Cannot view old MR changes

Summary

One of our customers tried viewing one of the Changes tab in an old MR and the UI shows the Something went wrong on our end. Please try again! error message.

They only experience this on old MR changes. They are currently using GitLab 13.9.4.

Steps to reproduce

Unfortunately, I wasn't able to reproduce the issue on my end.

What is the current bug behavior?

Shows an error message when viewing an old MR change.

What is the expected correct behavior?

Should show the MR change.

Relevant logs and/or screenshots

NoMethodError (undefined method `paths' for nil:NilClass):

app/services/discussions/capture_diff_note_positions_service.rb:29:in `block in build_discussions'
app/services/discussions/capture_diff_note_positions_service.rb:29:in `each'
app/services/discussions/capture_diff_note_positions_service.rb:29:in `flat_map'
app/services/discussions/capture_diff_note_positions_service.rb:29:in `build_discussions'
lib/gitlab/metrics/instrumentation.rb:160:in `block in build_discussions'
lib/gitlab/metrics/method_call.rb:27:in `measure'
lib/gitlab/metrics/instrumentation.rb:160:in `build_discussions'
app/services/discussions/capture_diff_note_positions_service.rb:12:in `execute'
lib/gitlab/metrics/instrumentation.rb:160:in `block in execute'
lib/gitlab/metrics/method_call.rb:27:in `measure'
lib/gitlab/metrics/instrumentation.rb:160:in `execute'
app/controllers/projects/merge_requests/diffs_controller.rb:188:in `update_diff_discussion_positions!'
ee/lib/gitlab/ip_address_state.rb:10:in `with'
ee/app/controllers/ee/application_controller.rb:44:in `set_current_ip_address'
app/controllers/application_controller.rb:482:in `set_current_admin'
lib/gitlab/session.rb:11:in `with_session' 

I asked them to run these Rails code:

## While inside the Rails console
p = Project.find_by_full_path('project-path')
m = MergeRequest.find_by(project_id: p.id, iid: 13)
active_diff_discussions = m.notes.new_diff_notes.discussions.select do |discussion|
    discussion.active?(m.diff_refs)
end

## Check the value for the first array
pp active_diff_discussions[0].diff_file

The value of active_diff_discussions[0].diff_file is nil causing the error above.

Output of checks

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`)


System information
System:		
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.7.2p137
Gem Version:	3.1.4
Bundler Version:2.1.4
Rake Version:	13.0.3
Redis Version:	6.0.10
Git Version:	2.29.0
Sidekiq Version:5.2.9
Go Version:	unknown

GitLab information
Version:	13.9.4-ee
Revision:	fb7eeedfaf4
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	12.5
URL:		xxx
HTTP Clone URL:	xxx
SSH Clone URL:	xxx
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	13.17.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

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)
Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 13.17.0 ? ... OK (13.17.0)
Running /opt/gitlab/embedded/service/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 ... 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... Anonymous. No bind_dn or password configured
LDAP users with access to your GitLab server (only showing the first 100 results)
User output sanitized. Found 22 users of 100 limit.
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
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
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
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30/548 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.7.2 ? ... yes (2.7.2)
Git version >= 2.29.0 ? ... yes (2.29.0)
Git user has default SSH configuration? ... yes
Active users: ... 22
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... no
Try fixing it:
Please migrate all projects to hashed storage
as legacy storage is deprecated in 13.0 and support will be removed in 14.0.
For more information see:
doc/administration/repository_storage_types.md
Elasticsearch version 7.x (6.4 - 6.x deprecated to be removed in 13.8)? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes