Comment on any expanded diff line on MRs

app/assets/javascripts/diffs/store/mutations.js

@@ -65,7 +65,13 @@ export default {
     const { highlightedDiffLines, parallelDiffLines } = diffFile;
 
     removeMatchLine(diffFile, lineNumbers, bottom);
-    const lines = addLineReferences(contextLines, lineNumbers, bottom);
+
+    const lines = addLineReferences(contextLines, lineNumbers, bottom).map(line => ({
+      ...line,
+      lineCode: line.lineCode || `${fileHash}_${line.oldLine}_${line.newLine}`,
+      discussions: line.discussions || [],
+    }));
+
     addContextLines({
       inlineLines: highlightedDiffLines,
       parallelLines: parallelDiffLines,

app/controllers/projects/blob_controller.rb

@@ -122,7 +122,7 @@ class Projects::BlobController < Projects::ApplicationController
     @lines.map! do |line|
       # These are marked as context lines but are loaded from blobs.
       # We also have context lines loaded from diffs in other places.
-      diff_line = Gitlab::Diff::Line.new(line, 'context', nil, nil, nil)
+      diff_line = Gitlab::Diff::Line.new(line, expanded_diff_line_type, nil, nil, nil)
       diff_line.rich_text = line
       diff_line
     end
@@ -132,6 +132,11 @@ class Projects::BlobController < Projects::ApplicationController
     render json: DiffLineSerializer.new.represent(@lines)
   end
 
+  def expanded_diff_line_type
+    # Context lines can't receive comments.
+    Feature.enabled?(:comment_in_any_diff_line, @project) ? nil : 'context'
+  end
+
   def add_match_line
     return unless @form.unfold?
 

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -22,6 +22,12 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
 
   def render_diffs
     @environment = @merge_request.environments_for(current_user).last
+    notes_grouped_by_path = renderable_notes.group_by { |note| note.position.file_path }
+
+    @diffs.diff_files.each do |diff_file|
+      notes = notes_grouped_by_path.fetch(diff_file.file_path, [])
+      notes.each { |note| diff_file.unfold_diff_lines(note.position) }
+    end
 
     @diffs.write_cache
 
@@ -108,4 +114,10 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
     @grouped_diff_discussions = @merge_request.grouped_diff_discussions(@compare.diff_refs)
     @notes = prepare_notes_for_rendering(@grouped_diff_discussions.values.flatten.flat_map(&:notes), @merge_request)
   end
+
+  def renderable_notes
+    define_diff_comment_vars unless @notes
+
+    @notes
+  end
 end

app/models/diff_note.rb

@@ -66,6 +66,10 @@ class DiffNote < Note
     self.original_position.diff_refs == diff_refs
   end
 
+  def discussion_first_note?
+    self == discussion.first_note
+  end
+
   private
 
   def enqueue_diff_file_creation_job
@@ -78,10 +82,11 @@ class DiffNote < Note
   end
 
   def should_create_diff_file?
-    on_text? && note_diff_file.nil? && self == discussion.first_note
+    on_text? && note_diff_file.nil? && discussion_first_note?
   end
 
   def fetch_diff_file
+    file =
       if note_diff_file
         diff = Gitlab::Git::Diff.new(note_diff_file.to_hash)
         Gitlab::Diff::File.new(diff,
@@ -98,6 +103,12 @@ class DiffNote < Note
       else
         original_position.diff_file(self.project.repository)
       end
+
+    # Since persisted diff files already have its content "unfolded"
+    # there's no need to make it pass through the unfolding process.
+    file&.unfold_diff_lines(position) unless note_diff_file
+
+    file
   end
 
   def supported?

app/services/merge_requests/reload_diffs_service.rb

@@ -29,10 +29,6 @@ module MergeRequests
 
     # rubocop: disable CodeReuse/ActiveRecord
     def clear_cache(new_diff)
-      # Executing the iteration we cache highlighted diffs for each diff file of
-      # MergeRequestDiff.
-      cacheable_collection(new_diff).write_cache
-
       # Remove cache for all diffs on this MR. Do not use the association on the
       # model, as that will interfere with other actions happening when
       # reloading the diff.

app/services/notes/base_service.rb

+# frozen_string_literal: true
+
+module Notes
+  class BaseService < ::BaseService
+    def clear_noteable_diffs_cache(note)
+      if note.is_a?(DiffNote) &&
+          note.discussion_first_note? &&
+          note.position.unfolded_diff?(project.repository)
+        note.noteable.diffs.clear_cache
+      end
+    end
+  end
+end

app/services/notes/create_service.rb

 # frozen_string_literal: true
 
 module Notes
-  class CreateService < ::BaseService
+  class CreateService < ::Notes::BaseService
     def execute
       merge_request_diff_head_sha = params.delete(:merge_request_diff_head_sha)
 
@@ -35,6 +35,7 @@ module Notes
 
       if !only_commands && note.save
         todo_service.new_note(note, current_user)
+        clear_noteable_diffs_cache(note)
       end
 
       if command_params.present?

app/services/notes/destroy_service.rb

 # frozen_string_literal: true
 
 module Notes
-  class DestroyService < BaseService
+  class DestroyService < ::Notes::BaseService
     def execute(note)
       TodoService.new.destroy_target(note) do |note|
         note.destroy
       end
+
+      clear_noteable_diffs_cache(note)
     end
   end
 end

changelogs/unreleased/osw-comment-on-any-line-on-diffs.yml

+---
+title: Allow commenting on any diff line in Merge Requests
+merge_request: 22914
+author:
+type: added

lib/gitlab/diff/file.rb

@@ -26,6 +26,7 @@ module Gitlab
         @repository = repository
         @diff_refs = diff_refs
         @fallback_diff_refs = fallback_diff_refs
+        @unfolded = false
 
         # Ensure items are collected in the the batch
         new_blob_lazy
@@ -135,6 +136,24 @@ module Gitlab
           Gitlab::Diff::Parser.new.parse(raw_diff.each_line, diff_file: self).to_a
       end
 
+      # Changes diff_lines according to the given position. That is,
+      # it checks whether the position requires blob lines into the diff
+      # in order to be presented.
+      def unfold_diff_lines(position)
+        return unless position
+
+        unfolder = Gitlab::Diff::LinesUnfolder.new(self, position)
+
+        if unfolder.unfold_required?
+          @diff_lines = unfolder.unfolded_diff_lines
+          @unfolded = true
+        end
+      end
+
+      def unfolded?
+        @unfolded
+      end
+
       def highlighted_diff_lines
         @highlighted_diff_lines ||=
           Gitlab::Diff::Highlight.new(self, repository: self.repository).highlight

lib/gitlab/diff/line.rb

@@ -3,9 +3,9 @@ module Gitlab
     class Line
       SERIALIZE_KEYS = %i(line_code rich_text text type index old_pos new_pos).freeze
 
-      attr_reader :line_code, :type, :index, :old_pos, :new_pos
+      attr_reader :line_code, :type, :old_pos, :new_pos
       attr_writer :rich_text
-      attr_accessor :text
+      attr_accessor :text, :index
 
       def initialize(text, type, index, old_pos, new_pos, parent_file: nil, line_code: nil, rich_text: nil)
         @text, @type, @index = text, type, index
@@ -19,7 +19,14 @@ module Gitlab
       end
 
       def self.init_from_hash(hash)
-        new(hash[:text], hash[:type], hash[:index], hash[:old_pos], hash[:new_pos], line_code: hash[:line_code], rich_text: hash[:rich_text])
+        new(hash[:text],
+            hash[:type],
+            hash[:index],
+            hash[:old_pos],
+            hash[:new_pos],
+            parent_file: hash[:parent_file],
+            line_code: hash[:line_code],
+            rich_text: hash[:rich_text])
       end
 
       def to_hash

lib/gitlab/diff/lines_unfolder.rb

+# frozen_string_literal: true
+
+# Given a position, calculates which Blob lines should be extracted, treated and
+# injected in the current diff file lines in order to present a "unfolded" diff.
+module Gitlab
+  module Diff
+    class LinesUnfolder
+      include Gitlab::Utils::StrongMemoize
+
+      UNFOLD_CONTEXT_SIZE = 3
+
+      def initialize(diff_file, position)
+        @diff_file = diff_file
+        @blob = diff_file.old_blob
+        @position = position
+        @generate_top_match_line = true
+        @generate_bottom_match_line = true
+
+        # These methods update `@generate_top_match_line` and
+        # `@generate_bottom_match_line`.
+        @from_blob_line = calculate_from_blob_line!
+        @to_blob_line = calculate_to_blob_line!
+      end
+
+      # Returns merged diff lines with required blob lines with correct
+      # positions.
+      def unfolded_diff_lines
+        strong_memoize(:unfolded_diff_lines) do
+          next unless unfold_required?
+
+          merged_diff_with_blob_lines
+        end
+      end
+
+      # Returns the extracted lines from the old blob which should be merged
+      # with the current diff lines.
+      def blob_lines
+        strong_memoize(:blob_lines) do
+          # Blob lines, unlike diffs, doesn't start with an empty space for
+          # unchanged line, so the parsing and highlighting step can get fuzzy
+          # without the following change.
+          line_prefix = ' '
+          blob_as_diff_lines = @blob.data.each_line.map { |line| "#{line_prefix}#{line}" }
+
+          lines = Gitlab::Diff::Parser.new.parse(blob_as_diff_lines, diff_file: @diff_file).to_a
+
+          from = from_blob_line - 1
+          to = to_blob_line - 1
+
+          lines[from..to]
+        end
+      end
+
+      def unfold_required?
+        strong_memoize(:unfold_required) do
+          next false unless @diff_file.text?
+          next false unless @position.unchanged?
+          next false if @diff_file.new_file? || @diff_file.deleted_file?
+          next false unless @position.old_line
+          # Invalid position (MR import scenario)
+          next false if @position.old_line > @blob.lines.size
+          next false if @diff_file.diff_lines.empty?
+          next false if @diff_file.line_for_position(@position)
+          next false unless unfold_line
+
+          true
+        end
+      end
+
+      private
+
+      attr_reader :from_blob_line, :to_blob_line
+
+      def merged_diff_with_blob_lines
+        lines = @diff_file.diff_lines
+        match_line = unfold_line
+        insert_index = bottom? ? -1 : match_line.index
+
+        lines -= [match_line] unless bottom?
+
+        lines.insert(insert_index, *blob_lines_with_matches)
+
+        # The inserted blob lines have invalid indexes, so we need
+        # to reindex them.
+        reindex(lines)
+
+        lines
+      end
+
+      # Returns 'unchanged' blob lines with recalculated `old_pos` and
+      # `new_pos` and the recalculated new match line (needed if we for instance
+      # we unfolded once, but there are still folded lines).
+      def blob_lines_with_matches
+        old_pos = from_blob_line
+        new_pos = from_blob_line + offset
+
+        new_blob_lines = []
+
+        new_blob_lines.push(top_blob_match_line) if top_blob_match_line
+
+        blob_lines.each do |line|
+          new_blob_lines << Gitlab::Diff::Line.new(line.text, line.type, nil, old_pos, new_pos,
+                                                   parent_file: @diff_file)
+
+          old_pos += 1
+          new_pos += 1
+        end
+
+        new_blob_lines.push(bottom_blob_match_line) if bottom_blob_match_line
+
+        new_blob_lines
+      end
+
+      def reindex(lines)
+        lines.each_with_index { |line, i| line.index = i }
+      end
+
+      def top_blob_match_line
+        strong_memoize(:top_blob_match_line) do
+          next unless @generate_top_match_line
+
+          old_pos = from_blob_line
+          new_pos = from_blob_line + offset
+
+          build_match_line(old_pos, new_pos)
+        end
+      end
+
+      def bottom_blob_match_line
+        strong_memoize(:bottom_blob_match_line) do
+          # The bottom line match addition is already handled on
+          # Diff::File#diff_lines_for_serializer
+          next if bottom?
+          next unless @generate_bottom_match_line
+
+          position = line_after_unfold_position.old_pos
+
+          old_pos = position
+          new_pos = position + offset
+
+          build_match_line(old_pos, new_pos)
+        end
+      end
+
+      def build_match_line(old_pos, new_pos)
+        blob_lines_length = blob_lines.length
+        old_line_ref = [old_pos, blob_lines_length].join(',')
+        new_line_ref = [new_pos, blob_lines_length].join(',')
+        new_match_line_str = "@@ -#{old_line_ref}+#{new_line_ref} @@"
+
+        Gitlab::Diff::Line.new(new_match_line_str, 'match', nil, old_pos, new_pos)
+      end
+
+      # Returns the first line position that should be extracted
+      # from `blob_lines`.
+      def calculate_from_blob_line!
+        return unless unfold_required?
+
+        from = comment_position - UNFOLD_CONTEXT_SIZE
+
+        # There's no line before the match if it's in the top-most
+        # position.
+        prev_line_number = line_before_unfold_position&.old_pos || 0
+
+        if from <= prev_line_number + 1
+          @generate_top_match_line = false
+          from = prev_line_number + 1
+        end
+
+        from
+      end
+
+      # Returns the last line position that should be extracted
+      # from `blob_lines`.
+      def calculate_to_blob_line!
+        return unless unfold_required?
+
+        to = comment_position + UNFOLD_CONTEXT_SIZE
+
+        return to if bottom?
+
+        next_line_number = line_after_unfold_position.old_pos
+
+        if to >= next_line_number - 1
+          @generate_bottom_match_line = false
+          to = next_line_number - 1
+        end
+
+        to
+      end
+
+      def offset
+        unfold_line.new_pos - unfold_line.old_pos
+      end
+
+      def line_before_unfold_position
+        return unless index = unfold_line&.index
+
+        @diff_file.diff_lines[index - 1] if index > 0
+      end
+
+      def line_after_unfold_position
+        return unless index = unfold_line&.index
+
+        @diff_file.diff_lines[index + 1] if index >= 0
+      end
+
+      def bottom?
+        strong_memoize(:bottom) do
+          @position.old_line > last_line.old_pos
+        end
+      end
+
+      # Returns the line which needed to be expanded in order to send a comment
+      # in `@position`.
+      def unfold_line
+        strong_memoize(:unfold_line) do
+          next last_line if bottom?
+
+          @diff_file.diff_lines.find do |line|
+            line.old_pos > comment_position && line.type == 'match'
+          end
+        end
+      end
+
+      def comment_position
+        @position.old_line
+      end
+
+      def last_line
+        @diff_file.diff_lines.last
+      end
+    end
+  end
+end

