Merge branch 'pr-importer-io-extra-error-handling' into 'master'

Ensure MR diffs always exist in the PR importer

Closes #47312

See merge request gitlab-org/gitlab-ce!19704

changelogs/unreleased/pr-importer-io-extra-error-handling.yml

+---
+title: Ensure MR diffs always exist in the PR importer
+merge_request:
+author:
+type: fixed

lib/gitlab/github_import/importer/pull_request_importer.rb

@@ -104,7 +104,8 @@ module Gitlab
           # first save the diff, then populate it.
           diff =
             if already_exists
-              merge_request.merge_request_diffs.take
+              merge_request.merge_request_diffs.take ||
+                merge_request.merge_request_diffs.build
             else
               merge_request.merge_request_diffs.build
             end

spec/lib/gitlab/github_import/importer/pull_request_importer_spec.rb

@@ -276,5 +276,17 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
       expect(diff.merge_request_diff_commits.exists?).to eq(true)
     end
+
+    context 'when the merge request exists' do
+      it 'creates the merge request diffs if they do not yet exist' do
+        mr, _ = importer.create_merge_request
+
+        mr.merge_request_diffs.delete_all
+
+        importer.insert_git_data(mr, true)
+
+        expect(mr.merge_request_diffs.exists?).to eq(true)
+      end
+    end
   end
 end