Use a path for the related merge requests endpoint

Hosts using a non-standard configuration may have a different
hostname/port/scheme than what may be configured on the GitLab
server. While expose_url should generate a proper URL, there are cases
where it may not work. Since we don't need the full URL, we can use the
relative path.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61280

app/views/projects/issues/show.html.haml

@@ -79,7 +79,7 @@
 
     = render_if_exists 'projects/issues/related_issues'
 
-    #js-related-merge-requests{ data: { endpoint: expose_url(api_v4_projects_issues_related_merge_requests_path(id: @project.id, issue_iid: @issue.iid)), project_namespace: @project.namespace.path, project_path: @project.path } }
+    #js-related-merge-requests{ data: { endpoint: expose_path(api_v4_projects_issues_related_merge_requests_path(id: @project.id, issue_iid: @issue.iid)), project_namespace: @project.namespace.path, project_path: @project.path } }
 
     - if can?(current_user, :download_code, @project)
       #related-branches{ data: { url: related_branches_project_issue_path(@project, @issue) } }

changelogs/unreleased/sh-fix-related-merge-requests-path.yml

+---
+title: Use a path for the related merge requests endpoint
+merge_request: 28171
+author:
+type: fixed

lib/api/helpers/related_resources_helpers.rb

@@ -13,6 +13,10 @@ module API
         available?(:merge_requests, project, options[:current_user])
       end
 
+      def expose_path(path)
+        Gitlab::Utils.append_path(Gitlab.config.gitlab.relative_url_root, path)
+      end
+
       def expose_url(path)
         url_options = Gitlab::Application.routes.default_url_options
         protocol, host, port, script_name = url_options.values_at(:protocol, :host, :port, :script_name)

spec/lib/api/helpers/related_resources_helpers_spec.rb

@@ -5,6 +5,40 @@ describe API::Helpers::RelatedResourcesHelpers do
     Class.new.include(described_class).new
   end
 
+  describe '#expose_path' do
+    let(:path) { '/api/v4/awesome_endpoint' }
+
+    context 'empty relative URL root' do
+      before do
+        stub_config_setting(relative_url_root: '')
+      end
+
+      it 'returns the existing path' do
+        expect(helpers.expose_path(path)).to eq(path)
+      end
+    end
+
+    context 'slash relative URL root' do
+      before do
+        stub_config_setting(relative_url_root: '/')
+      end
+
+      it 'returns the existing path' do
+        expect(helpers.expose_path(path)).to eq(path)
+      end
+    end
+
+    context 'with relative URL root' do
+      before do
+        stub_config_setting(relative_url_root: '/gitlab/root')
+      end
+
+      it 'returns the existing path' do
+        expect(helpers.expose_path(path)).to eq("/gitlab/root" + path)
+      end
+    end
+  end
+
   describe '#expose_url' do
     let(:path) { '/api/v4/awesome_endpoint' }
     subject(:url) { helpers.expose_url(path) }