Cache markdown when viewing a blob

In order to improve performance of
`Projects::BlobController#show.json` when viewing a markdown blob,
we need to utilize the cache so succeeding requests to the same
blob won't have to re-parse markdown.

Based on tests done locally, this improves the response time of
the endpoint when viewing a large markdown file from ~11s to ~4s.

app/models/blob_viewer/markup.rb

@@ -9,5 +9,15 @@ module BlobViewer
     self.extensions = Gitlab::MarkupHelper::EXTENSIONS
     self.file_types = %i(readme)
     self.binary = false
+
+    def banzai_render_context
+      {}.tap do |h|
+        h[:rendered] = blob.rendered_markup if blob.respond_to?(:rendered_markup)
+
+        if Feature.enabled?(:cached_markdown_blob, blob.project)
+          h[:cache_key] = ['blob', blob.id, 'commit', blob.commit_id]
+        end
+      end
+    end
   end
 end

app/views/projects/blob/viewers/_markup.html.haml

 - blob = viewer.blob
-- context = blob.respond_to?(:rendered_markup) ? { rendered: blob.rendered_markup } : {}
 .file-content.md
-  = markup(blob.name, blob.data, context)
+  = markup(blob.name, blob.data, viewer.banzai_render_context)

config/feature_flags/development/cached_markdown_blob.yml

+---
+name: cached_markdown_blob
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44300
+rollout_issue_url:
+type: development
+group: group::source code
+default_enabled: false

spec/models/blob_viewer/markup_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe BlobViewer::Markup do
+  include FakeBlobHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:blob) { fake_blob(path: 'CHANGELOG.md') }
+
+  subject { described_class.new(blob) }
+
+  describe '#banzai_render_context' do
+    it 'returns context needed for banzai rendering' do
+      expect(subject.banzai_render_context.keys).to eq([:cache_key])
+    end
+
+    context 'when blob does respond to rendered_markup' do
+      before do
+        allow(blob).to receive(:rendered_markup).and_return("some rendered markup")
+      end
+
+      it 'does sets rendered key' do
+        expect(subject.banzai_render_context.keys).to include(:rendered)
+      end
+    end
+
+    context 'when cached_markdown_blob feature flag is disabled' do
+      before do
+        stub_feature_flags(cached_markdown_blob: false)
+      end
+
+      it 'does not set cache_key key' do
+        expect(subject.banzai_render_context.keys).not_to include(:cache_key)
+      end
+    end
+  end
+end