Projects API returns 500 error due to orphaned namespaces

Summary

The /api/v4/projects endpoint can potentially return a 500 error when encountering an orphaned namespace:

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

This appears to be a regression due to a change we made in !17749 (merged)

Encountered (Zendesk, internal use only) by a 7,100-seat premium customer.

Steps to reproduce

(How one can reproduce the issue - this is very important)

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behavior, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug is fixed in a more recent version)

What is the current bug behavior?

(What actually happens)

What is the expected correct behavior?

(What you should see instead)

Relevant logs and/or screenshots

NoMethodError (undefined method `first_auto_devops_config' for nil:NilClass):
  /opt/gitlab/embedded/service/gitlab-rails/app/models/namespace.rb:304:in `block in first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/utils/strong_memoize.rb:30:in `strong_memoize'
  /opt/gitlab/embedded/service/gitlab-rails/app/models/namespace.rb:302:in `first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:161:in `block in first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/method_call.rb:36:in `measure'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:161:in `first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/app/models/project.rb:704:in `first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:161:in `block in first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/method_call.rb:36:in `measure'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:161:in `first_auto_devops_config'
  /opt/gitlab/embedded/service/gitlab-rails/app/models/project.rb:692:in `has_auto_devops_implicitly_enabled?'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:161:in `block in has_auto_devops_implicitly_enabled?'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/method_call.rb:36:in `measure'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:161:in `has_auto_devops_implicitly_enabled?'
  /opt/gitlab/embedded/service/gitlab-rails/app/models/project.rb:685:in `auto_devops_enabled?'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/delegator/plain_object.rb:8:in `delegate'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/entity.rb:512:in `delegate_attribute'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/delegator_exposure.rb:8:in `value'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/base.rb:82:in `valid_value'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/base.rb:66:in `serializable_value'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:41:in `block in serializable_value'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:120:in `block (2 levels) in map_entity_exposures'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/base.rb:116:in `block in with_attr_path'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/options.rb:90:in `with_attr_path'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/base.rb:115:in `with_attr_path'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:119:in `block in map_entity_exposures'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:118:in `each'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:118:in `each_with_object'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:118:in `map_entity_exposures'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/exposure/nesting_exposure.rb:40:in `serializable_value'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-entity-0.7.1/lib/grape_entity/entity.rb:489:in `serializable_hash'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/core_ext/object/json.rb:152:in `block in as_json'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/core_ext/object/json.rb:152:in `map'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/core_ext/object/json.rb:152:in `as_json'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/json/encoding.rb:35:in `encode'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/json/encoding.rb:22:in `encode'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/core_ext/object/json.rb:41:in `to_json'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/formatter/json.rb:6:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/formatter.rb:45:in `block (2 levels) in build_formatted_response'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/formatter.rb:45:in `collect'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/formatter.rb:45:in `block in build_formatted_response'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/notifications.rb:170:in `instrument'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/formatter.rb:44:in `build_formatted_response'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/formatter.rb:28:in `after'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/base.rb:34:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/base.rb:24:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/base.rb:31:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/base.rb:24:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/ee/lib/gitlab/middleware/ip_restrictor.rb:14:in `block in call'
  /opt/gitlab/embedded/service/gitlab-rails/ee/lib/gitlab/ip_address_state.rb:10:in `with'
  /opt/gitlab/embedded/service/gitlab-rails/ee/lib/gitlab/middleware/ip_restrictor.rb:13:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/api/api_guard.rb:164:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-oauth2-1.9.3/lib/rack/oauth2/server/resource.rb:20:in `_call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-oauth2-1.9.3/lib/rack/oauth2/server/resource/bearer.rb:8:in `_call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-oauth2-1.9.3/lib/rack/oauth2/server/abstract/handler.rb:17:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/error.rb:38:in `block in call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/error.rb:37:in `catch'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/error.rb:37:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/base.rb:24:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape_logging-1.7.0/lib/grape_logging/middleware/request_logger.rb:60:in `block in call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape_logging-1.7.0/lib/grape_logging/middleware/request_logger.rb:58:in `catch'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape_logging-1.7.0/lib/grape_logging/middleware/request_logger.rb:58:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/middleware/base.rb:24:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/head.rb:12:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/endpoint.rb:227:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/endpoint.rb:221:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router/route.rb:72:in `exec'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:121:in `process_route'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:74:in `block in identity'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:93:in `transaction'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:72:in `identity'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:57:in `block in call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:137:in `with_optimization'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/router.rb:56:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/api.rb:119:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/api.rb:45:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/grape-1.1.0/lib/grape/api.rb:40:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/routing/mapper.rb:19:in `block in <class:Constraints>'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/routing/mapper.rb:48:in `serve'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/journey/router.rb:52:in `block in serve'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/journey/router.rb:35:in `each'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/journey/router.rb:35:in `serve'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/routing/route_set.rb:840:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:192:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:169:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:192:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:169:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:192:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:169:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:192:in `call!'