500 error on diff of some MR: NoMethodError (undefined method `file_path' for #<String:0x00007faf3698ca28>)

Summary

Running on GitLab 11.6.10 (omnibus), when trying to display the diff from one of our MRs the server replies with a 500 error on diffs.json.

Steps to reproduce

Open diff in specific merge request. example url: https://gitlab.example.com/group/project/merge_requests/233/diffs

What is the current bug behavior?

500 on loading https://gitlab.example.com/group/project/merge_requests/233/diffs.json

Relevant logs and/or screenshots

Processing by Projects::MergeRequests::DiffsController#show as JSON
  Parameters: {"namespace_id"=>"group", "project_id"=>"project", "id"=>"233"}
Completed 500 Internal Server Error in 108ms (ActiveRecord: 14.9ms)
  
NoMethodError (undefined method `file_path' for #<String:0x00007faf3698ca28>):
  
lib/gitlab/diff/file_collection/base.rb:39:in `block in unfold_diff_files'
lib/gitlab/diff/file_collection/base.rb:39:in `each'
lib/gitlab/diff/file_collection/base.rb:39:in `group_by'
lib/gitlab/diff/file_collection/base.rb:39:in `unfold_diff_files'
app/controllers/projects/merge_requests/diffs_controller.rb:27:in `render_diffs'
app/controllers/projects/merge_requests/diffs_controller.rb:14:in `show'
lib/gitlab/i18n.rb:55:in `with_locale'
lib/gitlab/i18n.rb:61:in `with_user_locale'
app/controllers/application_controller.rb:420:in `set_locale'
lib/gitlab/middleware/multipart.rb:103:in `call'
lib/gitlab/request_profiler/middleware.rb:16:in `call'
lib/gitlab/middleware/go.rb:19:in `call'
lib/gitlab/etag_caching/middleware.rb:13:in `call'
lib/gitlab/middleware/correlation_id.rb:16:in `block in call'
lib/gitlab/correlation_id.rb:15:in `use_id'
lib/gitlab/middleware/correlation_id.rb:15:in `call'
lib/gitlab/middleware/read_only/controller.rb:40:in `call'
lib/gitlab/middleware/read_only.rb:18:in `call'
lib/gitlab/middleware/basic_health_check.rb:25:in `call'
lib/gitlab/request_context.rb:20:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:29:in `call'
lib/gitlab/middleware/release_env.rb:13:in `call'
Processing by Projects::MergeRequestsController#ci_environments_status as JSON
  Parameters: {"namespace_id"=>"group", "project_id"=>"project", "id"=>"233"}
Processing by Projects::MergeRequestsController#discussions as JSON
  Parameters: {"namespace_id"=>"group", "project_id"=>"project", "id"=>"233"}

I tried to look for the cause of the error and wrapped line 39 in to begin/rescue block to see content of positions variable:

        # This mutates `diff_files` lines.
        def unfold_diff_files(positions)
          begin
            positions_grouped_by_path = positions.group_by { |position| position.file_path }
          rescue
            raise positions.map{|i| "'#{i}'"}.join("\n")
          end
RuntimeError ('{"base_sha":"52f0b6cc23cd954270350719e4f979c876406dc7","start_sha":null,"head_sha":"9295621e95ea5be588aa9574515195518502f902","old_path":"path/to/file.txt","new_path":"path/to/file.txt","position_type":"text","old_line":162,"new_line":null}'
'{"base_sha":"52f0b6cc23cd954270350719e4f979c876406dc7","start_sha":null,"head_sha":"9295621e95ea5be588aa9574515195518502f902","old_path":"path/to/file.txt","new_path":"path/to/file.txt","position_type":"text","old_line":178,"new_line":null}'
'{"base_sha":"52f0b6cc23cd954270350719e4f979c876406dc7","start_sha":null,"head_sha":"9295621e95ea5be588aa9574515195518502f902","old_path":"path/to/file.txt","new_path":"path/to/file.txt","position_type":"text","old_line":105,"new_line":105}'):

There are json strings instead of objects and there is no file_path property. Only old_path and new_path (values are the same).

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:		Debian 7.0
Current User:	git
Using RVM:	no
Ruby Version:	2.5.3p105
Gem Version:	2.7.6
Bundler Version:1.16.6
Rake Version:	12.3.2
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.3
Go Version:	unknown

GitLab information
Version:	11.6.10
Revision:	c6b7579
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://gitlab.example.com
HTTP Clone URL:	https://gitlab.example.com/some-group/some-project.git
SSH Clone URL:	git@gitlab.example.com:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:


GitLab Shell
Version:	8.4.3
Repository storage paths:
  • default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

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 >= 8.4.3 ? ... OK (8.4.3)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK


Access to /var/opt/gitlab/.ssh/authorized_keys: 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: ... LDAP is disabled in config/gitlab.yml


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: ...
6/4 ... yes
2/7 ... yes
18/8 ... yes
6/9 ... yes
33/10 ... yes
18/11 ... yes
25/12 ... yes
8/13 ... yes
8/14 ... yes
44/15 ... yes
44/16 ... yes
44/17 ... yes
44/18 ... yes
44/19 ... yes
18/20 ... yes
26/21 ... yes
18/22 ... yes
18/23 ... yes
58/24 ... yes
58/25 ... yes
58/27 ... yes
58/28 ... yes
58/29 ... yes
65/30 ... yes
65/31 ... yes
36/33 ... yes
36/34 ... yes
66/35 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 42


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished