Use SIGKILL by default in Sidekiq::MemoryKiller

This makes the memory growth-triggered Sidekiq restarts more reliable by
reducing the chance that Sidekiq ends up in a state where it is not
accepting new jobs but also not shutting down: SIGKILL is more likely to
work than SIGTERM.

CHANGELOG

@@ -57,6 +57,7 @@ v 7.11.0 (unreleased)
   - Show user roles by comments.
   - Fix automatic blocking of auto-created users from Active Directory.
   - Call merge request web hook for each new commits (Arthur Gautier)
+  - Use SIGKILL by default in Sidekiq::MemoryKiller
 
 v 7.10.2
   - Fix CI links on MR page

lib/gitlab/sidekiq_middleware/memory_killer.rb

@@ -7,7 +7,7 @@ module Gitlab
       GRACE_TIME = (ENV['SIDEKIQ_MEMORY_KILLER_GRACE_TIME'] || 15 * 60).to_s.to_i
       # Wait 30 seconds for running jobs to finish during graceful shutdown
       SHUTDOWN_WAIT = (ENV['SIDEKIQ_MEMORY_KILLER_SHUTDOWN_WAIT'] || 30).to_s.to_i
-      SHUTDOWN_SIGNAL = (ENV['SIDEKIQ_MEMORY_KILLER_SHUTDOWN_SIGNAL'] || 'SIGTERM').to_s
+      SHUTDOWN_SIGNAL = (ENV['SIDEKIQ_MEMORY_KILLER_SHUTDOWN_SIGNAL'] || 'SIGKILL').to_s
 
       # Create a mutex used to ensure there will be only one thread waiting to
       # shut Sidekiq down