Make it possible to define downstream bridge variables

It is now possible to pass variables to a downstream pipeline from a
bridge job. Variables defined as YAML variables for this bridge job are
going to be populated downstream, and are going to be available in every
job created within a downstream pipeline. These variables are then going
to be passed to GitLab Runner as environment variables.

ee/lib/ee/gitlab/ci/config/entry/bridge.rb

@@ -14,7 +14,7 @@ module EE
             include ::Gitlab::Config::Entry::Attributable
 
             ALLOWED_KEYS = %i[trigger stage allow_failure only except
-                              when extends].freeze
+                              when extends variables].freeze
 
             validations do
               validates :config, allowed_keys: ALLOWED_KEYS
@@ -44,6 +44,9 @@ module EE
             entry :except, ::Gitlab::Ci::Config::Entry::Policy,
               description: 'Refs policy this job will be executed for.'
 
+            entry :variables, ::Gitlab::Ci::Config::Entry::Variables,
+              description: 'Environment variables available for this job.'
+
             helpers(*ALLOWED_KEYS)
             attributes(*ALLOWED_KEYS)
 
@@ -58,6 +61,7 @@ module EE
                 stage: stage_value,
                 when: when_value,
                 extends: extends_value,
+                variables: (variables_value if variables_defined?),
                 only: only_value,
                 except: except_value }.compact
             end

ee/spec/lib/ee/gitlab/ci/config/entry/bridge_spec.rb

@@ -54,7 +54,8 @@ describe EE::Gitlab::Ci::Config::Entry::Bridge do
         extends: '.some-key',
         stage: 'deploy',
         only: { variables: %w[$SOMEVARIABLE] },
-        except: { refs: %w[feature] } }
+        except: { refs: %w[feature] },
+        variables: { VARIABLE: '123' } }
     end
 
     it { is_expected.to be_valid }

ee/spec/models/ci/bridge_spec.rb

@@ -5,7 +5,9 @@ describe Ci::Bridge do
   set(:pipeline) { create(:ci_pipeline, project: project) }
 
   let(:bridge) do
-    create(:ci_bridge, status: :created, options: options, pipeline: pipeline)
+    create(:ci_bridge, :variables, status: :created,
+                                   options: options,
+                                   pipeline: pipeline)
   end
 
   let(:options) do
@@ -63,4 +65,18 @@ describe Ci::Bridge do
       end
     end
   end
+
+  describe '#yaml_variables' do
+    it 'returns YAML variables' do
+      expect(bridge.yaml_variables)
+        .to include(key: 'BRIDGE', value: 'cross', public: true)
+    end
+  end
+
+  describe '#downstream_variables' do
+    it 'returns variables that are going to be passed downstream' do
+      expect(bridge.downstream_variables)
+        .to include(key: 'BRIDGE', value: 'cross')
+    end
+  end
 end

ee/spec/services/ci/create_pipeline_service_spec.rb

@@ -77,6 +77,8 @@ describe Ci::CreatePipelineService, '#execute' do
           script: rspec
 
         deploy:
+          variables:
+            CROSS: downstream
           stage: deploy
           trigger: my/project
       YAML
@@ -94,6 +96,8 @@ describe Ci::CreatePipelineService, '#execute' do
       expect(bridge.stage).to eq 'deploy'
       expect(pipeline.statuses).to match_array [test, bridge]
       expect(bridge.options).to eq(trigger: { project: 'my/project' })
+      expect(bridge.yaml_variables)
+        .to include(key: 'CROSS', value: 'downstream', public: true)
     end
   end