Merge branch '26952-fix-viewing-legacy-diff-notes-in-discussion' into 'master'

Fix viewing some legacy diff notes in discussions

See merge request gitlab-org/gitlab!45920

app/models/diff_viewer/image.rb

@@ -10,5 +10,13 @@ module DiffViewer
     self.binary = true
     self.switcher_icon = 'doc-image'
     self.switcher_title = _('image diff')
+
+    def self.can_render?(diff_file, verify_binary: true)
+      # When both blobs are missing, we often still have a textual diff that can
+      # be displayed
+      return false if diff_file.old_blob.nil? && diff_file.new_blob.nil?
+
+      super
+    end
   end
 end

changelogs/unreleased/26952-fix-viewing-legacy-diff-notes-in-discussion.yml

+---
+title: Fix viewing GitHub-imported diff notes in discussions
+merge_request: 45920
+author:
+type: fixed

spec/models/diff_viewer/image_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe DiffViewer::Image do
+  describe '.can_render?' do
+    let(:diff_file) { double(Gitlab::Diff::File) }
+    let(:blob) { double(Gitlab::Git::Blob, binary_in_repo?: true, extension: 'png') }
+
+    subject { described_class.can_render?(diff_file, verify_binary: false) }
+
+    it 'returns false if both old and new blob are absent' do
+      allow(diff_file).to receive(:old_blob) { nil }
+      allow(diff_file).to receive(:new_blob) { nil }
+
+      is_expected.to be_falsy
+    end
+
+    it 'returns true if the old blob is present' do
+      allow(diff_file).to receive(:old_blob) { blob }
+      allow(diff_file).to receive(:new_blob) { nil }
+
+      is_expected.to be_truthy
+    end
+
+    it 'returns true if the new blob is present' do
+      allow(diff_file).to receive(:old_blob) { nil }
+      allow(diff_file).to receive(:new_blob) { blob }
+
+      is_expected.to be_truthy
+    end
+
+    it 'returns true if both old and new blobs are present' do
+      allow(diff_file).to receive(:old_blob) { blob }
+      allow(diff_file).to receive(:new_blob) { blob }
+
+      is_expected.to be_truthy
+    end
+  end
+end