Fixes #196172: manual IID should render conflict

When creating an issue via the API with a manual IID which is already
in use, the server would send a server error response instead of a
conflict response.

changelogs/unreleased/196172-issue-create-iid-conflict.yml

+---
+title: 'Fix Issue API: creating with manual IID returns conflict when IID already
+  in use'
+merge_request: 22788
+author: Mara Sophie Grosch
+type: fixed

lib/api/issues.rb

@@ -220,6 +220,7 @@ module API
 
         issue_params = convert_parameters_from_legacy_format(issue_params)
 
+        begin
           issue = ::Issues::CreateService.new(user_project,
                                               current_user,
                                               issue_params.merge(request: request, api: true)).execute
@@ -233,6 +234,9 @@ module API
           else
             render_validation_error!(issue)
           end
+        rescue ::ActiveRecord::RecordNotUnique
+          render_api_error!('Duplicated issue', 409)
+        end
       end
 
       desc 'Update an existing issue' do

spec/requests/api/issues/post_projects_issues_spec.rb

@@ -160,6 +160,16 @@ describe API::Issues do
           expect(json_response['iid']).not_to eq 9001
         end
       end
+
+      context 'when an issue with the same IID exists on database' do
+        it 'returns 409' do
+          post api("/projects/#{project.id}/issues", admin),
+            params: { title: 'new issue', iid: issue.iid }
+
+          expect(response).to have_gitlab_http_status(409)
+          expect(json_response['message']).to eq 'Duplicated issue'
+        end
+      end
     end
 
     it 'creates a new project issue' do