Enable feature flag ci_cross_pipeline_artifacts_download

Set feature flag to be enabled by default and add documentation.

app/models/ci/build_dependencies.rb

@@ -143,7 +143,7 @@ module Ci
 
     def specified_cross_pipeline_dependencies
       strong_memoize(:specified_cross_pipeline_dependencies) do
-        next [] unless Feature.enabled?(:ci_cross_pipeline_artifacts_download, processable.project, default_enabled: false)
+        next [] unless Feature.enabled?(:ci_cross_pipeline_artifacts_download, processable.project, default_enabled: true)
 
         specified_cross_dependencies.select { |dep| dep[:pipeline] && dep[:artifacts] }
       end

changelogs/unreleased/enable-ci-cross-pipeline-artifacts-download.yml

+---
+title: Allow job to download artifacts in parent-child pipeline hierarchy
+merge_request: 49837
+author:
+type: added

config/feature_flags/development/ci_cross_pipeline_artifacts_download.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/287622
 milestone: '13.7'
 type: development
 group: group::continuous integration
-default_enabled: false
+default_enabled: true

doc/ci/yaml/README.md

@@ -2088,12 +2088,59 @@ build_job:
   needs:
     - project: $CI_PROJECT_PATH
       job: $DEPENDENCY_JOB_NAME
-      ref: $CI_COMMIT_BRANCH
+      ref: $ARTIFACTS_DOWNLOAD_REF
       artifacts: true
 ```
 
 Downloading artifacts from jobs that are run in [`parallel:`](#parallel) is not supported.
 
+To download artifacts between [parent-child pipelines](../parent_child_pipelines.md) use [`needs:pipeline`](#artifact-downloads-to-child-pipelines).
+Downloading artifacts from the same ref as the currently running pipeline is not
+recommended because artifacts could be overridden by concurrent pipelines running
+on the same ref.
+
+##### Artifact downloads to child pipelines
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/255983) in GitLab v13.7.
+
+A [child pipeline](../parent_child_pipelines.md) can download artifacts from a job in
+its parent pipeline or another child pipeline in the same parent-child pipeline hierarchy.
+
+For example, with the following parent pipeline that has a job that creates some artifacts:
+
+```yaml
+create-artifact:
+  stage: build
+  script: echo 'sample artifact' > artifact.txt
+  artifacts:
+    paths: [artifact.txt]
+
+child-pipeline:
+  stage: test
+  trigger:
+    include: child.yml
+    strategy: depend
+  variables:
+    PARENT_PIPELINE_ID: $CI_PIPELINE_ID
+```
+
+A job in the child pipeline can download artifacts from the `create-artifact` job in
+the parent pipeline:
+
+```yaml
+use-artifact:
+  script: cat artifact.txt
+  needs:
+    - pipeline: $PARENT_PIPELINE_ID
+      job: create-artifact
+```
+
+The `pipeline` attribute accepts a pipeline ID and it must be a pipeline present
+in the same parent-child pipeline hierarchy of the given pipeline.
+
+The `pipeline` attribute does not accept the current pipeline ID (`$CI_PIPELINE_ID`).
+To download artifacts from a job in the current pipeline, use the basic form of [`needs`](#artifact-downloads-with-needs).
+
 ### `tags`
 
 Use `tags` to select a specific runner from the list of all runners that are