lib/gitlab/diff/position.rb

@@ -101,6 +101,10 @@ module Gitlab
         @diff_refs ||= DiffRefs.new(base_sha: base_sha, start_sha: start_sha, head_sha: head_sha)
       end
 
+      def unfolded_diff?(repository)
+        diff_file(repository)&.unfolded?
+      end
+
       def diff_file(repository)
         return @diff_file if defined?(@diff_file)
 
@@ -134,7 +138,13 @@ module Gitlab
         return unless diff_refs.complete?
         return unless comparison = diff_refs.compare_in(repository.project)
 
-        comparison.diffs(diff_options).diff_files.first
+        file = comparison.diffs(diff_options).diff_files.first
+
+        # We need to unfold diff lines according to the position in order
+        # to correctly calculate the line code and trace position changes.
+        file&.unfold_diff_lines(self)
+
+        file
       end
 
       def get_formatter_class(type)

spec/controllers/projects/blob_controller_spec.rb

@@ -141,6 +141,28 @@ describe Projects::BlobController do
           expect(lines.first).to have_key('rich_text')
         end
 
+        context 'comment in any diff line feature flag' do
+          it 'renders context lines when feature disabled' do
+            stub_feature_flags(comment_in_any_diff_line: false)
+
+            do_get(since: 1, to: 5, offset: 10, from_merge_request: true)
+            lines = JSON.parse(response.body)
+            all_context = lines.all? { |line| line['type'] == 'context' }
+
+            expect(all_context).to be(true)
+          end
+
+          it 'renders unchanged lines when feature enabled' do
+            stub_feature_flags(comment_in_any_diff_line: true)
+
+            do_get(since: 1, to: 5, offset: 10, from_merge_request: true)
+            lines = JSON.parse(response.body)
+            all_unchanged = lines.all? { |line| line['type'].nil? }
+
+            expect(all_unchanged).to be(true)
+          end
+        end
+
         context 'when rendering match lines' do
           it 'adds top match line when "since" is less than 1' do
             do_get(since: 5, to: 10, offset: 10, from_merge_request: true)
@@ -157,7 +179,7 @@ describe Projects::BlobController do
 
             match_line = JSON.parse(response.body).first
 
-            expect(match_line['type']).to eq('context')
+            expect(match_line['type']).to be_nil
           end
 
           it 'adds bottom match line when "t"o is less than blob size' do
@@ -177,7 +199,7 @@ describe Projects::BlobController do
 
             match_line = JSON.parse(response.body).last
 
-            expect(match_line['type']).to eq('context')
+            expect(match_line['type']).to be_nil
           end
         end
       end

spec/features/merge_request/user_posts_diff_notes_spec.rb

@@ -85,12 +85,13 @@ describe 'Merge request > User posts diff notes', :js do
       # `.line_holder` will be an unfolded line.
       let(:line_holder) { first('#a5cc2925ca8258af241be7e5b0381edf30266302 .line_holder') }
 
