Add 'MemoryKiller' Sidekiq middleware

When enabled, this middleware allows Sidekiq to detect that its RSS has
exceeded a maximum value, triggering a graceful shutdown. This
middleware should be combined with external process supervision that
will restart Sidekiq after the graceful shutdown, such as Runit.

CHANGELOG

@@ -4,7 +4,7 @@ v 7.6.0
   - Add CRON=1 backup setting for quiet backups
   -
   -
-  -
+  - Add optional Sidekiq MemoryKiller middleware (enabled via SIDEKIQ_MAX_RSS env variable)
   -
   -
   -

config/initializers/4_sidekiq.rb

@@ -15,6 +15,7 @@ Sidekiq.configure_server do |config|
   config.server_middleware do |chain|
     chain.add Gitlab::SidekiqMiddleware::ArgumentsLogger
+    chain.add Gitlab::SidekiqMiddleware::MemoryKiller if ENV['SIDEKIQ_MAX_RSS']
   end
 end

lib/gitlab/sidekiq_middleware/memory_killer.rb

+module Gitlab
+  module SidekiqMiddleware
+    class MemoryKiller
+      # Wait 30 seconds for running jobs to finish during graceful shutdown
+      GRACEFUL_SHUTDOWN_WAIT = 30
+
+      def call(worker, job, queue)
+        yield
+        current_rss = get_rss
+        return unless max_rss > 0 && current_rss > max_rss
+
+        Sidekiq.logger.warn "current RSS #{current_rss} exceeds maximum RSS "\
+         "#{max_rss}"
+        Sidekiq.logger.warn "sending SIGUSR1 to PID #{Process.pid}"
+        Process.kill('SIGUSR1', Process.pid)
+
+        Sidekiq.logger.warn "spawning thread that will send SIGTERM to PID "\
+          "#{Process.pid} in #{graceful_shutdown_wait} seconds"
+        Thread.new do
+          sleep(graceful_shutdown_wait)
+          Process.kill('SIGTERM', Process.pid)
+        end
+      end
+
+      private
+
+      def get_rss
+        output, status = Gitlab::Popen.popen(%W(ps -o rss= -p #{Process.pid}))
+        return 0 unless status.zero?
+
+        output.to_i
+      end
+
+      def max_rss
+        @max_rss ||= ENV['SIDEKIQ_MAX_RSS'].to_s.to_i
+      end
+
+      def graceful_shutdown_wait
+        @graceful_shutdown_wait ||= (
+          ENV['SIDEKIQ_GRACEFUL_SHUTDOWN_WAIT'] || GRACEFUL_SHUTDOWN_WAIT
+        ).to_i
+      end
+    end
+  end
+end