How does Gitlab-ci triggered subproject evaluate variables in a multiproject pipeline?

Question

Context

1. I have a parent (upstream) project with a gitlab ci pipeline
2. I have a child (downtream) project with a gitlab ci pipeline
3. parent triggers child
4. Both parent & child declare a VERSION representing the version of the specific project

Goal

1. Parent shall pass its version as CORE_VERSION to child (for reporting purposes)

Issue

Cannot pass variable - that references another variable - from upstream(parent) pipeline to downstream(child) one.

Question

1. how can I pass these variables properly?

Details

1. parent/upstream project - CI_COMMIT_SHORT_SHA=09cceef0
2. child/downstream project - CI_COMMIT_SHORT_SHA=c09dcc86
3. gitlab-ci of parent

[...]
# 
variables:
VERSION: 1.70.$CI_COMMIT_SHORT_SHA
# parent version declaration; value: 1.70.09cceef0 
[...]
trigger_childproject:
stage: qa
trigger:
  project: childproject
  branch: master
  strategy: depend
inherit: # only allow propagating these variables
  variables:
    - CI_COMMIT_BRANCH
    - VERSION
variables:
  # would like to access upstream project variables using the CORE_ prefix
  CORE_VERSION: $VERSION
  CORE_BRANCH: $CI_COMMIT_BRANCH

1. gitlab-ci.yml of child

[...]
# 
variables:
VERSION: 1.70.$CI_COMMIT_SHORT_SHA
# child version declaration; value expected to be: 1.70.c09dcc86 
[...]
job_of_child:
stage: test
script: scripts/run-test.sh

1. Source of run-test.sh with (+)Output with (#) Observations

+ Checking out c09dcc86 as master...
[...]
BUILD_REF=${CI_COMMIT_SHORT_SHA:-$(git log --oneline -n1 | grep -Eo '^\w+')}
+ BUILD_REF=c09dcc86
# BUILD_REF has received the child's CI_COMMIT_SHORT_SHA; as expected

VERSION=${VERSION:-"1.0.$BUILD_REF"}
+ VERSION=1.70.09cceef0
# VERSION has received parent project's version
# unexpected, but apparently parent VERSION overrides child declaration
# OK i guess.

[...]
CORE_VERSION=${CORE_VERSION:-?}
+ CORE_VERSION=1.70.c09dcc86
# and this is where it is "messed up" 
# The CORE_VERSION - apparently/as far as i can tell - is interpreted as:
# CORE_VERSION: $VERSION # from parent's trigger
# CORE_VERSION=1.70.$CI_COMMIT_SHORT_SHA # VERSION is interpreted as declared in parent
# CORE_VERSION=1.70.c09dcc86 # ie. CI_COMMIT_SHORT_SHA is taken from child

echo ":thumbsup: $TARGET_URL[$CORE_VERSION] QA/diff-hga[$VERSION]: SUCCESS"
+ :thumbsup: parent[1.70.c09dcc86] checked by child[1.70.09cceef0]: SUCCESS
# this is where this whole message is confusing, because parent reports version of child and vice versa :)

Related discussions

1. https://gitlab.com/groups/gitlab-org/-/epics/4529
2. https://gitlab.com/gitlab-org/gitlab/-/issues/10126
3. Unable to pass variable to downstream pipeline on GitLab CI

Answer 1

Thanks @sytech for the information.

Reading through the above linked resources, I also ended up with the conclusion: it's an "issue" of how GitLab treats variables; evaluates them; passes them from context to context.

The main issues were

1. overlapping variable names - in my case: VERSION
2. "recursive" / dynamic definition

My solution

I ended up doing the following

parent-gitlab-ci.yml

# parent / global declaration
variables:
  VERSION: 1.70.$CI_COMMIT_SHORT_SHA
  [...]
  UPSTREAM_CI_COMMIT_BRANCH: $CI_COMMIT_BRANCH
  UPSTREAM_CI_COMMIT_SHORT_SHA: $CI_COMMIT_SHORT_SHA
  # this still doesn't work properly; but ok; child script relies on UPSTREAM_CI_COMMIT_SHORT_SHA
  UPSTREAM_VERSION: $VERSION
[...]
# in the trigger
trigger_pmp-qa:
  stage: qa
  trigger:
    project: child
    branch: master
    strategy: depend
  inherit: # only allow propagating the declared vars
    variables: 
      - UPSTREAM_CI_COMMIT_BRANCH
      - UPSTREAM_CI_COMMIT_SHORT_SHA
      # this still doesn't work properly; but ok; child script relies on UPSTREAM_CI_COMMIT_SHORT_SHA
      - UPSTREAM_VERSION
  variables: # plus statically declared vars
    SLACK_ENABLED: 'true'

child script

# references UPSTREAM_CI_COMMIT_SHORT_SHA; 
# does not see "UPSTREAM_VERSION" because of expansion issues
# but it's ok :smilecry:
CORE_VERSION=${UPSTREAM_VERSION:-1.??.${UPSTREAM_CI_COMMIT_SHORT_SHA:-?}}