Remove unused data from discussions endpoint

We don't need a series of attributes to render diff files on
discussions.json request. Therefore this MR removes lots of unnecessary
attributes from the request, mainly the highlighted diff lines, which
are pretty expensive.

app/serializers/diff_file_base_entity.rb

+# frozen_string_literal: true
+
+class DiffFileBaseEntity < Grape::Entity
+  include RequestAwareEntity
+  include BlobHelper
+  include SubmoduleHelper
+  include DiffHelper
+  include TreeHelper
+  include ChecksCollaboration
+  include Gitlab::Utils::StrongMemoize
+
+  expose :content_sha
+  expose :submodule?, as: :submodule
+
+  expose :submodule_link do |diff_file|
+    memoized_submodule_links(diff_file).first
+  end
+
+  expose :submodule_tree_url do |diff_file|
+    memoized_submodule_links(diff_file).last
+  end
+
+  expose :edit_path, if: -> (_, options) { options[:merge_request] } do |diff_file|
+    merge_request = options[:merge_request]
+
+    options = merge_request.persisted? ? { from_merge_request_iid: merge_request.iid } : {}
+
+    next unless merge_request.source_project
+
+    project_edit_blob_path(merge_request.source_project,
+      tree_join(merge_request.source_branch, diff_file.new_path),
+      options)
+  end
+
+  expose :old_path_html do |diff_file|
+    old_path, _ = mark_inline_diffs(diff_file.old_path, diff_file.new_path)
+    old_path
+  end
+
+  expose :new_path_html do |diff_file|
+    _, new_path = mark_inline_diffs(diff_file.old_path, diff_file.new_path)
+    new_path
+  end
+
+  expose :formatted_external_url, if: -> (_, options) { options[:environment] } do |diff_file|
+    options[:environment].formatted_external_url
+  end
+
+  expose :external_url, if: -> (_, options) { options[:environment] } do |diff_file|
+    options[:environment].external_url_for(diff_file.new_path, diff_file.content_sha)
+  end
+
+  expose :blob, using: BlobEntity
+
+  expose :can_modify_blob do |diff_file|
+    merge_request = options[:merge_request]
+
+    next unless diff_file.blob
+
+    if merge_request&.source_project && current_user
+      can_modify_blob?(diff_file.blob, merge_request.source_project, merge_request.source_branch)
+    else
+      false
+    end
+  end
+
+  expose :file_hash do |diff_file|
+    Digest::SHA1.hexdigest(diff_file.file_path)
+  end
+
+  expose :file_path
+  expose :old_path
+  expose :new_path
+  expose :new_file?, as: :new_file
+  expose :collapsed?, as: :collapsed
+  expose :text?, as: :text
+  expose :diff_refs
+  expose :stored_externally?, as: :stored_externally
+  expose :external_storage
+  expose :renamed_file?, as: :renamed_file
+  expose :deleted_file?, as: :deleted_file
+  expose :mode_changed?, as: :mode_changed
+  expose :a_mode
+  expose :b_mode
+
+  private
+
+  def memoized_submodule_links(diff_file)
+    strong_memoize(:submodule_links) do
+      if diff_file.submodule?
+        submodule_links(diff_file.blob, diff_file.content_sha, diff_file.repository)
+      else
+        []
+      end
+    end
+  end
+
+  def current_user
+    request.current_user
+  end
+end

app/serializers/diff_file_entity.rb

 # frozen_string_literal: true
 
-class DiffFileEntity < Grape::Entity
-  include RequestAwareEntity
+class DiffFileEntity < DiffFileBaseEntity
   include CommitsHelper
-  include DiffHelper
-  include SubmoduleHelper
-  include BlobHelper
   include IconsHelper
-  include TreeHelper
-  include ChecksCollaboration
-  include Gitlab::Utils::StrongMemoize
 
-  expose :submodule?, as: :submodule
-
-  expose :submodule_link do |diff_file|
-    memoized_submodule_links(diff_file).first
-  end
-
-  expose :submodule_tree_url do |diff_file|
-    memoized_submodule_links(diff_file).last
-  end
-
-  expose :blob, using: BlobEntity
-
-  expose :can_modify_blob do |diff_file|
-    merge_request = options[:merge_request]
-
-    next unless diff_file.blob
-
-    if merge_request&.source_project && current_user
-      can_modify_blob?(diff_file.blob, merge_request.source_project, merge_request.source_branch)
-    else
-      false
-    end
-  end
-
-  expose :file_hash do |diff_file|
-    Digest::SHA1.hexdigest(diff_file.file_path)
-  end
-
-  expose :file_path
   expose :too_large?, as: :too_large
-  expose :collapsed?, as: :collapsed
-  expose :new_file?, as: :new_file
-
-  expose :deleted_file?, as: :deleted_file
-  expose :renamed_file?, as: :renamed_file
-  expose :mode_changed?, as: :mode_changed
-  expose :old_path
-  expose :new_path
-  expose :mode_changed?, as: :mode_changed
-  expose :a_mode
-  expose :b_mode
-  expose :text?, as: :text
   expose :added_lines
   expose :removed_lines
-  expose :diff_refs
-  expose :content_sha
-  expose :stored_externally?, as: :stored_externally
-  expose :external_storage
 
   expose :load_collapsed_diff_url, if: -> (diff_file, options) { diff_file.text? && options[:merge_request] } do |diff_file|
     merge_request = options[:merge_request]
@@ -76,36 +24,6 @@ class DiffFileEntity < Grape::Entity
     )
   end
 
-  expose :formatted_external_url, if: -> (_, options) { options[:environment] } do |diff_file|
-    options[:environment].formatted_external_url
-  end
-
-  expose :external_url, if: -> (_, options) { options[:environment] } do |diff_file|
-    options[:environment].external_url_for(diff_file.new_path, diff_file.content_sha)
-  end
-
-  expose :old_path_html do |diff_file|
-    old_path, _ = mark_inline_diffs(diff_file.old_path, diff_file.new_path)
-    old_path
-  end
-
-  expose :new_path_html do |diff_file|
-    _, new_path = mark_inline_diffs(diff_file.old_path, diff_file.new_path)
-    new_path
-  end
-
-  expose :edit_path, if: -> (_, options) { options[:merge_request] } do |diff_file|
-    merge_request = options[:merge_request]
-
-    options = merge_request.persisted? ? { from_merge_request_iid: merge_request.iid } : {}
-
-    next unless merge_request.source_project
-
-    project_edit_blob_path(merge_request.source_project,
-      tree_join(merge_request.source_branch, diff_file.new_path),
-      options)
-  end
-
   expose :view_path, if: -> (_, options) { options[:merge_request] } do |diff_file|
     merge_request = options[:merge_request]
 
@@ -146,18 +64,4 @@ class DiffFileEntity < Grape::Entity
 
   # Used for parallel diffs
   expose :parallel_diff_lines, using: DiffLineParallelEntity, if: -> (diff_file, _) { diff_file.text? }
-
-  def current_user
-    request.current_user
-  end
-
-  def memoized_submodule_links(diff_file)
-    strong_memoize(:submodule_links) do
-      if diff_file.submodule?
-        submodule_links(diff_file.blob, diff_file.content_sha, diff_file.repository)
-      else
-        []
-      end
-    end
-  end
 end

app/serializers/discussion_diff_file_entity.rb

+# frozen_string_literal: true
+
+class DiscussionDiffFileEntity < DiffFileBaseEntity
+end

app/serializers/discussion_entity.rb

@@ -36,7 +36,7 @@ class DiscussionEntity < Grape::Entity
     new_project_issue_path(discussion.project, merge_request_to_resolve_discussions_of: discussion.noteable.iid, discussion_to_resolve: discussion.id)
   end
 
-  expose :diff_file, using: DiffFileEntity, if: -> (d, _) { d.diff_discussion? }
+  expose :diff_file, using: DiscussionDiffFileEntity, if: -> (d, _) { d.diff_discussion? }
 
   expose :diff_discussion?, as: :diff_discussion
 
@@ -46,19 +46,6 @@ class DiscussionEntity < Grape::Entity
 
   expose :truncated_diff_lines, using: DiffLineEntity, if: -> (d, _) { d.diff_discussion? && d.on_text? && (d.expanded? || render_truncated_diff_lines?) }
 
