Keep trailing newline when picking conflict sections

If our side of the conflict file has a trailing newline, and we are picking
sections, not editing the whole content, then add a trailing newline back.

app/services/merge_requests/conflicts/resolve_service.rb

@@ -37,10 +37,12 @@ module MergeRequests
       private
 
       def write_resolved_file_to_index(merge_index, rugged, file, params)
-        new_file = if params[:sections]
-                     file.resolve_lines(params[:sections]).map(&:text).join("\n")
+        if params[:sections]
+          new_file = file.resolve_lines(params[:sections]).map(&:text).join("\n")
+
+          new_file << "\n" if file.our_blob.data.ends_with?("\n")
         elsif params[:content]
-                     file.resolve_content(params[:content])
+          new_file = file.resolve_content(params[:content])
         end
 
         our_path = file.our_path

changelogs/unreleased/32983-merge-conflict-resolution-removed-the-newline-in-the-end-of-file.yml

+---
+title: Keep trailing newline when resolving conflicts by picking sides
+merge_request:
+author:

spec/services/merge_requests/conflicts/resolve_service_spec.rb

@@ -26,6 +26,10 @@ describe MergeRequests::Conflicts::ResolveService do
   describe '#execute' do
     let(:service) { described_class.new(merge_request) }
 
+    def blob_content(project, ref, path)
+      project.repository.blob_at(ref, path).data
+    end
+
     context 'with section params' do
       let(:params) do
         {
@@ -66,6 +70,35 @@ describe MergeRequests::Conflicts::ResolveService do
         end
       end
 
+      context 'when some files have trailing newlines' do
+        let!(:source_head) do
+          branch = 'conflict-resolvable'
+          path = 'files/ruby/popen.rb'
+          popen_content = blob_content(project, branch, path)
+
+          project.repository.update_file(
+            user,
+            path,
+            popen_content.chomp("\n"),
+            message: 'Remove trailing newline from popen.rb',
+            branch_name: branch
+          )
+        end
+
+        before do
+          service.execute(user, params)
+        end
+
+        it 'preserves trailing newlines from our side of the conflicts' do
+          head_sha = merge_request.source_branch_head.sha
+          popen_content = blob_content(project, head_sha, 'files/ruby/popen.rb')
+          regex_content = blob_content(project, head_sha, 'files/ruby/regex.rb')
+
+          expect(popen_content).not_to end_with("\n")
+          expect(regex_content).to end_with("\n")
+        end
+      end
+
       context 'when the source project is a fork and does not contain the HEAD of the target branch' do
         let!(:target_head) do
           project.repository.create_file(
@@ -142,10 +175,13 @@ describe MergeRequests::Conflicts::ResolveService do
       end
 
       it 'sets the content to the content given' do
-        blob = merge_request.source_project.repository.blob_at(merge_request.source_branch_head.sha,
-                                                               'files/ruby/popen.rb')
+        blob = blob_content(
+          merge_request.source_project,
+          merge_request.source_branch_head.sha,
+          'files/ruby/popen.rb'
+        )
 
-        expect(blob.data).to eq(popen_content)
+        expect(blob).to eq(popen_content)
       end
     end