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diff --git a/spec/services/system_notes/merge_requests_service_spec.rb b/spec/services/system_notes/merge_requests_service_spec.rb
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+++ b/spec/services/system_notes/merge_requests_service_spec.rb
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+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ::SystemNotes::MergeRequestsService do
+  include Gitlab::Routing
+
+  let_it_be(:group) { create(:group) }
+  let_it_be(:project) { create(:project, :repository, group: group) }
+  let_it_be(:author) { create(:user) }
+
+  let(:noteable) { create(:merge_request, source_project: project, target_project: project) }
+
+  let(:service) { described_class.new(noteable: noteable, project: project, author: author) }
+
+  describe '.merge_when_pipeline_succeeds' do
+    let(:pipeline) { build(:ci_pipeline) }
+
+    subject { service.merge_when_pipeline_succeeds(pipeline.sha) }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'merge' }
+    end
+
+    it "posts the 'merge when pipeline succeeds' system note" do
+      expect(subject.note).to match(%r{enabled an automatic merge when the pipeline for (\w+/\w+@)?\h{40} succeeds})
+    end
+  end
+
+  describe '.cancel_merge_when_pipeline_succeeds' do
+    subject { service.cancel_merge_when_pipeline_succeeds }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'merge' }
+    end
+
+    it "posts the 'merge when pipeline succeeds' system note" do
+      expect(subject.note).to eq "canceled the automatic merge"
+    end
+  end
+
+  describe '.abort_merge_when_pipeline_succeeds' do
+    subject { service.abort_merge_when_pipeline_succeeds('merge request was closed') }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'merge' }
+    end
+
+    it "posts the 'merge when pipeline succeeds' system note" do
+      expect(subject.note).to eq "aborted the automatic merge because merge request was closed"
+    end
+  end
+
+  describe '.handle_merge_request_wip' do
+    context 'adding wip note' do
+      let(:noteable) { create(:merge_request, source_project: project, title: 'WIP Lorem ipsum') }
+
+      subject { service.handle_merge_request_wip }
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'title' }
+      end
+
+      it 'sets the note text' do
+        expect(subject.note).to eq 'marked as a **Work In Progress**'
+      end
+    end
+
+    context 'removing wip note' do
+      subject { service.handle_merge_request_wip }
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'title' }
+      end
+
+      it 'sets the note text' do
+        expect(subject.note).to eq 'unmarked as a **Work In Progress**'
+      end
+    end
+  end
+
+  describe '.add_merge_request_wip_from_commit' do
+    subject { service.add_merge_request_wip_from_commit(noteable.diff_head_commit) }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'title' }
+    end
+
+    it "posts the 'marked as a Work In Progress from commit' system note" do
+      expect(subject.note).to match(
+        /marked as a \*\*Work In Progress\*\* from #{Commit.reference_pattern}/
+      )
+    end
+  end
+
+  describe '.resolve_all_discussions' do
+    subject { service.resolve_all_discussions }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'discussion' }
+    end
+
+    it 'sets the note text' do
+      expect(subject.note).to eq 'resolved all threads'
+    end
+  end
+
+  describe '.diff_discussion_outdated' do
+    let(:discussion) { create(:diff_note_on_merge_request, project: project).to_discussion }
+    let(:merge_request) { discussion.noteable }
+    let(:change_position) { discussion.position }
+
+    def reloaded_merge_request
+      MergeRequest.find(merge_request.id)
+    end
+
+    let(:service) { described_class.new(project: project, author: author) }
+
+    subject { service.diff_discussion_outdated(discussion, change_position) }
+
+    it_behaves_like 'a system note' do
+      let(:expected_noteable) { discussion.first_note.noteable }
+      let(:action)            { 'outdated' }
+    end
+
+    context 'when the change_position is valid for the discussion' do
+      it 'creates a new note in the discussion' do
+        # we need to completely rebuild the merge request object, or the `@discussions` on the merge request are not reloaded.
+        expect { subject }.to change { reloaded_merge_request.discussions.first.notes.size }.by(1)
+      end
+
+      it 'links to the diff in the system note' do
+        diff_id = merge_request.merge_request_diff.id
+        line_code = change_position.line_code(project.repository)
+        link = diffs_project_merge_request_path(project, merge_request, diff_id: diff_id, anchor: line_code)
+
+        expect(subject.note).to eq("changed this line in [version 1 of the diff](#{link})")
+      end
+
+      context 'discussion is on an image' do
+        let(:discussion) { create(:image_diff_note_on_merge_request, project: project).to_discussion }
+
+        it 'links to the diff in the system note' do
+          diff_id = merge_request.merge_request_diff.id
+          file_hash = change_position.file_hash
+          link = diffs_project_merge_request_path(project, merge_request, diff_id: diff_id, anchor: file_hash)
+
+          expect(subject.note).to eq("changed this file in [version 1 of the diff](#{link})")
+        end
+      end
+    end
+
+    context 'when the change_position does not point to a valid version' do
+      before do
+        allow(merge_request).to receive(:version_params_for).and_return(nil)
+      end
+
+      it 'creates a new note in the discussion' do
+        # we need to completely rebuild the merge request object, or the `@discussions` on the merge request are not reloaded.
+        expect { subject }.to change { reloaded_merge_request.discussions.first.notes.size }.by(1)
+      end
+
+      it 'does not create a link' do
+        expect(subject.note).to eq('changed this line in version 1 of the diff')
+      end
+    end
+  end
+
+  describe '.change_branch' do
+    subject { service.change_branch('target', old_branch, new_branch) }
+
+    let(:old_branch) { 'old_branch'}
+    let(:new_branch) { 'new_branch'}
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'branch' }
+    end
+
+    context 'when target branch name changed' do
+      it 'sets the note text' do
+        expect(subject.note).to eq "changed target branch from `#{old_branch}` to `#{new_branch}`"
+      end
+    end
+  end
+
+  describe '.change_branch_presence' do
+    subject { service.change_branch_presence(:source, 'feature', :delete) }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'branch' }
+    end
+
+    context 'when source branch deleted' do
+      it 'sets the note text' do
+        expect(subject.note).to eq "deleted source branch `feature`"
+      end
+    end
+  end
+
+  describe '.new_issue_branch' do
+    let(:branch) { '1-mepmep' }
+
+    subject { service.new_issue_branch(branch, branch_project: branch_project) }
+
+    shared_examples_for 'a system note for new issue branch' do
+      it_behaves_like 'a system note' do
+        let(:action) { 'branch' }
+      end
+
+      context 'when a branch is created from the new branch button' do
+        it 'sets the note text' do
+          expect(subject.note).to start_with("created branch [`#{branch}`]")
+        end
+      end
+    end
+
+    context 'branch_project is set' do
+      let(:branch_project) { create(:project, :repository) }
+
+      it_behaves_like 'a system note for new issue branch'
+    end
+
+    context 'branch_project is not set' do
+      let(:branch_project) { nil }
+
+      it_behaves_like 'a system note for new issue branch'
+    end
+  end
+
+  describe '.new_merge_request' do
+    subject { service.new_merge_request(merge_request) }
+
+    let!(:merge_request) { create(:merge_request, source_project: project, source_branch: generate(:branch), target_project: project) }
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'merge' }
+    end
+
+    it 'sets the new merge request note text' do
+      expect(subject.note).to eq("created merge request #{merge_request.to_reference(project)} to address this issue")
+    end
+  end
+end