Highlight huge diffs as plain text without loading a blob

Feature flag: limited_diff_highlighting

When a huge file changes, its whole content highlighted
as a plain text anyway:
https://gitlab.com/gitlab-org/gitlab/-/issues/300228

config/feature_flags/development/limited_diff_highlighting.yml

+---
+name: limited_diff_highlighting
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53768
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/323566
+milestone: '13.12'
+type: development
+group: group::source code
+default_enabled: true

lib/gitlab/diff/highlight.rb

@@ -87,6 +87,7 @@ module Gitlab
 
       def highlight_line(diff_line)
         return unless diff_file && diff_file.diff_refs
+        return diff_line_highlighting(diff_line, plain: true) if blobs_too_large?
 
         if Feature.enabled?(:diff_line_syntax_highlighting, project, default_enabled: :yaml)
           diff_line_highlighting(diff_line)
@@ -95,9 +96,10 @@ module Gitlab
         end
       end
 
-      def diff_line_highlighting(diff_line)
+      def diff_line_highlighting(diff_line, plain: false)
         rich_line = syntax_highlighter(diff_line).highlight(
           diff_line.text(prefix: false),
+          plain: plain,
           context: { line_number: diff_line.line }
         )
 
@@ -158,6 +160,13 @@ module Gitlab
         blob.load_all_data!
         blob.present.highlight.lines
       end
+
+      def blobs_too_large?
+        return false unless Feature.enabled?(:limited_diff_highlighting, project, default_enabled: :yaml)
+        return true if Gitlab::Highlight.too_large?(diff_file.old_blob&.size)
+
+        Gitlab::Highlight.too_large?(diff_file.new_blob&.size)
+      end
     end
   end
 end

lib/gitlab/highlight.rb

@@ -10,6 +10,10 @@ module Gitlab
         .highlight(blob_content, continue: false, plain: plain)
     end
 
+    def self.too_large?(size)
+      size.to_i > Gitlab.config.extra['maximum_text_highlight_size_kilobytes']
+    end
+
     attr_reader :blob_name
 
     def initialize(blob_name, blob_content, language: nil)
@@ -22,7 +26,7 @@ module Gitlab
     def highlight(text, continue: false, plain: false, context: {})
       @context = context
 
-      plain ||= text.length > maximum_text_highlight_size
+      plain ||= self.class.too_large?(text.length)
 
       highlighted_text = highlight_text(text, continue: continue, plain: plain)
       highlighted_text = link_dependencies(text, highlighted_text) if blob_name
@@ -80,10 +84,6 @@ module Gitlab
       Gitlab::DependencyLinker.link(blob_name, text, highlighted_text)
     end
 
-    def maximum_text_highlight_size
-      Gitlab.config.extra['maximum_text_highlight_size_kilobytes']
-    end
-
     def add_highlight_timeout_metric
       return unless Feature.enabled?(:track_highlight_timeouts)
 

spec/lib/gitlab/diff/highlight_spec.rb

@@ -5,7 +5,8 @@ require 'spec_helper'
 RSpec.describe Gitlab::Diff::Highlight do
   include RepoHelpers
 
-  let(:project) { create(:project, :repository) }
+  let_it_be(:project) { create(:project, :repository) }
+
   let(:commit) { project.commit(sample_commit.id) }
   let(:diff) { commit.raw_diffs.first }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: commit.diff_refs, repository: project.repository) }
@@ -156,5 +157,34 @@ RSpec.describe Gitlab::Diff::Highlight do
         it_behaves_like 'without inline diffs'
       end
     end
+
+    context 'when blob is too large' do
+      let(:subject) { described_class.new(diff_file, repository: project.repository).highlight }
+
+      before do
+        allow(Gitlab::Highlight).to receive(:too_large?).and_return(true)
+      end
+
+      it 'blobs are highlighted as plain text without loading all data' do
+        expect(diff_file.blob).not_to receive(:load_all_data!)
+
+        expect(subject[2].rich_text).to eq(%Q{ <span id="LC7" class="line" lang="">  def popen(cmd, path=nil)</span>\n})
+        expect(subject[2].rich_text).to be_html_safe
+      end
+
+      context 'when limited_diff_highlighting is disabled' do
+        before do
+          stub_feature_flags(limited_diff_highlighting: false)
+          stub_feature_flags(diff_line_syntax_highlighting: false)
+        end
+
+        it 'blobs are highlighted as plain text with loading all data' do
+          expect(diff_file.blob).to receive(:load_all_data!).twice
+
+          code = %Q{ <span id="LC7" class="line" lang="">  def popen(cmd, path=nil)</span>\n}
+          expect(subject[2].rich_text).to eq(code)
+        end
+      end
+    end
   end
 end