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/omniauth-1.9.0/lib/omniauth/strategy.rb:169:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/rails_queue_duration.rb:27:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/transaction.rb:62:in `run'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/rack_middleware.rb:17:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/request_profiler/middleware.rb:17:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/ee/lib/gitlab/jira/middleware.rb:19:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/go.rb:20:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/etag_caching/middleware.rb:13:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/correlation_id.rb:16:in `block in call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/gitlab-labkit-0.5.2/lib/labkit/correlation/correlation_id.rb:18:in `use_id'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/correlation_id.rb:15:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/batch-loader-1.4.0/lib/batch_loader/middleware.rb:11:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/apollo_upload_server-2.0.0.beta.3/lib/apollo_upload_server/middleware.rb:20:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/multipart.rb:117:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-attack-4.4.1/lib/rack/attack.rb:107:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/warden-1.2.8/lib/warden/manager.rb:36:in `block in call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/warden-1.2.8/lib/warden/manager.rb:34:in `catch'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/warden-1.2.8/lib/warden/manager.rb:34:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-cors-1.0.2/lib/rack/cors.rb:97:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/tempfile_reaper.rb:15:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/etag.rb:25:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/conditional_get.rb:25:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/head.rb:12:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/http/content_security_policy.rb:18:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/read_only/controller.rb:48:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/read_only.rb:18:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/session/abstract/id.rb:232:in `context'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/session/abstract/id.rb:226:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/cookies.rb:670:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/callbacks.rb:98:in `run_callbacks'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/callbacks.rb:26:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/debug_exceptions.rb:61:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/basic_health_check.rb:25:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/railties-5.2.3/lib/rails/rack/logger.rb:38:in `call_app'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/railties-5.2.3/lib/rails/rack/logger.rb:26:in `block in call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/tagged_logging.rb:71:in `block in tagged'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/tagged_logging.rb:28:in `tagged'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/activesupport-5.2.3/lib/active_support/tagged_logging.rb:71:in `tagged'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/railties-5.2.3/lib/rails/rack/logger.rb:26:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/request_context.rb:32:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/request_store-1.3.1/lib/request_store/middleware.rb:9:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/request_id.rb:27:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/method_override.rb:22:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/runtime.rb:22:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/actionpack-5.2.3/lib/action_dispatch/middleware/executor.rb:14:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/sendfile.rb:111:in `call'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/requests_rack_middleware.rb:49:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/sentry-raven-2.9.0/lib/raven/integrations/rack.rb:51:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/railties-5.2.3/lib/rails/engine.rb:524:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/railties-5.2.3/lib/rails/railtie.rb:190:in `public_send'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/railties-5.2.3/lib/rails/railtie.rb:190:in `method_missing'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/middleware/release_env.rb:12:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/urlmap.rb:68:in `block in call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/urlmap.rb:53:in `each'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/rack-2.0.7/lib/rack/urlmap.rb:53:in `call'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:606:in `process_client'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-worker-killer-0.4.4/lib/unicorn/worker_killer.rb:52:in `process_client'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:701:in `worker_loop'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:549:in `spawn_missing_workers'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:563:in `maintain_worker_count'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:293:in `join'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/bin/unicorn:126:in `<top (required)>'
  /opt/gitlab/embedded/bin/unicorn:23:in `load'
  /opt/gitlab/embedded/bin/unicorn:23:in `<top (required)>'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/cli/exec.rb:74:in `load'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/cli/exec.rb:74:in `kernel_load'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/cli/exec.rb:28:in `run'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/cli.rb:463:in `exec'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/vendor/thor/lib/thor/invocation.rb:126:in `invoke_command'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/vendor/thor/lib/thor.rb:387:in `dispatch'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/cli.rb:27:in `dispatch'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/vendor/thor/lib/thor/base.rb:466:in `start'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/cli.rb:18:in `start'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/exe/bundle:30:in `block in <top (required)>'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/lib/bundler/friendly_errors.rb:124:in `with_friendly_errors'
  /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/bundler-1.17.3/exe/bundle:22:in `<top (required)>'
  /opt/gitlab/embedded/bin/bundle:23:in `load'
  /opt/gitlab/embedded/bin/bundle:23:in `<main>'

Output of checks

This happens on v12.4.1-ee.

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

(If you can, link to the line of code that might be responsible for the problem)