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.

4 files changed, 107 insertions, 111 deletions

diff --git a/app/serializers/diff_file_base_entity.rb b/app/serializers/diff_file_base_entity.rb
new file mode 100644
index 00000000000..06a8db78476
--- /dev/null
+++ b/app/serializers/diff_file_base_entity.rb
@@ -0,0 +1,101 @@
+# 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
diff --git a/app/serializers/diff_file_entity.rb b/app/serializers/diff_file_entity.rb
index e570039d47e..f0881829efd 100644
--- a/app/serializers/diff_file_entity.rb
+++ b/app/serializers/diff_file_entity.rb
@@ -1,64 +1,12 @@
 # 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
diff --git a/app/serializers/discussion_diff_file_entity.rb b/app/serializers/discussion_diff_file_entity.rb
new file mode 100644
index 00000000000..419e7edf94f
--- /dev/null
+++ b/app/serializers/discussion_diff_file_entity.rb
@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+class DiscussionDiffFileEntity < DiffFileBaseEntity
+end
diff --git a/app/serializers/discussion_entity.rb b/app/serializers/discussion_entity.rb
index b6786a0d597..b2d9d52bd22 100644
--- a/app/serializers/discussion_entity.rb
+++ b/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