Properly handle multiple X-Forwarded-For addresses in runner IP

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24624 extracted
the X-Forwarded-For address directly, but this didn't consider the case
where multiple proxies are in the chain. To fix this, we use the Rails
implementation to filter trusted proxies, as documented by Grape:
https://github.com/ruby-grape/grape#remote-ip

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58103

changelogs/unreleased/sh-fix-issue-58103.yml

+---
+title: Properly handle multiple X-Forwarded-For addresses in runner IP
+merge_request: 25511
+author:
+type: fixed

lib/api/helpers/runner.rb

@@ -26,7 +26,7 @@ module API
       end
       def get_runner_ip
-        { ip_address: request.env["HTTP_X_FORWARDED_FOR"] || request.ip }
+        { ip_address: env["action_dispatch.remote_ip"].to_s || request.ip }
       end
       def current_runner

spec/requests/api/runner_spec.rb

@@ -526,6 +526,15 @@ describe API::Runner, :clean_gitlab_redis_shared_state do
             expect(runner.reload.ip_address).to eq('123.222.123.222')
           end
+          it "handles multiple X-Forwarded-For addresses" do
+            post api('/jobs/request'),
+              params: { token: runner.token },
+              headers: { 'User-Agent' => user_agent, 'X-Forwarded-For' => '123.222.123.222, 127.0.0.1' }
+
+            expect(response).to have_gitlab_http_status 201
+            expect(runner.reload.ip_address).to eq('123.222.123.222')
+          end
+
           context 'when concurrently updating a job' do
             before do
               expect_any_instance_of(Ci::Build).to receive(:run!)