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Summary

The problem is two-sided for both creating an environment and stopping the environment when the URL set is not resolvable.

1. When creating an environment with an url set to a hostname that is not resolved, the link to the URL does not appear in the environment page
2. When an environment was successfully created (URL resolvable at the moment of creation) and then trying to stop the environment, a 500 error appears

Steps to reproduce

To reproduce problem 1 above:

1. Create a new project
2. Add a simple .gitlab-ci.yml file:

stages:
  - start
  - stop

start:dev:
  stage: start
  environment:
    name: demonstrates-environment-dns-issue
    url: https://this-does-not-resolve.com
    on_stop: stop:dev
  script:
    - echo starting

stop:dev:
  stage: stop
  environment:
    name: demonstrates-environment-dns-issue
    action: stop
  when: manual
  script:
    - echo stopping

3. Go to environment page => the environment link is not shown

To reproduce problem 2 above:

1. Create a new project
2. Add a .gitlab-ci.yml file as above, but replace https://this-does-not-resolve.com with a DNS that you control.
3. Start the job, create the environment
4. Remove the A record for the URL you've set up in step 2
5. Try to stop the environment => 500, with the logs pasted below

Example Project

For the problem 1 above, see cirimie/repro-environment-urls> environment 936581

I've reproduced problem 2 on the latest (12.2) version locally with the logs pasted below.

What is the current bug behavior?

1. Environment link is not visible when environment URL is not resolvable
2. Started environment cannot be stopped when environment URL was resolvable when environment was created but is not resolvable when stopping

What is the expected correct behavior?

Either, this should be properly documented (as a limitation, or why this happens) or:

1. Environment link should be visible when environment URL was not resolvable
2. Started environment can be stopped even if URL is not resolvable anymore

Relevant logs and/or screenshots

==> /var/log/gitlab/gitlab-rails/production.log <==
Started POST "/root/repro-environment-addressable-urls/environments/1/stop" for 192.168.56.101 at 2019-08-28 18:36:18 +0100
Processing by Projects::EnvironmentsController#stop as HTML
  Parameters: {"authenticity_token"=>"[FILTERED]", "namespace_id"=>"root", "project_id"=>"repro-environment-addressable-urls", "id"=>"1"}
Completed 500 Internal Server Error in 331ms (ActiveRecord: 27.5ms | Elasticsearch: 0.0ms)
  
StateMachines::InvalidTransition (Cannot transition state via :stop from :available (Reason(s): External url is blocked: Host cannot be resolved or invalid)):
  
app/models/environment.rb:148:in `stop_with_action!'
lib/gitlab/metrics/instrumentation.rb:161:in `block in stop_with_action!'
lib/gitlab/metrics/method_call.rb:36:in `measure'
lib/gitlab/metrics/instrumentation.rb:161:in `stop_with_action!'
app/controllers/projects/environments_controller.rb:93:in `stop'
ee/lib/gitlab/ip_address_state.rb:10:in `with'
ee/app/controllers/ee/application_controller.rb:28:in `set_current_ip_address'
lib/gitlab/session.rb:11:in `with_session'
app/controllers/application_controller.rb:445:in `set_session_storage'
lib/gitlab/i18n.rb:55:in `with_locale'
lib/gitlab/i18n.rb:61:in `with_user_locale'
app/controllers/application_controller.rb:439:in `set_locale'
lib/gitlab/middleware/rails_queue_duration.rb:27:in `call'
lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
lib/gitlab/metrics/transaction.rb:57:in `run'
lib/gitlab/metrics/rack_middleware.rb:17:in `call'
lib/gitlab/middleware/multipart.rb:103:in `call'
lib/gitlab/request_profiler/middleware.rb:16:in `call'
ee/lib/gitlab/jira/middleware.rb:17:in `call'
lib/gitlab/middleware/go.rb:20:in `call'
lib/gitlab/etag_caching/middleware.rb:13:in `call'
lib/gitlab/middleware/correlation_id.rb:16:in `block in call'
lib/gitlab/middleware/correlation_id.rb:15:in `call'
lib/gitlab/middleware/read_only/controller.rb:42: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:26:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:29:in `call'
lib/gitlab/middleware/release_env.rb:12:in `call'

Possible fixes

Line 36 in the environment model has an addressable_url check, which runs when the state machine tries to switch states and it will fail if the DNS can't be resolved, producing the 500 above