Support merge to ref for merge-commit and squash

Adds the ground work for writing into
the merge ref refs/merge-requests/:iid/merge the
merge result between source and target branches of
a MR, without further side-effects such as
mailing, MR updates and target branch changes.

3 files changed, 234 insertions, 0 deletions

diff --git a/spec/lib/gitlab/git/repository_spec.rb b/spec/lib/gitlab/git/repository_spec.rb
index 8a9e78ba3c3..b3a728c139e 100644
--- a/spec/lib/gitlab/git/repository_spec.rb
+++ b/spec/lib/gitlab/git/repository_spec.rb
@@ -1704,6 +1704,37 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
   end
 
+  describe '#merge_to_ref' do
+    let(:repository) { mutable_repository }
+    let(:branch_head) { '6d394385cf567f80a8fd85055db1ab4c5295806f' }
+    let(:left_sha) { 'cfe32cf61b73a0d5e9f13e774abde7ff789b1660' }
+    let(:right_branch) { '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)
+    end
+
+    def merge_to_ref
+      repository.merge_to_ref(user, left_sha, right_branch, target_ref, 'Merge message')
+    end
+
+    it 'generates a commit in the target_ref' do
+      expect(repository.ref_exists?(target_ref)).to be(false)
+
+      commit_sha = merge_to_ref
+      ref_head = repository.commit(target_ref)
+
+      expect(commit_sha).to be_present
+      expect(repository.ref_exists?(target_ref)).to be(true)
+      expect(ref_head.id).to eq(commit_sha)
+    end
+
+    it 'does not change the right branch HEAD' do
+      expect { merge_to_ref }.not_to change { repository.find_branch(right_branch).target }
+    end
+  end
+
   describe '#merge' do
     let(:repository) { mutable_repository }
     let(:source_sha) { '913c66a37b4a45b9769037c55c2d238bd0942d2e' }
diff --git a/spec/models/repository_spec.rb b/spec/models/repository_spec.rb
index f78760bf567..17201d8b90a 100644
--- a/spec/models/repository_spec.rb
+++ b/spec/models/repository_spec.rb
@@ -1373,6 +1373,29 @@ describe Repository do
     end
   end
 
