Fix the last-ditch memory killer pgroup SIGKILL

changelogs/unreleased/more-pgroup-fix.yml

+---
+title: Fix the last-ditch memory killer pgroup SIGKILL
+merge_request: 25940
+author:
+type: fixed

lib/gitlab/sidekiq_middleware/memory_killer.rb

@@ -63,7 +63,7 @@ module Gitlab
         sleep(time)
 
         Sidekiq.logger.warn "sending Sidekiq worker PGRP-#{pid} #{signal} (#{explanation})"
-        Process.kill(signal, "-#{pid}")
+        Process.kill(signal, 0)
       end
 
       def wait_and_signal(time, signal, explanation)

spec/lib/gitlab/sidekiq_middleware/memory_killer_spec.rb

@@ -54,7 +54,7 @@ describe Gitlab::SidekiqMiddleware::MemoryKiller do
 
       expect(Process).to receive(:kill).with('SIGTSTP', pid).ordered
       expect(Process).to receive(:kill).with('SIGTERM', pid).ordered
-      expect(Process).to receive(:kill).with('SIGKILL', "-#{pid}").ordered
+      expect(Process).to receive(:kill).with('SIGKILL', 0).ordered
 
       run
     end