Fix Error 500 when viewing old merge requests with bad diff data

Customers running old versions of GitLab may have MergeRequestDiffs with
the text ["--broken diff"] due to text generated by gitlab_git 1.0.3.
To avoid the Error 500, verify that each element is a type that gitlab_git
will accept before attempting to create a DiffCollection.

Closes #20776

1 files changed, 10 insertions, 0 deletions

diff --git a/spec/models/merge_request_diff_spec.rb b/spec/models/merge_request_diff_spec.rb
index 530a7def553..96f1f60dbc0 100644
--- a/spec/models/merge_request_diff_spec.rb
+++ b/spec/models/merge_request_diff_spec.rb
@@ -44,6 +44,16 @@ describe MergeRequestDiff, models: true do
       end
     end
 
+    context 'when the raw diffs have invalid content' do
+      before { mr_diff.update_attributes(st_diffs: ["--broken-diff"]) }
+
+      it 'returns an empty DiffCollection' do
+        expect(mr_diff.raw_diffs.to_a).to be_empty
+        expect(mr_diff.raw_diffs).to be_a(Gitlab::Git::DiffCollection)
+        expect(mr_diff.raw_diffs).to be_empty
+      end
+    end
+
     context 'when the raw diffs exist' do
       it 'returns the diffs' do
         expect(mr_diff.raw_diffs).to be_a(Gitlab::Git::DiffCollection)