Fix Error 500 when using closes_issues API with an external issue tracker

Closes #18484

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.9.0 (unreleased)
+  - Fix Error 500 when using closes_issues API with an external issue tracker
   - Bulk assign/unassign labels to issues.
   - Ability to prioritize labels !4009 / !3205 (Thijs Wouters)
   - Fix endless redirections when accessing user OAuth applications when they are disabled

doc/api/merge_requests.md

@@ -572,7 +572,7 @@ GET /projects/:id/merge_requests/:merge_request_id/closes_issues
 curl -H "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v3/projects/76/merge_requests/1/closes_issues
 ```
 
-Example response:
+Example response when the GitLab issue tracker is used:
 
 ```json
 [
@@ -618,6 +618,17 @@ Example response:
 ]
 ```
 
+Example response when an external issue tracker (e.g. JIRA) is used:
+
+```json
+[
+   {
+       "id" : "PROJECT-123",
+       "title" : "Title of this issue"
+   }
+]
+```
+
 ## Subscribe to a merge request
 
 Subscribes the authenticated user to a merge request to receive notification. If

lib/api/entities.rb

@@ -179,6 +179,11 @@ module API
       expose :upvotes, :downvotes
     end
 
+    class ExternalIssue < Grape::Entity
+      expose :title
+      expose :id
+    end
+
     class MergeRequest < ProjectEntity
       expose :target_branch, :source_branch
       expose :upvotes, :downvotes

lib/api/helpers.rb

@@ -418,5 +418,13 @@ module API
     def send_git_archive(repository, ref:, format:)
       header(*Gitlab::Workhorse.send_git_archive(repository, ref: ref, format: format))
     end
+
+    def issue_entity(project)
+      if project.has_external_issue_tracker?
+        Entities::ExternalIssue
+      else
+        Entities::Issue
+      end
+    end
   end
 end

lib/api/merge_requests.rb

@@ -329,7 +329,7 @@ module API
         get "#{path}/closes_issues" do
           merge_request = user_project.merge_requests.find(params[:merge_request_id])
           issues = ::Kaminari.paginate_array(merge_request.closes_issues(current_user))
-          present paginate(issues), with: Entities::Issue, current_user: current_user
+          present paginate(issues), with: issue_entity(user_project), current_user: current_user
         end
       end
     end

spec/requests/api/merge_requests_spec.rb

@@ -563,6 +563,21 @@ describe API::API, api: true  do
       expect(json_response).to be_an Array
       expect(json_response.length).to eq(0)
     end
+
+    it 'handles external issues' do
+      jira_project = create(:jira_project, :public, name: 'JIR_EXT1')
+      issue = ExternalIssue.new("#{jira_project.name}-123", jira_project)
+      merge_request = create(:merge_request, :simple, author: user, assignee: user, source_project: jira_project)
+      merge_request.update_attribute(:description, "Closes #{issue.to_reference(jira_project)}")
+
+      get api("/projects/#{jira_project.id}/merge_requests/#{merge_request.id}/closes_issues", user)
+
+      expect(response.status).to eq(200)
+      expect(json_response).to be_an Array
+      expect(json_response.length).to eq(1)
+      expect(json_response.first['title']).to eq(issue.title)
+      expect(json_response.first['id']).to eq(issue.id)
+    end
   end
 
   describe 'POST :id/merge_requests/:merge_request_id/subscription' do