Add `shared_projects` endpoint

This endpoint lists projects shared with a group visible to the
current user.

The `filter` and `sort` params are supported like on the `children` endpoint.

app/controllers/groups/shared_projects_controller.rb

+module Groups
+  class SharedProjectsController < Groups::ApplicationController
+    respond_to :json
+    before_action :group
+    skip_cross_project_access_check :index
+
+    def index
+      shared_projects = GroupProjectsFinder.new(
+        group: group,
+        current_user: current_user,
+        params: finder_params,
+        options: { only_shared: true }
+      ).execute
+      serializer = GroupChildSerializer.new(current_user: current_user)
+                     .with_pagination(request, response)
+
+      render json: serializer.represent(shared_projects)
+    end
+
+    private
+
+    def finder_params
+      @finder_params ||= begin
+                           # Make the `search` param consitent for the frontend,
+                           # which will be using `filter`.
+                           params[:search] ||= params[:filter] if params[:filter]
+                           params.permit(:sort, :search)
+                         end
+    end
+  end
+end

config/routes/group.rb

@@ -30,6 +30,7 @@ constraints(::Constraints::GroupUrlConstrainer.new) do
     resource :variables, only: [:show, :update]
 
     resources :children, only: [:index]
+    resources :shared_projects, only: [:index]
 
     resources :labels, except: [:show] do
       post :toggle_subscription, on: :member

spec/controllers/groups/shared_projects_controller_spec.rb

+require 'spec_helper'
+
+describe Groups::SharedProjectsController do
+  def get_shared_projects(params = {})
+    get :index, params.reverse_merge(format: :json, group_id: group.full_path)
+  end
+
+  def share_project(project)
+    Projects::GroupLinks::CreateService.new(
+      project,
+      user,
+      link_group_access: ProjectGroupLink::DEVELOPER
+    ).execute(group)
+  end
+
+  let(:group) { create(:group) }
+  let(:user) { create(:user) }
+  let!(:shared_project) do
+    shared_project = create(:project, namespace: user.namespace)
+    share_project(shared_project)
+
+    shared_project
+  end
+
+  let(:json_project_ids) { json_response.map { |project_info| project_info['id'] } }
+
+  before do
+    sign_in(user)
+  end
+
+  describe 'GET #index' do
+    it 'returns only projects shared with the group' do
+      create(:project, namespace: group)
+
+      get_shared_projects
+
+      expect(json_project_ids).to contain_exactly(shared_project.id)
+    end
+
+    it 'allows fitlering shared projects' do
+      project = create(:project, :archived, namespace: user.namespace, name: "Searching for")
+      share_project(project)
+
+      get_shared_projects(filter: 'search')
+
+      expect(json_project_ids).to contain_exactly(project.id)
+    end
+
+    it 'allows sorting projects' do
+      shared_project.update!(name: 'bbb')
+      second_project = create(:project, namespace: user.namespace, name: 'aaaa')
+      share_project(second_project)
+
+      get_shared_projects(sort: 'name_asc')
+
+      expect(json_project_ids).to eq([second_project.id, shared_project.id])
+    end
+  end
+end