-  expose :image_diff_html, if: -> (d, _) { d.diff_discussion? && d.on_image? } do |discussion|
-    diff_file = discussion.diff_file
-    partial = diff_file.new_file? || diff_file.deleted_file? ? 'single_image_diff' : 'replaced_image_diff'
-    options[:context].render_to_string(
-      partial: "projects/diffs/#{partial}",
-      locals: { diff_file: diff_file,
-                position: discussion.position.to_json,
-                click_to_comment: false },
-      layout: false,
-      formats: [:html]
-    )
-  end
-
   expose :for_commit?, as: :for_commit
   expose :commit_id
 

changelogs/unreleased/osw-remove-unnused-data-from-diff-discussions.yml

+---
+title: Remove unused data from discussions endpoint
+merge_request: 23570
+author:
+type: performance

spec/javascripts/diffs/mock_data/diff_discussions.js

@@ -489,8 +489,6 @@ export default {
   diff_discussion: true,
   truncated_diff_lines:
     '<tr class="line_holder new" id="">\n<td class="diff-line-num new old_line" data-linenumber="1">\n \n</td>\n<td class="diff-line-num new new_line" data-linenumber="1">\n1\n</td>\n<td class="line_content new noteable_line"><span id="LC1" class="line" lang="plaintext">  - Bad dates</span>\n</td>\n</tr>\n<tr class="line_holder new" id="">\n<td class="diff-line-num new old_line" data-linenumber="1">\n \n</td>\n<td class="diff-line-num new new_line" data-linenumber="2">\n2\n</td>\n<td class="line_content new noteable_line"><span id="LC2" class="line" lang="plaintext"></span>\n</td>\n</tr>\n',
-  image_diff_html:
-    '<div class="image js-replaced-image" data="">\n<div class="two-up view">\n<div class="wrap">\n<div class="frame deleted">\n<img alt="CHANGELOG" src="http://localhost:3000/gitlab-org/gitlab-test/raw/e63f41fe459e62e1228fcef60d7189127aeba95a/CHANGELOG" />\n</div>\n<p class="image-info hide">\n<span class="meta-filesize">22.3 KB</span>\n|\n<strong>W:</strong>\n<span class="meta-width"></span>\n|\n<strong>H:</strong>\n<span class="meta-height"></span>\n</p>\n</div>\n<div class="wrap">\n<div class="added frame js-image-frame" data-note-type="DiffNote" data-position="{&quot;base_sha&quot;:&quot;e63f41fe459e62e1228fcef60d7189127aeba95a&quot;,&quot;start_sha&quot;:&quot;d9eaefe5a676b820c57ff18cf5b68316025f7962&quot;,&quot;head_sha&quot;:&quot;c48ee0d1bf3b30453f5b32250ce03134beaa6d13&quot;,&quot;old_path&quot;:&quot;CHANGELOG&quot;,&quot;new_path&quot;:&quot;CHANGELOG&quot;,&quot;position_type&quot;:&quot;text&quot;,&quot;old_line&quot;:null,&quot;new_line&quot;:2}">\n<img alt="CHANGELOG" draggable="false" src="http://localhost:3000/gitlab-org/gitlab-test/raw/c48ee0d1bf3b30453f5b32250ce03134beaa6d13/CHANGELOG" />\n</div>\n\n<p class="image-info hide">\n<span class="meta-filesize">22.3 KB</span>\n|\n<strong>W:</strong>\n<span class="meta-width"></span>\n|\n<strong>H:</strong>\n<span class="meta-height"></span>\n</p>\n</div>\n</div>\n<div class="swipe view hide">\n<div class="swipe-frame">\n<div class="frame deleted">\n<img alt="CHANGELOG" src="http://localhost:3000/gitlab-org/gitlab-test/raw/e63f41fe459e62e1228fcef60d7189127aeba95a/CHANGELOG" />\n</div>\n<div class="swipe-wrap">\n<div class="added frame js-image-frame" data-note-type="DiffNote" data-position="{&quot;base_sha&quot;:&quot;e63f41fe459e62e1228fcef60d7189127aeba95a&quot;,&quot;start_sha&quot;:&quot;d9eaefe5a676b820c57ff18cf5b68316025f7962&quot;,&quot;head_sha&quot;:&quot;c48ee0d1bf3b30453f5b32250ce03134beaa6d13&quot;,&quot;old_path&quot;:&quot;CHANGELOG&quot;,&quot;new_path&quot;:&quot;CHANGELOG&quot;,&quot;position_type&quot;:&quot;text&quot;,&quot;old_line&quot;:null,&quot;new_line&quot;:2}">\n<img alt="CHANGELOG" draggable="false" src="http://localhost:3000/gitlab-org/gitlab-test/raw/c48ee0d1bf3b30453f5b32250ce03134beaa6d13/CHANGELOG" />\n</div>\n\n</div>\n<span class="swipe-bar">\n<span class="top-handle"></span>\n<span class="bottom-handle"></span>\n</span>\n</div>\n</div>\n<div class="onion-skin view hide">\n<div class="onion-skin-frame">\n<div class="frame deleted">\n<img alt="CHANGELOG" src="http://localhost:3000/gitlab-org/gitlab-test/raw/e63f41fe459e62e1228fcef60d7189127aeba95a/CHANGELOG" />\n</div>\n<div class="added frame js-image-frame" data-note-type="DiffNote" data-position="{&quot;base_sha&quot;:&quot;e63f41fe459e62e1228fcef60d7189127aeba95a&quot;,&quot;start_sha&quot;:&quot;d9eaefe5a676b820c57ff18cf5b68316025f7962&quot;,&quot;head_sha&quot;:&quot;c48ee0d1bf3b30453f5b32250ce03134beaa6d13&quot;,&quot;old_path&quot;:&quot;CHANGELOG&quot;,&quot;new_path&quot;:&quot;CHANGELOG&quot;,&quot;position_type&quot;:&quot;text&quot;,&quot;old_line&quot;:null,&quot;new_line&quot;:2}">\n<img alt="CHANGELOG" draggable="false" src="http://localhost:3000/gitlab-org/gitlab-test/raw/c48ee0d1bf3b30453f5b32250ce03134beaa6d13/CHANGELOG" />\n</div>\n\n<div class="controls">\n<div class="transparent"></div>\n<div class="drag-track">\n<div class="dragger" style="left: 0px;"></div>\n</div>\n<div class="opaque"></div>\n</div>\n</div>\n</div>\n</div>\n<div class="view-modes hide">\n<ul class="view-modes-menu">\n<li class="two-up" data-mode="two-up">2-up</li>\n<li class="swipe" data-mode="swipe">Swipe</li>\n<li class="onion-skin" data-mode="onion-skin">Onion skin</li>\n</ul>\n</div>\n',
 };
 
 export const imageDiffDiscussions = [

spec/serializers/diff_file_entity_spec.rb

@@ -13,39 +13,6 @@ describe DiffFileEntity do
 
   subject { entity.as_json }
 
-  shared_examples 'diff file entity' do
-    it 'exposes correct attributes' do
-      expect(subject).to include(
-        :submodule, :submodule_link, :submodule_tree_url, :file_path,
-        :deleted_file, :old_path, :new_path, :mode_changed,
-        :a_mode, :b_mode, :text, :old_path_html,
-        :new_path_html, :highlighted_diff_lines, :parallel_diff_lines,
-        :blob, :file_hash, :added_lines, :removed_lines, :diff_refs, :content_sha,
-        :stored_externally, :external_storage, :too_large, :collapsed, :new_file,
-        :context_lines_path
-      )
-    end
-
-    it 'includes viewer' do
-      expect(subject[:viewer].with_indifferent_access)
-          .to match_schema('entities/diff_viewer')
-    end
-
-    # Converted diff files from GitHub import does not contain blob file
-    # and content sha.
-    context 'when diff file does not have a blob and content sha' do
-      it 'exposes some attributes as nil' do
-        allow(diff_file).to receive(:content_sha).and_return(nil)
-        allow(diff_file).to receive(:blob).and_return(nil)
-
-        expect(subject[:context_lines_path]).to be_nil
-        expect(subject[:view_path]).to be_nil
-        expect(subject[:highlighted_diff_lines]).to be_nil
-        expect(subject[:can_modify_blob]).to be_nil
-      end
-    end
-  end
-
   context 'when there is no merge request' do
     it_behaves_like 'diff file entity'
   end

