Extend MergeToRefService for creating merge ref from the other ref

Currently, MergeToRefService is specifically designed for
createing merge commits from source branch and target branch of
merge reqeusts. We extend this behavior to source branch and any
target ref paths.

Gemfile

@@ -429,7 +429,7 @@ group :ed25519 do
 end
 # Gitaly GRPC client
-gem 'gitaly-proto', '~> 1.32.0', require: 'gitaly'
+gem 'gitaly-proto', '~> 1.36.0', require: 'gitaly'
 gem 'grpc', '~> 1.19.0'

Gemfile.lock

@@ -303,7 +303,7 @@ GEM
       gettext_i18n_rails (>= 0.7.1)
       po_to_json (>= 1.0.0)
       rails (>= 3.2.0)
-    gitaly-proto (1.32.0)
+    gitaly-proto (1.36.0)
       grpc (~> 1.0)
     github-markup (1.7.0)
     gitlab-labkit (0.3.0)
@@ -1092,7 +1092,7 @@ DEPENDENCIES
   gettext (~> 3.2.2)
   gettext_i18n_rails (~> 1.8.0)
   gettext_i18n_rails_js (~> 1.3)
-  gitaly-proto (~> 1.32.0)
+  gitaly-proto (~> 1.36.0)
   github-markup (~> 1.7.0)
   gitlab-labkit (~> 0.3.0)
   gitlab-markup (~> 1.7.0)

app/models/repository.rb

@@ -839,10 +839,10 @@ class Repository
     end
   end
-  def merge_to_ref(user, source_sha, merge_request, target_ref, message)
+  def merge_to_ref(user, source_sha, merge_request, target_ref, message, first_parent_ref)
     branch = merge_request.target_branch
-    raw.merge_to_ref(user, source_sha, branch, target_ref, message)
+    raw.merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
   end
   def ff_merge(user, source, target_branch, merge_request: nil)

app/services/merge_requests/merge_to_ref_service.rb

 # frozen_string_literal: true
 module MergeRequests
-  # Performs the merge between source SHA and the target branch. Instead
+  # Performs the merge between source SHA and the target branch or the specified first parent ref. Instead
   # of writing the result to the MR target branch, it targets the `target_ref`.
   #
   # Ideally this should leave the `target_ref` state with the same state the
@@ -56,12 +56,22 @@ module MergeRequests
       raise_error(error) if error
     end
+    ##
+    # The parameter `target_ref` is where the merge result will be written.
+    # Default is the merge ref i.e. `refs/merge-requests/:iid/merge`.
     def target_ref
-      merge_request.merge_ref_path
+      params[:target_ref] || merge_request.merge_ref_path
+    end
+
+    ##
+    # The parameter `first_parent_ref` is the main line of the merge commit.
+    # Default is the target branch ref of the merge request.
+    def first_parent_ref
+      params[:first_parent_ref] || merge_request.target_branch_ref
     end
     def commit
-      repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message)
+      repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message, first_parent_ref)
     rescue Gitlab::Git::PreReceiveError => error
       raise MergeError, error.message
     end

changelogs/unreleased/create-merge-train-ref-ce.yml

+---
+title: Extend `MergeToRefService` to create merge ref from an arbitrary ref
+merge_request: 30361
+author:
+type: added

lib/gitlab/git/repository.rb

@@ -536,9 +536,9 @@ module Gitlab
         tags.find { |tag| tag.name == name }
       end
-      def merge_to_ref(user, source_sha, branch, target_ref, message)
+      def merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
         wrapped_gitaly_errors do
-          gitaly_operation_client.user_merge_to_ref(user, source_sha, branch, target_ref, message)
+          gitaly_operation_client.user_merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
         end
       end

lib/gitlab/gitaly_client/operation_service.rb

@@ -100,14 +100,15 @@ module Gitlab
         end
       end
-      def user_merge_to_ref(user, source_sha, branch, target_ref, message)
+      def user_merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
         request = Gitaly::UserMergeToRefRequest.new(
           repository: @gitaly_repo,
           source_sha: source_sha,
           branch: encode_binary(branch),
           target_ref: encode_binary(target_ref),
           user: Gitlab::Git::User.from_gitlab(user).to_gitaly,
-          message: encode_binary(message)
+          message: encode_binary(message),
+          first_parent_ref: encode_binary(first_parent_ref)
         )
         response = GitalyClient.call(@repository.storage, :operation_service, :user_merge_to_ref, request)

spec/lib/gitlab/git/repository_spec.rb

@@ -1694,14 +1694,15 @@ describe Gitlab::Git::Repository, :seed_helper do
     let(:branch_head) { '6d394385cf567f80a8fd85055db1ab4c5295806f' }
     let(:left_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
     let(:right_branch) { 'test-master' }
+    let(:first_parent_ref) { 'refs/heads/test-master' }
     let(:target_ref) { 'refs/merge-requests/999/merge' }
     before do
-      repository.create_branch(right_branch, branch_head) unless repository.branch_exists?(right_branch)
+      repository.create_branch(right_branch, branch_head) unless repository.ref_exists?(first_parent_ref)
     end
     def merge_to_ref
-      repository.merge_to_ref(user, left_sha, right_branch, target_ref, 'Merge message')
+      repository.merge_to_ref(user, left_sha, right_branch, target_ref, 'Merge message', first_parent_ref)
     end
     it 'generates a commit in the target_ref' do
@@ -1716,7 +1717,7 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
     it 'does not change the right branch HEAD' do
-      expect { merge_to_ref }.not_to change { repository.find_branch(right_branch).target }
+      expect { merge_to_ref }.not_to change { repository.commit(first_parent_ref).sha }
     end
   end

spec/lib/gitlab/gitaly_client/operation_service_spec.rb

@@ -79,13 +79,13 @@ describe Gitlab::GitalyClient::OperationService do
     end
     describe '#user_merge_to_ref' do
-      let(:branch) { 'my-branch' }
+      let(:first_parent_ref) { 'refs/heads/my-branch' }
       let(:source_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
       let(:ref) { 'refs/merge-requests/x/merge' }
       let(:message) { 'validación' }
       let(:response) { Gitaly::UserMergeToRefResponse.new(commit_id: 'new-commit-id') }
-      subject { client.user_merge_to_ref(user, source_sha, branch, ref, message) }
+      subject { client.user_merge_to_ref(user, source_sha, nil, ref, message, first_parent_ref) }
       it 'sends a user_merge_to_ref message' do
         expect_any_instance_of(Gitaly::OperationService::Stub)

spec/models/repository_spec.rb

@@ -1420,12 +1420,13 @@ describe Repository do
                              source_project: project)
     end
-    it 'writes merge of source and target to MR merge_ref_path' do
+    it 'writes merge of source SHA and first parent ref to MR merge_ref_path' do
       merge_commit_id = repository.merge_to_ref(user,
                                                 merge_request.diff_head_sha,
                                                 merge_request,
                                                 merge_request.merge_ref_path,
-                                                'Custom message')
+                                                'Custom message',
+                                                merge_request.target_branch_ref)
       merge_commit = repository.commit(merge_commit_id)

spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -22,7 +22,6 @@ describe MergeRequests::MergeToRefService do
   shared_examples_for 'successfully merges to ref with merge method' do
     it 'writes commit to merge ref' do
       repository = project.repository
-      target_ref = merge_request.merge_ref_path
       expect(repository.ref_exists?(target_ref)).to be(false)
@@ -33,7 +32,7 @@ describe MergeRequests::MergeToRefService do
       expect(result[:status]).to eq(:success)
       expect(result[:commit_id]).to be_present
       expect(result[:source_id]).to eq(merge_request.source_branch_sha)
-      expect(result[:target_id]).to eq(merge_request.target_branch_sha)
+      expect(result[:target_id]).to eq(repository.commit(first_parent_ref).sha)
       expect(repository.ref_exists?(target_ref)).to be(true)
       expect(ref_head.id).to eq(result[:commit_id])
     end
@@ -74,17 +73,22 @@ describe MergeRequests::MergeToRefService do
   describe '#execute' do
     let(:service) do
-      described_class.new(project, user, commit_message: 'Awesome message',
-                                         should_remove_source_branch: true)
+      described_class.new(project, user, **params)
     end
+    let(:params) { { commit_message: 'Awesome message', should_remove_source_branch: true } }
+
     def process_merge_to_ref
       perform_enqueued_jobs do
         service.execute(merge_request)
       end
     end
-    it_behaves_like 'successfully merges to ref with merge method'
+    it_behaves_like 'successfully merges to ref with merge method' do
+      let(:first_parent_ref) { 'refs/heads/master' }
+      let(:target_ref) { merge_request.merge_ref_path }
+    end
+
     it_behaves_like 'successfully evaluates pre-condition checks'
     context 'commit history comparison with regular MergeService' do
@@ -129,14 +133,22 @@ describe MergeRequests::MergeToRefService do
       context 'when semi-linear merge method' do
         let(:merge_method) { :rebase_merge }
-        it_behaves_like 'successfully merges to ref with merge method'
+        it_behaves_like 'successfully merges to ref with merge method' do
+          let(:first_parent_ref) { 'refs/heads/master' }
+          let(:target_ref) { merge_request.merge_ref_path }
+        end
+
         it_behaves_like 'successfully evaluates pre-condition checks'
       end
       context 'when fast-forward merge method' do
         let(:merge_method) { :ff }
-        it_behaves_like 'successfully merges to ref with merge method'
+        it_behaves_like 'successfully merges to ref with merge method' do
+          let(:first_parent_ref) { 'refs/heads/master' }
+          let(:target_ref) { merge_request.merge_ref_path }
+        end
+
         it_behaves_like 'successfully evaluates pre-condition checks'
       end
@@ -178,5 +190,34 @@ describe MergeRequests::MergeToRefService do
       it { expect(todo).not_to be_done }
     end
+
+    describe 'cascading merge refs' do
+      set(:project) { create(:project, :repository) }
+      let(:params) { { commit_message: 'Cascading merge', first_parent_ref: first_parent_ref, target_ref: target_ref } }
+
+      context 'when first merge happens' do
+        let(:merge_request) do
+          create(:merge_request, source_project: project, source_branch: 'feature',
+                                 target_project: project, target_branch: 'master')
+        end
+
+        it_behaves_like 'successfully merges to ref with merge method' do
+          let(:first_parent_ref) { 'refs/heads/master' }
+          let(:target_ref) { 'refs/merge-requests/1/train' }
+        end
+
+        context 'when second merge happens' do
+          let(:merge_request) do
+            create(:merge_request, source_project: project, source_branch: 'improve/awesome',
+                                   target_project: project, target_branch: 'master')
+          end
+
+          it_behaves_like 'successfully merges to ref with merge method' do
+            let(:first_parent_ref) { 'refs/merge-requests/1/train' }
+            let(:target_ref) { 'refs/merge-requests/2/train' }
+          end
+        end
+      end
+    end
   end
 end