-      it 'does not allow commenting on the left side' do
-        should_not_allow_commenting(line_holder, 'left')
+      it 'allows commenting on the left side' do
+        should_allow_commenting(line_holder, 'left')
       end
 
-      it 'does not allow commenting on the right side' do
-        should_not_allow_commenting(line_holder, 'right')
+      it 'allows commenting on the right side' do
+        # Automatically shifts comment box to left side.
+        should_allow_commenting(line_holder, 'right')
       end
     end
   end
@@ -147,8 +148,8 @@ describe 'Merge request > User posts diff notes', :js do
       # `.line_holder` will be an unfolded line.
       let(:line_holder) { first('.line_holder[id="a5cc2925ca8258af241be7e5b0381edf30266302_1_1"]') }
 
-      it 'does not allow commenting' do
-        should_not_allow_commenting line_holder
+      it 'allows commenting' do
+        should_allow_commenting line_holder
       end
     end
 

spec/javascripts/diffs/store/mutations_spec.js

@@ -98,7 +98,7 @@ describe('DiffsStoreMutations', () => {
     it('should call utils.addContextLines with proper params', () => {
       const options = {
         lineNumbers: { oldLineNumber: 1, newLineNumber: 2 },
-        contextLines: [{ oldLine: 1 }],
+        contextLines: [{ oldLine: 1, newLine: 1, lineCode: 'ff9200_1_1', discussions: [] }],
         fileHash: 'ff9200',
         params: {
           bottom: true,
@@ -110,7 +110,7 @@ describe('DiffsStoreMutations', () => {
         parallelDiffLines: [],
       };
       const state = { diffFiles: [diffFile] };
-      const lines = [{ oldLine: 1 }];
+      const lines = [{ oldLine: 1, newLine: 1 }];
 
       const findDiffFileSpy = spyOnDependency(mutations, 'findDiffFile').and.returnValue(diffFile);
       const removeMatchLineSpy = spyOnDependency(mutations, 'removeMatchLine');

spec/lib/gitlab/diff/file_spec.rb

@@ -41,6 +41,52 @@ describe Gitlab::Diff::File do
     end
   end
 
+  describe '#unfold_diff_lines' do
+    let(:unfolded_lines) { double('expanded-lines') }
+    let(:unfolder) { instance_double(Gitlab::Diff::LinesUnfolder) }
+    let(:position) { instance_double(Gitlab::Diff::Position, old_line: 10) }
+
+    before do
+      allow(Gitlab::Diff::LinesUnfolder).to receive(:new) { unfolder }
+    end
+
+    context 'when unfold required' do
+      before do
+        allow(unfolder).to receive(:unfold_required?) { true }
+        allow(unfolder).to receive(:unfolded_diff_lines) { unfolded_lines }
+      end
+
+      it 'changes @unfolded to true' do
+        diff_file.unfold_diff_lines(position)
+
+        expect(diff_file).to be_unfolded
+      end
+
+      it 'updates @diff_lines' do
+        diff_file.unfold_diff_lines(position)
+
+        expect(diff_file.diff_lines).to eq(unfolded_lines)
+      end
+    end
+
+    context 'when unfold not required' do
+      before do
+        allow(unfolder).to receive(:unfold_required?) { false }
+      end
+
+      it 'keeps @unfolded false' do
+        diff_file.unfold_diff_lines(position)
+
+        expect(diff_file).not_to be_unfolded
+      end
+
+      it 'does not update @diff_lines' do
+        expect { diff_file.unfold_diff_lines(position) }
+          .not_to change(diff_file, :diff_lines)
+      end
+    end
+  end
+
   describe '#mode_changed?' do
     it { expect(diff_file.mode_changed?).to be_falsey }
   end

spec/services/merge_requests/reload_diffs_service_spec.rb

@@ -31,32 +31,11 @@ describe MergeRequests::ReloadDiffsService, :use_clean_rails_memory_store_cachin
     end
 
