Adjust applied suggestion reverting previous changes

1. Avoid suggestions being applied on the same file
from reverting previous changes
2. Gracefully use and handle file changes error
when updating the file (though, it does not totally
solves the sync problem for multiple suggestion
applications at once)

1 files changed, 172 insertions, 1 deletions

diff --git a/spec/services/suggestions/apply_service_spec.rb b/spec/services/suggestions/apply_service_spec.rb
index 3a483717756..e5ca1c155ed 100644
--- a/spec/services/suggestions/apply_service_spec.rb
+++ b/spec/services/suggestions/apply_service_spec.rb
@@ -117,13 +117,184 @@ describe Suggestions::ApplyService do
         expect(commit.committer_email).to eq(user.commit_email)
         expect(commit.author_name).to eq(user.name)
       end
+
+      context 'when it fails to apply because the file was changed' do
+        it 'returns error message' do
+          service = instance_double(Files::UpdateService)
+
+          expect(Files::UpdateService).to receive(:new)
+            .and_return(service)
+
+          allow(service).to receive(:execute)
+            .and_raise(Files::UpdateService::FileChangedError)
+
+          result = subject.execute(suggestion)
+
+          expect(result).to eq(message: 'The file has been changed', status: :error)
+        end
+      end
+
+      context 'when diff ref from position is different from repo diff refs' do
+        it 'returns error message' do
+          outdated_refs = Gitlab::Diff::DiffRefs.new(base_sha: 'foo', start_sha: 'bar', head_sha: 'outdated')
+
+          allow(suggestion).to receive(:appliable?) { true }
+          allow(suggestion.position).to receive(:diff_refs) { outdated_refs }
+
+          result = subject.execute(suggestion)
+
+          expect(result).to eq(message: 'The file has been changed', status: :error)
+        end
+      end
+
+      context 'multiple suggestions applied' do
+        let(:expected_content) do
+          <<-CONTENT.strip_heredoc
+              require 'fileutils'
+              require 'open3'
+
+              module Popen
+                extend self
+
+
+                def popen(cmd, path=nil)
+                  unless cmd.is_a?(Array)
+                    # v1 change
+                  end
+
+                  path ||= Dir.pwd
+                  # v1 change
+                  vars = {
+                    "PWD" => path
+                  }
+
+                  options = {
+                    chdir: path
+                  }
+                  # v2 change
+                  unless File.directory?(path)
+                    FileUtils.mkdir_p(path)
+                  end
+
+                  @cmd_output = ""
+                  # v2 change
+
+                  Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
+                    @cmd_output << stdout.read
+                    @cmd_output << stderr.read
+                    @cmd_status = wait_thr.value.exitstatus
+                  end
+
+                  return @cmd_output, @cmd_status
+                end
+              end
+          CONTENT
+        end
+
+        let(:merge_request) do
+          create(:merge_request, source_project: project,
+                                 target_project: project)
+        end
+
+        def create_suggestion(diff, old_line: nil, new_line: nil, from_content:, to_content:, path:)
+          position = Gitlab::Diff::Position.new(old_path: path,
+                                                new_path: path,
+                                                old_line: old_line,
+                                                new_line: new_line,
+                                                diff_refs: diff.diff_refs)
+
+          suggestion_note = create(:diff_note_on_merge_request, noteable: merge_request,
+                                                                original_position: position,
+                                                                position: position,
+                                                                project: project)
+          create(:suggestion, note: suggestion_note,
+                              from_content: from_content,
+                              to_content: to_content)
+        end
+
+        def apply_suggestion(suggestion)
+          suggestion.note.reload
+          merge_request.reload
+          merge_request.clear_memoized_shas
+
+          result = subject.execute(suggestion)
+          refresh = MergeRequests::RefreshService.new(project, user)
+          refresh.execute(merge_request.diff_head_sha,
+                          suggestion.commit_id,
+                          merge_request.source_branch_ref)
+
+          result
+        end
+
+        def fetch_raw_diff(suggestion)
+          project.reload.commit(suggestion.commit_id).diffs.diff_files.first.diff.diff
+        end
+
+        it 'applies multiple suggestions in subsequent versions correctly' do
+          diff = merge_request.merge_request_diff
+          path = 'files/ruby/popen.rb'
+
+          suggestion_1_changes = { old_line: nil,
+                                   new_line: 13,
+                                   from_content: "\n",
+                                   to_content: "# v1 change\n",
+                                   path: path }
+
+          suggestion_2_changes = { old_line: 24,
+                                   new_line: 31,
+                                   from_content: "       @cmd_output << stderr.read\n",
+                                   to_content: "# v2 change\n",
+                                   path: path }
+
+          suggestion_1 = create_suggestion(diff, suggestion_1_changes)
+          suggestion_2 = create_suggestion(diff, suggestion_2_changes)
+
+          apply_suggestion(suggestion_1)
+
+          suggestion_1_diff = fetch_raw_diff(suggestion_1)
+
+          # rubocop: disable Layout/TrailingWhitespace
+          expected_suggestion_1_diff = <<-CONTENT.strip_heredoc
+            @@ -10,7 +10,7 @@ module Popen
+                 end
+             
+                 path ||= Dir.pwd
+            -
+            +# v1 change
+                 vars = {
+                   "PWD" => path
+                 }
+          CONTENT
+          # rubocop: enable Layout/TrailingWhitespace
+
+          apply_suggestion(suggestion_2)
+
+          suggestion_2_diff = fetch_raw_diff(suggestion_2)
+
+          # rubocop: disable Layout/TrailingWhitespace
+          expected_suggestion_2_diff = <<-CONTENT.strip_heredoc
+            @@ -28,7 +28,7 @@ module Popen
+             
+                 Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
+                   @cmd_output << stdout.read
+            -      @cmd_output << stderr.read
+            +# v2 change
+                   @cmd_status = wait_thr.value.exitstatus
+                 end
+          CONTENT
+          # rubocop: enable Layout/TrailingWhitespace
+
+          expect(suggestion_1_diff.strip).to eq(expected_suggestion_1_diff.strip)
+          expect(suggestion_2_diff.strip).to eq(expected_suggestion_2_diff.strip)
+        end
+      end
     end
 
     context 'fork-project' do
       let(:project) { create(:project, :public, :repository) }
 
       let(:forked_project) do
-        fork_project_with_submodules(project, user)
+        fork_project_with_submodules(project, user, repository: project.repository)
       end
 
       let(:merge_request) do