spec/serializers/discussion_diff_file_entity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe DiscussionDiffFileEntity do
+  include RepoHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:repository) { project.repository }
+  let(:commit) { project.commit(sample_commit.id) }
+  let(:diff_refs) { commit.diff_refs }
+  let(:diff) { commit.raw_diffs.first }
+  let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: diff_refs, repository: repository) }
+  let(:entity) { described_class.new(diff_file, request: {}) }
+
+  subject { entity.as_json }
+
+  context 'when there is no merge request' do
+    it_behaves_like 'diff file discussion entity'
+  end
+
+  context 'when there is a merge request' do
+    let(:user) { create(:user) }
+    let(:request) { EntityRequest.new(project: project, current_user: user) }
+    let(:merge_request) { create(:merge_request, source_project: project, target_project: project) }
+    let(:entity) { described_class.new(diff_file, request: request, merge_request: merge_request) }
+
+    it_behaves_like 'diff file discussion entity'
+
+    it 'exposes additional attributes' do
+      expect(subject).to include(:edit_path)
+    end
+
+    it 'exposes no diff lines' do
+      expect(subject).not_to include(:highlighted_diff_lines,
+                                     :parallel_diff_lines)
+    end
+  end
+end

spec/serializers/discussion_entity_spec.rb

@@ -74,13 +74,5 @@ describe DiscussionEntity do
         :active
       )
     end
-
-    context 'when diff file is a image' do
-      it 'exposes image attributes' do
-        allow(discussion).to receive(:on_image?).and_return(true)
-
-        expect(subject.keys).to include(:image_diff_html)
-      end
-    end
   end
 end

spec/support/shared_examples/serializers/diff_file_entity_examples.rb

+# frozen_string_literal: true
+
+shared_examples 'diff file base entity' do
+  it 'exposes essential attributes' do
+    expect(subject).to include(:content_sha, :submodule, :submodule_link,
+                               :submodule_tree_url, :old_path_html,
+                               :new_path_html, :blob, :can_modify_blob,
+                               :file_hash, :file_path, :old_path, :new_path,
+                               :collapsed, :text, :diff_refs, :stored_externally,
+                               :external_storage, :renamed_file, :deleted_file,
+                               :mode_changed, :a_mode, :b_mode, :new_file)
+  end
+
+  # Converted diff files from GitHub import does not contain blob file
+  # and content sha.
+  context 'when diff file does not have a blob and content sha' do
+    it 'exposes some attributes as nil' do
+      allow(diff_file).to receive(:content_sha).and_return(nil)
+      allow(diff_file).to receive(:blob).and_return(nil)
+
+      expect(subject[:context_lines_path]).to be_nil
+      expect(subject[:view_path]).to be_nil
+      expect(subject[:highlighted_diff_lines]).to be_nil
+      expect(subject[:can_modify_blob]).to be_nil
+    end
+  end
+end
+
+shared_examples 'diff file entity' do
+  it_behaves_like 'diff file base entity'
+
+  it 'exposes correct attributes' do
+    expect(subject).to include(:too_large, :added_lines, :removed_lines,
+                               :context_lines_path, :highlighted_diff_lines,
+                               :parallel_diff_lines)
+  end
+
+  it 'includes viewer' do
+    expect(subject[:viewer].with_indifferent_access)
+        .to match_schema('entities/diff_viewer')
+  end
+end
+
+shared_examples 'diff file discussion entity' do
+  it_behaves_like 'diff file base entity'
+end