Merge branch '240921-fix-merge-request-diff-file-invalid-base64' into 'master'

Fall back to the raw diff if base64 encoding fails

Closes #240921

See merge request gitlab-org/gitlab!40598

app/models/merge_request_diff_file.rb

@@ -25,6 +25,16 @@ class MergeRequestDiffFile < ApplicationRecord
         super
       end
 
-    binary? ? content.unpack1('m0') : content
+    return content unless binary?
+
+    # If the data isn't valid base64, return it as-is, since it's almost certain
+    # to be a valid diff. Parsing it as a diff will fail if it's something else.
+    #
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/240921
+    begin
+      content.unpack1('m0')
+    rescue ArgumentError
+      content
+    end
   end
 end

changelogs/unreleased/240921-fix-merge-request-diff-file-invalid-base64.yml

+---
+title: Fix reading some merge request diffs
+merge_request: 40598
+author:
+type: fixed

spec/models/merge_request_diff_file_spec.rb

@@ -25,6 +25,14 @@ RSpec.describe MergeRequestDiffFile do
         it 'unpacks from base 64' do
           expect(subject.diff).to eq(unpacked)
         end
+
+        context 'invalid base64' do
+          let(:packed) { '---/dev/null' }
+
+          it 'returns the raw diff' do
+            expect(subject.diff).to eq(packed)
+          end
+        end
       end
 
       context 'when the diff is not marked as binary' do