Expand pipeline variables passed downstream

doc/ci/multi_project_pipelines.md

@@ -134,6 +134,34 @@ staging:
 The `ENVIRONMENT` variable will be passed to every job defined in a downstream
 pipeline. It will be available as an environment variable when GitLab Runner picks a job.
 
+In the following configuration, the `MY_VARIABLE` variable will be passed
+downstream, because jobs inherit variables declared in top-level `variables`:
+
+```yaml
+variables:
+  MY_VARIABLE: my-value
+
+my-pipeline:
+  variables:
+    ENVIRONMENT: something
+  trigger: my/project
+```
+
+You might want to pass some information about the upstream pipeline using, for
+example, predefined variables. In order to do that, you can use interpolation
+to pass any variable. For example:
+
+```yaml
+my-pipeline:
+  variables:
+    UPSTREAM_BRANCH: $CI_COMMIT_REF_NAME
+  trigger: my/project
+```
+
+In this scenario, the `UPSTREAM_BRANCH` variable with a value related to the
+upstream pipeline will be passed to a `downstream` job, and will be available
+within the context of all downstream builds.
+
 ### Limitations
 
 Because bridge jobs are a little different to regular jobs, it is not

ee/app/models/ee/ci/bridge.rb

@@ -44,7 +44,11 @@ module EE
       end
 
       def downstream_variables
-        yaml_variables.to_a.map { |hash| hash.except(:public) }
+        scoped_variables.to_runner_variables.yield_self do |all_variables|
+          yaml_variables.to_a.map do |hash|
+            { key: hash[:key], value: ::ExpandVariables.expand(hash[:value], all_variables) }
+          end
+        end
       end
     end
   end

ee/changelogs/unreleased/feature-gb-expand-pipeline-variables-passed-downstream.yml

+---
+title: Expand pipeline variables passed downstream
+merge_request: 13197
+author:
+type: added

ee/spec/models/ci/bridge_spec.rb

@@ -82,6 +82,28 @@ describe Ci::Bridge do
       expect(bridge.downstream_variables)
         .to include(key: 'BRIDGE', value: 'cross')
     end
+
+    context 'when using variables interpolation' do
+      before do
+        bridge.yaml_variables << { key: 'EXPANDED', value: '$BRIDGE-bridge', public: true }
+      end
+
+      it 'correctly expands variables with interpolation' do
+        expect(bridge.downstream_variables)
+          .to include(key: 'EXPANDED', value: 'cross-bridge')
+      end
+    end
+
+    context 'when recursive interpolation has been used' do
+      before do
+        bridge.yaml_variables << { key: 'EXPANDED', value: '$EXPANDED', public: true }
+      end
+
+      it 'does not expand variable recursively' do
+        expect(bridge.downstream_variables)
+          .to include(key: 'EXPANDED', value: '$EXPANDED')
+      end
+    end
   end
 
   describe 'metadata support' do

ee/spec/services/ci/create_cross_project_pipeline_service_spec.rb

@@ -154,5 +154,33 @@ describe Ci::CreateCrossProjectPipelineService, '#execute' do
           .to have_attributes(key: 'BRIDGE', value: 'var')
       end
     end
+
+    context 'when pipeline variables are defined' do
+      before do
+        upstream_pipeline.variables.create(key: 'PIPELINE_VARIABLE', value: 'my-value')
+      end
+
+      it 'does not pass pipeline variables directly downstream' do
+        pipeline = service.execute(bridge)
+
+        pipeline.variables.map(&:key).tap do |variables|
+          expect(variables).not_to include 'PIPELINE_VARIABLE'
+        end
+      end
+
+      context 'when using YAML variables interpolation' do
+        before do
+          bridge.yaml_variables = [{ key: 'BRIDGE', value: '$PIPELINE_VARIABLE-var', public: true }]
+        end
+
+        it 'makes it possible to pass pipeline variable downstream' do
+          pipeline = service.execute(bridge)
+
+          pipeline.variables.find_by(key: 'BRIDGE').tap do |variable|
+            expect(variable.value).to eq 'my-value-var'
+          end
+        end
+      end
+    end
   end
 end