Usage of spec:include: within regular includes results in unexpected outcomes

spec:include: is specifically built for pipeline inputs.

We blocked usage for component inputs, but not for using it with regular inputs on included files. While this is not officially supported, some people are already getting value out of certain usage patterns. As seen in the original issue description below, there's also some unexpected outcomes that we should improve.

Original description:

---

Evidently 'local' has problem with branches or paths, because when I switch to 'project', the same file suddenly is found correctly.

If I include from a different repository in a non-default branch, and the inclusion chain reaches a path that exists in that branch but not in the default branch, GitLab says the path does not exist.

Related docs: https://docs.gitlab.com/ci/inputs/#define-pipeline-inputs-in-external-files

Reproducing

Branches:

• my-org/cicd@my-ref: has jobs/comment-inputs.yml
• my-org/cicd@main: does not have jobs/comment-inputs.yml

Error:

Unable to run pipeline jobs/comment.yml: Local file jobs/comment-inputs.yml does not exist!, unknown input arguments: foo

my-org/my-project

.gitlab-ci.yml

spec:
  include:
    - project: 'my-org/cicd'
      ref: my-ref
      file: '/jobs/comment-inputs.yml'
---
include:
  - project: 'my-org/cicd'
    ref: my-ref # important, as the file does not exist in 'main'
    file:
      - '/jobs/comment.yml'
    inputs:
      foo: $[[ inputs.foo ]]

my-org/cicd

jobs/comment-inputs.yml

inputs:
  foo:
    type: string
    default: ""

jobs/comment.yml

spec:
  include:
    - local: jobs/comment-inputs.yml

Workaround

All works when I change 'local' to 'project':

jobs/comment.yml

spec:
  include:
    - project: my-org/cicd
      ref: my-ref
      file: jobs/comment-inputs.yml