Notes will sometimes stop loading for a merge request

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Summary

For unknown reasons merge requests with notes and diffnotes will no longer load for a merge requests. This has been resolved in the past by deleting and re-creating the merge request.

This does not seem to correlate with the number of notes or diffnotes.

Steps to reproduce

• Create merge request
• add still unknown element to MR
• comments will no longer load

Example Project

Unable to reproduce consistently

What is the current bug behavior?

500 error and comments will not longer load

This happens on merge requests with as few as 18 notes.

What is the expected correct behavior?

notes will load

Relevant logs and/or screenshots

Click to expand

app/models/concerns/discussion_on_diff.rb:50:in `truncated_diff_lines', app/serializers/base_serializer.rb:16:in `represent', app/controllers/concerns/notes_actions.rb:31:in `index', lib/gitlab/ip_address_state.rb:11:in `with', ee/app/controllers/ee/application_controller.rb:45:in `set_current_ip_address', app/controllers/application_controller.rb:487:in `set_current_admin', lib/gitlab/session.rb:11:in `with_session', app/controllers/application_controller.rb:478:in `set_session_storage', lib/gitlab/i18n.rb:114:in `with_locale', lib/gitlab/i18n.rb:120:in `with_user_locale', app/controllers/application_controller.rb:469:in `set_locale', app/controllers/application_controller.rb:462:in `set_current_context', lib/gitlab/middleware/action_controller_static_context.rb:23:in `call', ee/lib/omni_auth/strategies/group_saml.rb:41:in `other_phase', lib/gitlab/metrics/elasticsearch_rack_middleware.rb:16:in `call', lib/gitlab/middleware/sidekiq_shard_awareness_validation.rb:20:in `block in call', lib/gitlab/sidekiq_sharding/validator.rb:42:in `enabled', lib/gitlab/middleware/sidekiq_shard_awareness_validation.rb:20:in `call', lib/gitlab/middleware/memory_report.rb:13:in `call', lib/gitlab/middleware/speedscope.rb:13:in `call', lib/gitlab/database/load_balancing/rack_middleware.rb:23:in `call', lib/gitlab/middleware/rails_queue_duration.rb:33:in `call', lib/gitlab/etag_caching/middleware.rb:21:in `call', lib/gitlab/metrics/rack_middleware.rb:16:in `block in call', lib/gitlab/metrics/web_transaction.rb:46:in `run', lib/gitlab/metrics/rack_middleware.rb:16:in `call', lib/gitlab/middleware/go.rb:21:in `call', lib/gitlab/middleware/query_analyzer.rb:11:in `block in call', lib/gitlab/database/query_analyzer.rb:83:in `within', lib/gitlab/middleware/query_analyzer.rb:11:in `call', lib/ci/job_token/middleware.rb:11:in `call', lib/gitlab/middleware/multipart.rb:173:in `call', lib/gitlab/middleware/read_only/controller.rb:50:in `call', lib/gitlab/middleware/read_only.rb:18:in `call', lib/gitlab/middleware/unauthenticated_session_expiry.rb:18:in `call', lib/gitlab/middleware/same_site_cookies.rb:27:in `call', lib/gitlab/middleware/path_traversal_check.rb:40:in `call', lib/gitlab/middleware/handle_malformed_strings.rb:21:in `call', lib/gitlab/middleware/basic_health_check.rb:25:in `call', lib/gitlab/middleware/handle_ip_spoof_attack_error.rb:25:in `call', lib/gitlab/middleware/request_context.rb:15:in `call', lib/gitlab/middleware/webhook_recursion_detection.rb:15:in `call', config/initializers/fix_local_cache_middleware.rb:11:in `call', lib/gitlab/middleware/compressed_json.rb:44:in `call', lib/gitlab/middleware/rack_multipart_tempfile_factory.rb:19:in `call', lib/gitlab/middleware/sidekiq_web_static.rb:20:in `call', lib/gitlab/metrics/requests_rack_middleware.rb:83:in `call', lib/gitlab/middleware/release_env.rb:12:in `call

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Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

 (For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`)  (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)  

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes