Check if file has been modified for each action provided.

When commiting multiple files we're now checking if any of those files
has been modified by another commit and we're rejecting the new commit
in this case.

app/services/files/multi_service.rb

@@ -20,6 +20,7 @@ module Files
       params[:actions].each do |action|
         validate_action!(action)
+        validate_file_status(action)
       end
     end
@@ -28,5 +29,16 @@ module Files
         raise_error("Unknown action '#{action[:action]}'")
       end
     end
+
+    def validate_file_status(action)
+      return unless action[:last_commit_id]
+
+      current_commit = Gitlab::Git::Commit.last_for_path(
+        @start_project.repository, @start_branch, action[:file_path])
+
+      if current_commit.sha != action[:last_commit_id]
+        raise_error("The file has changed since you started editing it: #{action[:file_path]}")
+      end
+    end
   end
 end

changelogs/unreleased/15922-validate-file-status-when-commiting-multiple-files.yml

+---
+title: 'Validate file status when commiting multiple files'
+merge_request: 15922
+author:
+type: added

doc/api/commits.md

@@ -84,6 +84,7 @@ POST /projects/:id/repository/commits
 | `previous_path` | string | no | Original full path to the file being moved. Ex. `lib/class1.rb` |
 | `content` | string | no | File content, required for all except `delete`. Optional for `move` |
 | `encoding` | string | no | `text` or `base64`. `text` is default. |
+| `last_commit_id` | string | no | Last known file commit id |
 ```bash
 PAYLOAD=$(cat << 'JSON'

spec/services/files/multi_service_spec.rb

@@ -6,14 +6,20 @@ describe Files::MultiService do
   let(:project) { create(:project, :repository) }
   let(:user) { create(:user) }
   let(:branch_name) { project.default_branch }
+  let(:file_path) { 'files/ruby/popen.rb' }
   let(:action) { 'update' }
+  let!(:original_commit_id) do
+    Gitlab::Git::Commit.last_for_path(project.repository, branch_name, file_path).sha
+  end
+
   let(:actions) do
     [
       {
         action: action,
-        file_path: 'files/ruby/popen.rb',
-        content: 'New content'
+        file_path: file_path,
+        content: 'New content',
+        last_commit_id: original_commit_id
       }
     ]
   end
@@ -50,5 +56,40 @@ describe Files::MultiService do
         expect(results[:message]).to match(/Unknown action/)
       end
     end
+
+    describe 'Updating files' do
+      context 'when the file has been previously updated' do
+        before do
+          update_file(file_path)
+        end
+
+        it 'rejects the commit' do
+          results = subject.execute
+
+          expect(results[:status]).to eq(:error)
+          expect(results[:message]).to match(file_path)
+        end
+      end
+
+      context 'when the file have not been modified' do
+        it 'accepts the commit' do
+          results = subject.execute
+
+          expect(results[:status]).to eq(:success)
+        end
+      end
+    end
+  end
+
+  def update_file(path)
+    params = {
+      file_path: path,
+      start_branch: branch_name,
+      branch_name: branch_name,
+      commit_message: 'Update file',
+      file_content: 'New content'
+    }
+
+    Files::UpdateService.new(project, user, params).execute
   end
 end