500 error when posting a note to an MR diff

Summary

A user encountered a 500 error when posting a new comment to a diff for an existing MR. After this error, attempting to view the MR results in a 500 error.

Relevant logs and/or screenshots

gitlab-rails/production.log: (user-specific information has been replaced, but message text is intact).

Started POST "/groupname/projectname/notes?html=true" for 103.23.18.44 at 2018-07-16 11:58:43 +1200
Processing by Projects::NotesController#create as JSON
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"[FILTERED]", "view"=>"parallel", "line_type"=>"new", "merge_request_diff_head_sha"=>"aa5e7b3811753dbeea64b49dd3ad8379cb10dcf1", "in_reply_to_discussion_id"=>"
", "note_project_id"=>"", "target_type"=>"merge_request", "target_id"=>"2717", "note"=>{"noteable_type"=>"MergeRequest", "noteable_id"=>"2717", "commit_id"=>"d40bfe0e9c86dea81cb717e076a016062bd1d61b", "type"=>
"DiffNote", "line_code"=>"3071fa8fbb802efe5a64bc87759514a6bbe5c5b9_160_158", "position"=>"{\"base_sha\":\"e73a815d1c800046e7c8a5f8a71155d2471f1495\",\"start_sha\":\"e73a815d1c800046e7c8a5f8a71155d2471f1495\",\
"head_sha\":\"d40bfe0e9c86dea81cb717e076a016062bd1d61b\",\"old_path\":\"path/to/source\",\"new_path\":\"path/to/source\",\"position_type\":\"text\",\"old_line\":160,\"new_line\":158}", "note"=>"so, where is `simmr-ns` coming from now?"}, "html"=>"true", "namespace_id"=>"groupname", "project_id"=>"projectname"}
Completed 500 Internal Server Error in 67ms (ActiveRecord: 6.3ms)

NoMethodError (undefined method `index' for nil:NilClass):
  lib/gitlab/diff/file.rb:81:in `diff_hunk'
  app/models/diff_note.rb:65:in `create_diff_file'
  config/initializers/forbid_sidekiq_in_transactions.rb:49:in `block in committed!'
  config/initializers/forbid_sidekiq_in_transactions.rb:11:in `skipping_transaction_check'
  config/initializers/forbid_sidekiq_in_transactions.rb:49:in `committed!'
  app/services/notes/create_service.rb:34:in `execute'
  app/controllers/concerns/notes_actions.rb:41:in `create'
  lib/gitlab/i18n.rb:50:in `with_locale'
  lib/gitlab/i18n.rb:56:in `with_user_locale'
  app/controllers/application_controller.rb:362:in `set_locale'
  lib/gitlab/middleware/multipart.rb:95:in `call'
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
  lib/gitlab/middleware/go.rb:17:in `call'
  lib/gitlab/etag_caching/middleware.rb:11:in `call'
  lib/gitlab/middleware/read_only/controller.rb:28:in `call'
  lib/gitlab/middleware/read_only.rb:16:in `call'
  lib/gitlab/request_context.rb:18:in `call'
  lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'
  lib/gitlab/middleware/release_env.rb:10:in `call'

After encountering this error, GET requests for the MR fail too:

Started GET "/groupname/projectname/merge_requests/1273/diffs?commit_id=d40bfe0e9c86dea81cb717e076a016062bd1d61b&w=1" for 103.23.18.44 at 2018-07-16 11:58:48 +1200
Processing by Projects::MergeRequestsController#show as HTML
  Parameters: {"commit_id"=>"d40bfe0e9c86dea81cb717e076a016062bd1d61b", "w"=>"1", "tab"=>"diffs", "namespace_id"=>"groupname", "project_id"=>"projectname", "id"=>"1273"}
Completed 500 Internal Server Error in 844ms (ActiveRecord: 39.2ms)

ActionView::Template::Error (undefined method `meta?' for nil:NilClass):
    1: - diff_file = discussion.diff_file
    2: - blob = discussion.blob
    3: - discussions = { discussion.original_line_code => [discussion] }
    4: - diff_file_class = diff_file.text? ? 'text-file' : 'js-image-file'
    5: - diff_data = {}
    6: - expanded = discussion.expanded? || local_assigns.fetch(:expanded, nil)
  lib/gitlab/diff/file.rb:54:in `line_code'
  app/models/diff_note.rb:85:in `original_line_code'
  app/models/concerns/discussion_on_diff.rb:8:in `original_line_code'
  app/views/discussions/_diff_with_notes.html.haml:3:in `_app_views_discussions__diff_with_notes_html_haml___1934583260500465353_70166110670440'
  app/views/discussions/_discussion.html.haml:52:in `_app_views_discussions__discussion_html_haml___3300946306980297002_70166168804540'
  app/views/shared/notes/_notes.html.haml:6:in `block in _app_views_shared_notes__notes_html_haml__4216686508215865599_70166119321020'
  app/views/shared/notes/_notes.html.haml:2:in `each'
  app/views/shared/notes/_notes.html.haml:2:in `_app_views_shared_notes__notes_html_haml__4216686508215865599_70166119321020'
  app/views/shared/notes/_notes_with_form.html.haml:6:in `_app_views_shared_notes__notes_with_form_html_haml__3828359774013864509_70166118991480'
  app/views/projects/merge_requests/_discussion.html.haml:11:in `_app_views_projects_merge_requests__discussion_html_haml__809004878834630127_70166118771760'
  app/views/projects/merge_requests/show.html.haml:80:in `_app_views_projects_merge_requests_show_html_haml___3096220450810035097_70166161581240'
  app/controllers/projects/merge_requests_controller.rb:52:in `block (2 levels) in show'
  app/controllers/projects/merge_requests_controller.rb:34:in `show'
  lib/gitlab/i18n.rb:50:in `with_locale'
  lib/gitlab/i18n.rb:56:in `with_user_locale'
  app/controllers/application_controller.rb:362:in `set_locale'
  lib/gitlab/middleware/multipart.rb:95:in `call'
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
  lib/gitlab/middleware/go.rb:17:in `call'
  lib/gitlab/etag_caching/middleware.rb:11:in `call'
  lib/gitlab/middleware/read_only/controller.rb:28:in `call'
  lib/gitlab/middleware/read_only.rb:16:in `call'
  lib/gitlab/request_context.rb:18:in `call'
  lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'
  lib/gitlab/middleware/release_env.rb:10:in `call'