+  describe '#merge_to_ref' do
+    let(:merge_request) do
+      create(:merge_request, source_branch: 'feature',
+                             target_branch: 'master',
+                             source_project: project)
+    end
+
+    it 'writes merge of source and target 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')
+
+      merge_commit = repository.commit(merge_commit_id)
+
+      expect(merge_commit.message).to eq('Custom message')
+      expect(merge_commit.author_name).to eq(user.name)
+      expect(merge_commit.author_email).to eq(user.commit_email)
+      expect(repository.blob_at(merge_commit.id, 'files/ruby/feature.rb')).to be_present
+    end
+  end
+
   describe '#ff_merge' do
     before do
       repository.add_branch(user, 'ff-target', 'feature~5')
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
new file mode 100644
index 00000000000..435a863cbd4
--- /dev/null
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -0,0 +1,180 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe MergeRequests::MergeToRefService do
+  set(:user) { create(:user) }
+  let(:merge_request) { create(:merge_request, :simple) }
+  let(:project) { merge_request.project }
+
+  before do
+    project.add_maintainer(user)
+  end
+
+  describe '#execute' do
+    let(:service) do
+      described_class.new(project, user,
+                          commit_message: 'Awesome message',
+                          'should_remove_source_branch' => true)
+    end
+
+    def process_merge_to_ref
+      perform_enqueued_jobs do
+        service.execute(merge_request)
+      end
+    end
+
+    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)
+
+      result = service.execute(merge_request)
+
+      ref_head = repository.commit(target_ref)
+
+      expect(result[:status]).to eq(:success)
+      expect(result[:commit_id]).to be_present
+      expect(repository.ref_exists?(target_ref)).to be(true)
+      expect(ref_head.id).to eq(result[:commit_id])
+    end
+
+    it 'does not send any mail' do
+      expect { process_merge_to_ref }.not_to change { ActionMailer::Base.deliveries.count }
+    end
+
+    it 'does not change the MR state' do
+      expect { process_merge_to_ref }.not_to change { merge_request.state }
+    end
+
+    it 'does not create notes' do
+      expect { process_merge_to_ref }.not_to change { merge_request.notes.count }
+    end
+
+    it 'does not delete the source branch' do
+      expect(DeleteBranchService).not_to receive(:new)
+
+      process_merge_to_ref
+    end
+
+    it 'returns an error when the failing to process the merge' do
+      allow(project.repository).to receive(:merge_to_ref).and_return(nil)
+
+      result = service.execute(merge_request)
+
+      expect(result[:status]).to eq(:error)
+      expect(result[:message]).to eq('Conflicts detected during merge')
+    end
+
+    context 'commit history comparison with regular MergeService' do
+      let(:merge_ref_service) do
+        described_class.new(project, user, {})
+      end
+
+      let(:merge_service) do
+        MergeRequests::MergeService.new(project, user, {})
+      end
+
+      shared_examples_for 'MergeService for target ref' do
+        it 'target_ref has the same state of target branch' do
+          repo = merge_request.target_project.repository
+
+          process_merge_to_ref
+          merge_service.execute(merge_request)
+
+          ref_commits = repo.commits(merge_request.merge_ref_path, limit: 3)
+          target_branch_commits = repo.commits(merge_request.target_branch, limit: 3)
+
+          ref_commits.zip(target_branch_commits).each do |ref_commit, target_branch_commit|
+            expect(ref_commit.parents).to eq(target_branch_commit.parents)
+          end
+        end
+      end
+
+      context 'when merge commit' do
+        it_behaves_like 'MergeService for target ref'
+      end
+
+      context 'when merge commit with squash' do
+        before do
+          merge_request.update!(squash: true, source_branch: 'master', target_branch: 'feature')
+        end
+
+        it_behaves_like 'MergeService for target ref'
+      end
+    end
+
+    context 'merge pre-condition checks' do
+      before do
+        merge_request.project.update!(merge_method: merge_method)
+      end
+
+      context 'when semi-linear merge method' do
+        let(:merge_method) { :rebase_merge }
+
+        it 'return error when MR should be able to fast-forward' do
+          allow(merge_request).to receive(:should_be_rebased?) { true }
+
+          error_message = 'Fast-forward merge is not possible. Please update your source branch.'
+
+          result = service.execute(merge_request)
+
+          expect(result[:status]).to eq(:error)
+          expect(result[:message]).to eq(error_message)
+        end
+      end
+
+      context 'when fast-forward merge method' do
+        let(:merge_method) { :ff }
+
+        it 'returns error' do
+          error_message = "Fast-forward to #{merge_request.merge_ref_path} is currently not supported."
+
+          result = service.execute(merge_request)
+
+          expect(result[:status]).to eq(:error)
+          expect(result[:message]).to eq(error_message)
+        end
+      end
+
+      context 'when MR is not mergeable to ref' do
+        let(:merge_method) { :merge }
+
+        it 'returns error' do
+          allow(merge_request).to receive(:mergeable_to_ref?) { false }
+
+          error_message = "Merge request is not mergeable to #{merge_request.merge_ref_path}"
+
+          result = service.execute(merge_request)
+
+          expect(result[:status]).to eq(:error)
+          expect(result[:message]).to eq(error_message)
+        end
+      end
+    end
+
+    context 'does not close related todos' do
+      let(:merge_request) { create(:merge_request, assignee: user, author: user) }
+      let(:project) { merge_request.project }
+      let!(:todo) do
+        create(:todo, :assigned,
+               project: project,
+               author: user,
+               user: user,
+               target: merge_request)
+      end
+
+      before do
+        allow(service).to receive(:execute_hooks)
+
+        perform_enqueued_jobs do
+          service.execute(merge_request)
+          todo.reload
+        end
+      end
+
+      it { expect(todo).not_to be_done }
+    end
+  end
+end