Intermittent failures when adding comments to MR diffs and publishing reviews

Summary

Users are experiencing intermittent issues when trying to add comments to specific lines in MR diffs and publishing reviews. The issue appears to be related to outdated diffs and unchanged lines.

Reported Issues

1. Comment submission failures: When attempting to add a comment to a specific line in the diff, users receive errors. Comments can be added successfully to other parts of the MR, but fail for certain lines.
2. Pending comments not visible: After submitting comments, they don't appear on the diff page as expected, though they do collect under the "Your review" button in the "Submit your review" panel.
3. 500 error on review publish: When attempting to publish the review, users encounter a 500 error at drafts_service.js:23.

Technical Details

• Correlation ID: 9a902963f1e6e6a5-ATL
• Related Sentry Issues:
  • https://new-sentry.gitlab.net/organizations/gitlab/issues/2757157/
  • https://new-sentry.gitlab.net/organizations/gitlab/issues/2823475/
• Related Issue: #510123 - "Submit Review does nothing; browser JS console indicates 500 error at drafts_service.js:23"
• Affected MR: !215053

Root Cause Hypothesis

The issue appears to be related to a combination of unchanged lines and outdated diffs that are not being properly guarded against. The Git service may have been overloaded, causing downstream effects in diff handling.

Reproduction

The issue is intermittent but reproducible on specific MRs. Users report:

• Successfully commenting on some lines while others fail
• Retrying multiple times over 10+ minutes without success
• Issue resolving after page refresh in some cases
• 500 errors when attempting to publish reviews

Impact

This affects the code review workflow, preventing users from effectively reviewing and commenting on MR changes.