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Loading and saving comments in the diff of merge requests fails with error 500

Summary

The loading of comments in merge requests fails with an error 500 if at least one comment attached to the diff exists. Creating a comment in the diff fails, too.

Steps to reproduce

  1. Open an existing merge request (in open, closed, or merged state) with at least one comment attached to the diff.
  2. Loading of the comments fails (with an appropriate red message box).
  3. Create a merge request with a non-empty changeset.
  4. Switch to the "Diff" pane.
  5. Trying to add a comment fails (with an appropriate red message box).

What is the current bug behavior?

The browsers debug console shows that GETing from "some-group/some-project/-/merge_requests/123/discussions.json?per_page=20" failed with 500.

The browsers debug console shows that POSTing to "some-group/some-project/notes?target_id=1234&target_type=merge_request" failed with 500.

What is the expected correct behavior?

Comments in the diff of a merge request can be fetched and created correctly.

Relevant logs and/or screenshots

Excerpt from the production.log when opening a MR with comments in the diff 
Completed 500 Internal Server Error in 685ms (ActiveRecord: 261.0ms | Elasticsearch: 0.0ms | Allocations: 140266)

Psych::DisallowedClass (Tried to load unspecified class: Gitlab::Diff::Position):


app/models/diff_discussion.rb:13:in position' app/models/diff_discussion.rb:51:in cache_key'
app/controllers/concerns/issuable_actions.rb:186:in render_mr_discussions' app/controllers/concerns/issuable_actions.rb:174:in discussions'
app/controllers/projects/merge_requests_controller.rb:377:in discussions' app/controllers/application_controller.rb:582:in block in allow_gitaly_ref_name_caching'
lib/gitlab/gitaly_client.rb:323:in allow_ref_name_caching' app/controllers/application_controller.rb:581:in allow_gitaly_ref_name_caching'
app/controllers/application_controller.rb:531:in set_current_admin' lib/gitlab/session.rb:11:in with_session'
app/controllers/application_controller.rb:522:in set_session_storage' lib/gitlab/i18n.rb:107:in with_locale'
lib/gitlab/i18n.rb:113:in `with_user_locale'
Excerpt from the production.log when creating a comment in the diff 
Completed 500 Internal Server Error in 65ms (ActiveRecord: 4.2ms | Elasticsearch: 0.0ms | Allocations: 22029)

Psych::DisallowedClass (Tried to load unspecified class: Gitlab::Diff::Position):


lib/gitlab/markdown_cache/active_record/extension.rb:21:in attributes' app/services/notes/build_service.rb:33:in new_note'
app/services/notes/build_service.rb:27:in execute' app/services/notes/create_service.rb:8:in execute'
app/controllers/concerns/notes_actions.rb:43:in create' app/controllers/application_controller.rb:531:in set_current_admin'
lib/gitlab/session.rb:11:in with_session' app/controllers/application_controller.rb:522:in set_session_storage'
lib/gitlab/i18n.rb:107:in with_locale' lib/gitlab/i18n.rb:113:in with_user_locale'

Output of checks

This issue happens in a Debian package installation.

Results of GitLab environment info

Output of `gitlab-rake gitlab:env:info` 
System information
System:
Current User:   git
Using RVM:      no
Ruby Version:   2.7.4p191
Gem Version:    3.2.5
Bundler Version:2.2.5
Rake Version:   13.0.3
Redis Version:  6.0.16
Sidekiq Version:6.4.1
Go Version:     unknown

GitLab information
Version:        15.3.2
Revision:       Unknown
Directory:      /usr/share/gitlab
DB Adapter:     PostgreSQL
DB Version:     13.8
URL:            https://localhost
HTTP Clone URL: https://localhost/some-group/some-project.git
SSH Clone URL:  git@localhost:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: no


GitLab Shell
Version:        14.11.0
Repository storage paths:
default:      /var/lib/gitlab/repositories
GitLab Shell path:              /usr/share/gitlab-shell

Results of GitLab application Check

Expand for output related to the GitLab application check 
Checking GitLab subtasks ...

Checking GitLab Shell ...


GitLab Shell: ... GitLab Shell version >= 14.10.0 ? ... OK (14.11.0)
Running /usr/share/gitlab-shell/bin/check
Internal API available: OK

Redis available via internal API: OK

gitlab-shell self-check successful


Checking GitLab Shell ... Finished


Checking Gitaly ...


Gitaly: ... default ... OK


Checking Gitaly ... Finished


Checking Sidekiq ...


Sidekiq: ... Running? ... yes
Number of Sidekiq processes (cluster/worker) ... 0/1


Checking Sidekiq ... Finished


Checking Incoming Email ...


Incoming Email: ... Reply by email is disabled in config/gitlab.yml


Checking Incoming Email ... Finished


Checking LDAP ...


LDAP: ... Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
// redacted


Checking LDAP ... Finished


Checking GitLab App ...


Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Systemd unit files or init script exist? ... yes
Systemd unit files or init script up-to-date? ... yes
Projects have namespace: ...
// redacted, all projects have yes
Redis version >= 6.0.0? ... yes
Ruby version >= 2.7.2 ? ... yes (2.7.4)
Git user has default SSH configuration? ... yes
Active users: ... 8
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished

Possible fixes

Googeling the issue I noticed that this might be a yaml serialization related issue. However, Gitlab::Diff::Position is definitely included in the whitelist config.active_record.yaml_column_permitted_classes in "config/application.rb".