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:		Ubuntu 16.04
Current User:	git
Using RVM:	no
Ruby Version:	2.4.4p296
Gem Version:	2.7.6
Bundler Version:1.16.2
Rake Version:	12.3.1
Redis Version:	3.2.11
Git Version:	2.17.1
Sidekiq Version:5.1.3
Go Version:	unknown

GitLab information
Version:	11.0.3
Revision:	aa62075
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://git.montoux.com
HTTP Clone URL:	https://git.montoux.com/some-group/some-project.git
SSH Clone URL:	ssh://git@git.montoux.com:34/some-group/some-project.git
Using LDAP:	no
Using Omniauth:	no


GitLab Shell
Version:	7.1.4
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 Shell ...

GitLab Shell version >= 7.1.4 ? ... OK (7.1.4)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:root, or git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
2/3 ... ok
2/4 ... ok
2/5 ... ok
2/6 ... ok
6/9 ... ok
9/10 ... ok
11/11 ... ok
6/12 ... ok
13/13 ... ok
13/14 ... ok
16/16 ... ok
13/17 ... ok
13/18 ... ok
13/19 ... ok
20/21 ... ok
20/22 ... ok
20/23 ... ok
20/24 ... ok
13/35 ... ok
20/36 ... ok
20/37 ... ok
13/38 ... ok
13/39 ... ok
13/40 ... ok
2/41 ... ok
13/42 ... ok
13/43 ... ok
13/44 ... ok
13/45 ... ok
13/47 ... ok
13/48 ... ok
13/50 ... ok
13/51 ... ok
13/52 ... ok
21/53 ... ok
20/54 ... ok
20/55 ... ok
20/56 ... ok
20/57 ... ok
20/58 ... ok
13/59 ... ok
21/60 ... ok
2/61 ... ok
13/62 ... ok
13/64 ... ok
13/65 ... ok
13/67 ... ok
13/68 ... ok
13/69 ... ok
2/70 ... ok
13/71 ... ok
28/73 ... ok
28/74 ... ok
13/75 ... ok
29/76 ... ok
28/77 ... ok
13/78 ... ok
32/79 ... ok
20/80 ... ok
32/81 ... ok
32/82 ... ok
32/83 ... ok
32/84 ... ok
13/87 ... ok
13/88 ... ok
20/89 ... ok
25/90 ... repository is empty
20/91 ... ok
13/92 ... ok
13/93 ... ok
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 Sidekiq ...


Running? ... yes
Number of Sidekiq processes ... 1


Checking Sidekiq ... Finished


Reply by email is disabled in config/gitlab.yml
Checking LDAP ...


LDAP is disabled in config/gitlab.yml


Checking LDAP ... Finished


Checking GitLab ...


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: ...
2/3 ... yes
2/4 ... yes
2/5 ... yes
2/6 ... yes
6/9 ... yes
9/10 ... yes
11/11 ... yes
6/12 ... yes
13/13 ... yes
13/14 ... yes
16/16 ... yes
13/17 ... yes
13/18 ... yes
13/19 ... yes
20/21 ... yes
20/22 ... yes
20/23 ... yes
20/24 ... yes
13/35 ... yes
20/36 ... yes
20/37 ... yes
13/38 ... yes
13/39 ... yes
13/40 ... yes
2/41 ... yes
13/42 ... yes
13/43 ... yes
13/44 ... yes
13/45 ... yes
13/47 ... yes
13/48 ... yes
13/50 ... yes
13/51 ... yes
13/52 ... yes
21/53 ... yes
20/54 ... yes
20/55 ... yes
20/56 ... yes
20/57 ... yes
20/58 ... yes
13/59 ... yes
21/60 ... yes
2/61 ... yes
13/62 ... yes
13/64 ... yes
13/65 ... yes
13/67 ... yes
13/68 ... yes
13/69 ... yes
2/70 ... yes
13/71 ... yes
28/73 ... yes
28/74 ... yes
13/75 ... yes
29/76 ... yes
28/77 ... yes
13/78 ... yes
32/79 ... yes
20/80 ... yes
32/81 ... yes
32/82 ... yes
32/83 ... yes
32/84 ... yes
13/87 ... yes
13/88 ... yes
20/89 ... yes
25/90 ... yes
20/91 ... yes
13/92 ... yes
13/93 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.4.4)
Git version >= 2.9.5 ? ... yes (2.17.1)
Git user has default SSH configuration? ... yes
Active users: ... 15


Checking GitLab ... Finished

Possible fixes

Possibly relevant issues: