Merge branch 'mc/feature/bridge-branch-variable-expansion' into 'master'

Add variable expansion to bridge branch property

Closes #10126

See merge request gitlab-org/gitlab!17430

doc/ci/multi_project_pipelines.md

@@ -116,7 +116,8 @@ staging:
 ```
 
 Use a `project` keyword to specify full path to a downstream project. Use
-a `branch` keyword to specify a branch name.
+a `branch` keyword to specify a branch name. Variable expansion is supported in
+the `branch` property.
 
 GitLab will use a commit that is currently on the HEAD of the branch when
 creating a downstream pipeline.

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

@@ -115,7 +115,12 @@ module EE
       end
 
       def target_ref
-        options&.dig(:trigger, :branch)
+        branch = options&.dig(:trigger, :branch)
+        return unless branch
+
+        scoped_variables.to_runner_variables.yield_self do |all_variables|
+          ::ExpandVariables.expand(branch, all_variables)
+        end
       end
 
       def dependent?

ee/changelogs/unreleased/mc-feature-bridge-branch-variable-expansion.yml

+---
+title: Support variable expansion in branch property of bridge jobs.
+merge_request: 17430
+author:
+type: changed

ee/spec/models/ci/bridge_spec.rb

@@ -178,6 +178,14 @@ describe Ci::Bridge do
       it 'returns a ref name' do
         expect(bridge.target_ref).to eq 'master'
       end
+
+      context 'when using variable expansion' do
+        let(:options) { { trigger: { project: 'my/project', branch: '$BRIDGE-master' } } }
+
+        it 'correctly expands variables' do
+          expect(bridge.target_ref).to eq('cross-master')
+        end
+      end
     end
 
     context 'when trigger does not have project defined' do