Document Dynamic child pipelines

- Add docs to parent/child pipelines
- Add docs to yaml reference

changelogs/unreleased/switch-ff-ci-dynamic-child-pipeline.yml

+---
+title: Enable feature Dynamic Child Pipeline creation via artifact
+merge_request: 26648
+author:
+type: added

doc/ci/parent_child_pipelines.md

@@ -126,7 +126,22 @@ microservice_a:
       script: ...
     ```
 
+## Dynamic child pipelines
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/35632) in GitLab 12.9.
+
+Instead of running a child pipeline from a static YAML file, you can define a job that runs
+your own script to generate a YAML file, which is then [used to trigger a child pipeline](yaml/README.md#trigger-child-pipeline-with-generated-configuration-file).
+
+This technique can be very powerful in generating pipelines targeting content that changed or to
+build a matrix of targets and architectures.
+
 ## Limitations
 
 A parent pipeline can trigger many child pipelines, but a child pipeline cannot trigger
-further child pipelines. See the [related issue](https://gitlab.com/gitlab-org/gitlab/issues/29651) for discussion on possible future improvements.
+further child pipelines. See the [related issue](https://gitlab.com/gitlab-org/gitlab/issues/29651)
+for discussion on possible future improvements.
+
+When triggering dynamic child pipelines, if the job containing the CI config artifact is not a predecessor of the
+trigger job, the child pipeline will fail to be created, causing also the parent pipeline to fail.
+In the future we want to validate the trigger job's dependencies [at the time the parent pipeline is created](https://gitlab.com/gitlab-org/gitlab/-/issues/209070) rather than when the child pipeline is created.

doc/ci/yaml/README.md

@@ -2880,6 +2880,31 @@ trigger_job:
     strategy: depend
 ```
 
+##### Trigger child pipeline with generated configuration file
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/35632) in GitLab 12.9.
+
+You can also trigger a child pipeline from a [dynamically generated configuration file](../parent_child_pipelines.md#dynamic-child-pipelines):
+
+```yaml
+generate-config:
+  stage: build
+  script: generate-ci-config > generated-config.yml
+  artifacts:
+    paths:
+      - generated-config.yml
+
+child-pipeline:
+  stage: test
+  trigger:
+    include:
+      - artifact: generated-config.yml
+        job: generate-config
+```
+
+The `generated-config.yml` is extracted from the artifacts and used as the configuration
+for triggering the child pipeline.
+
 #### Linking pipelines with `trigger:strategy`
 
 By default, the `trigger` job completes with the `success` status

lib/gitlab/ci/config/external/file/artifact.rb

@@ -30,7 +30,7 @@ module Gitlab
 
             def matching?
               super &&
-                Feature.enabled?(:ci_dynamic_child_pipeline, project)
+                Feature.enabled?(:ci_dynamic_child_pipeline, project, default_enabled: true)
             end
 
             private