     context 'cache clearing' do
-      before do
-        allow_any_instance_of(Gitlab::Diff::File).to receive(:text?).and_return(true)
-        allow_any_instance_of(Gitlab::Diff::File).to receive(:diffable?).and_return(true)
-      end
-
-      it 'retrieves the diff files to cache the highlighted result' do
-        new_diff = merge_request.create_merge_request_diff
-        cache_key = new_diff.diffs_collection.cache_key
-
-        expect(merge_request).to receive(:create_merge_request_diff).and_return(new_diff)
-        expect(Rails.cache).to receive(:read).with(cache_key).and_call_original
-        expect(Rails.cache).to receive(:write).with(cache_key, anything, anything).and_call_original
-
-        subject.execute
-      end
-
       it 'clears the cache for older diffs on the merge request' do
         old_diff = merge_request.merge_request_diff
         old_cache_key = old_diff.diffs_collection.cache_key
-        new_diff = merge_request.create_merge_request_diff
-        new_cache_key = new_diff.diffs_collection.cache_key
 
-        expect(merge_request).to receive(:create_merge_request_diff).and_return(new_diff)
         expect(Rails.cache).to receive(:delete).with(old_cache_key).and_call_original
-        expect(Rails.cache).to receive(:read).with(new_cache_key).and_call_original
-        expect(Rails.cache).to receive(:write).with(new_cache_key, anything, anything).and_call_original
 
         subject.execute
       end

spec/services/notes/create_service_spec.rb

@@ -57,6 +57,57 @@ describe Notes::CreateService do
       end
     end
 
+    context 'noteable highlight cache clearing' do
+      let(:project_with_repo) { create(:project, :repository) }
+      let(:merge_request) do
+        create(:merge_request, source_project: project_with_repo,
+                               target_project: project_with_repo)
+      end
+
+      let(:position) do
+        Gitlab::Diff::Position.new(old_path: "files/ruby/popen.rb",
+                                   new_path: "files/ruby/popen.rb",
+                                   old_line: nil,
+                                   new_line: 14,
+                                   diff_refs: merge_request.diff_refs)
+      end
+
+      let(:new_opts) do
+        opts.merge(in_reply_to_discussion_id: nil,
+                   type: 'DiffNote',
+                   noteable_type: 'MergeRequest',
+                   noteable_id: merge_request.id,
+                   position: position.to_h)
+      end
+
+      before do
+        allow_any_instance_of(Gitlab::Diff::Position)
+          .to receive(:unfolded_diff?) { true }
+      end
+
+      it 'clears noteable diff cache when it was unfolded for the note position' do
+        expect_any_instance_of(Gitlab::Diff::HighlightCache).to receive(:clear)
+
+        described_class.new(project_with_repo, user, new_opts).execute
+      end
+
+      it 'does not clear cache when note is not the first of the discussion' do
+        prev_note =
+          create(:diff_note_on_merge_request, noteable: merge_request,
+                                              project: project_with_repo)
+        reply_opts =
+          opts.merge(in_reply_to_discussion_id: prev_note.discussion_id,
+                     type: 'DiffNote',
+                     noteable_type: 'MergeRequest',
+                     noteable_id: merge_request.id,
+                     position: position.to_h)
+
+        expect(merge_request).not_to receive(:diffs)
+
+        described_class.new(project_with_repo, user, reply_opts).execute
+      end
+    end
+
     context 'note diff file' do
       let(:project_with_repo) { create(:project, :repository) }
       let(:merge_request) do

spec/services/notes/destroy_service_spec.rb

@@ -21,5 +21,38 @@ describe Notes::DestroyService do
       expect { described_class.new(project, user).execute(note) }
         .to change { user.todos_pending_count }.from(1).to(0)
     end
+
+    context 'noteable highlight cache clearing' do
+      let(:repo_project) { create(:project, :repository) }
+      let(:merge_request) do
+        create(:merge_request, source_project: repo_project,
+                               target_project: repo_project)
+      end
+
+      let(:note) do
+        create(:diff_note_on_merge_request, project: repo_project,
+                                            noteable: merge_request)
+      end
+
+      before do
+        allow(note.position).to receive(:unfolded_diff?) { true }
+      end
+
+      it 'clears noteable diff cache when it was unfolded for the note position' do
+        expect(merge_request).to receive_message_chain(:diffs, :clear_cache)
+
+        described_class.new(repo_project, user).execute(note)
+      end
+
+      it 'does not clear cache when note is not the first of the discussion' do
+        reply_note = create(:diff_note_on_merge_request, in_reply_to: note,
+                                                         project: repo_project,
+                                                         noteable: merge_request)
+
+        expect(merge_request).not_to receive(:diffs)
+
+        described_class.new(repo_project, user).execute(reply_note)
+      end
+    end
   end
 end