Render MR page when no persisted diff_refs or diffs size

app/models/merge_request_diff.rb

@@ -182,7 +182,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
   def diffs(diff_options = nil)
-    if without_files? && comparison = diff_refs.compare_in(project)
+    if without_files? && comparison = diff_refs&.compare_in(project)
       # It should fetch the repository when diffs are cleaned by the system.
       # We don't keep these for storage overload purposes.
       # See https://gitlab.com/gitlab-org/gitlab-ce/issues/37639

changelogs/unreleased/osw-fallback-to-collection-when-no-diff-refs.yml

+---
+title: Render MR page when diffs cannot be fetched from the database or the git repository
+merge_request: 20680
+author:
+type: fixed

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -53,6 +53,23 @@ describe Projects::MergeRequestsController do
     it_behaves_like "loads labels", :show
     describe 'as html' do
+      context 'when diff files were cleaned' do
+        render_views
+
+        it 'renders page when diff size is not persisted and diff_refs does not exist' do
+          diff = merge_request.merge_request_diff
+
+          diff.clean!
+          diff.update!(real_size: nil,
+                       start_commit_sha: nil,
+                       base_commit_sha: nil)
+
+          go(format: :html)
+
+          expect(response).to be_success
+        end
+      end
+
       it "renders merge request page" do
         go(format: :html)

spec/models/merge_request_diff_spec.rb

@@ -82,6 +82,14 @@ describe MergeRequestDiff do
         diff.diffs
       end
+
+      it 'returns persisted diffs if diff refs does not exist' do
+        expect(diff).to receive(:load_diffs)
+
+        diff.update!(start_commit_sha: nil, base_commit_sha: nil)
+
+        diff.diffs
+      end
     end
   end