
    GROUP_FILE_VARIABLE: "$GROUP_FILE_VARIABLE"
    PROJECT_FILE_VARIABLE: "$PROJECT_FILE_VARIABLE"
    PIPELINE_FILE_VARIABLE: "$PIPELINE_FILE_VARIABLE"
  trigger:
    include:
      - local: child-pipeline.yml

trigger_cross_project:
  trigger:
    project: test-group/downstream-project

trigger_cross_project_with_interpolation:
  variables:
    GROUP_FILE_VARIABLE: "$GROUP_FILE_VARIABLE"
    PROJECT_FILE_VARIABLE: "$PROJECT_FILE_VARIABLE"
    PIPELINE_FILE_VARIABLE: "$PIPELINE_FILE_VARIABLE"
  trigger:
    project: test-group/downstream-project

# child-pipeline.yml
echo:
  script:
    - echo "GROUP_FILE_VARIABLE $GROUP_FILE_VARIABLE"
    - echo "PROJECT_FILE_VARIABLE $PROJECT_FILE_VARIABLE"
    - echo "PIPELINE_FILE_VARIABLE $PIPELINE_FILE_VARIABLE"

downstream-project CI yml

echo:
  script:
    - echo "GROUP_FILE_VARIABLE $GROUP_FILE_VARIABLE"
    - echo "PROJECT_FILE_VARIABLE $PROJECT_FILE_VARIABLE"
    - echo "PIPELINE_FILE_VARIABLE $PIPELINE_FILE_VARIABLE"

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #390252 (closed)