Resource Group API to list all upcoming jobs

Problem

When oldest_first or newest_first process mode is selected in a resource group, sometimes users need to know the upcoming jobs list to figure out which job is the current blocker. For example, #202186 (comment 690048551).

Users should be able to figure out which job is currently blocking the resource group.

Proposal

We add the following endpoint in Resource Group API:

GET /projects/:id/resource_groups/:key/upcoming_jobs

This returns the list of upcoming jobs to the resource group.

Implementation

It's easy to implement, as the following:

diff --git a/lib/api/ci/resource_groups.rb b/lib/api/ci/resource_groups.rb
index 02a5e92bd83..bb01381f268 100644
--- a/lib/api/ci/resource_groups.rb
+++ b/lib/api/ci/resource_groups.rb
@@ -3,6 +3,8 @@
 module API
   module Ci
     class ResourceGroups < ::API::Base
+      include PaginationParams
+
       before { authenticate! }
 
       feature_category :continuous_delivery
@@ -41,6 +43,23 @@ class ResourceGroups < ::API::Base
             render_validation_error!(resource_group)
           end
         end
+
+        desc 'List upcoming jobs of a resource group' do
+          success Entities::Ci::JobBasic
+        end
+        params do
+          requires :key, type: String, desc: 'The key of the resource group'
+          use :pagination
+        end
+        get ':id/resource_groups/:key/upcoming_jobs' do
+          authorize! :read_resource_group, resource_group
+          authorize! :read_build, user_project
+
+          upcoming_processables = resource_group.upcoming_processables
+          upcoming_processables = upcoming_processables.preload(:user, pipeline: :project)
+
+          present paginate(upcoming_processables), with: Entities::Ci::JobBasic
+        end
       end
 
       helpers do