Unicorn reinicia com o mesmo PID na USR2

Question

Depois de migrar recentemente o nosso aplicativo Rails para a MRI 2.2.0 (que pode ou não estar relacionada), estamos com problemas com nossas reinicializações de tempo zero de inatividade.

Quando envio o sinal USR2 para o processo mestre (PID 19216 neste exemplo), obtenho o seguinte no log do Unicorn:

I, [2015-03-11T23:47:33.992274 #6848]  INFO -- : executing ["/srv/ourapp/shared/bundle/ruby/2.2.0/bin/unicorn", "/srv/ourapp/current/config.ru", "-Dc", "/srv/ourapp/shared/config/unicorn.rb", {10=>#<Kgio::UNIXServer:/srv/ourapp/shared/sockets/unicorn.sock>}] (in /srv/ourapp/releases/a0e8b5df474ad5129200654f92a76af00a750f47)
I, [2015-03-11T23:47:36.504235 #6848]  INFO -- : inherited addr=/srv/ourapp/shared/sockets/unicorn.sock fd=10
/srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/lib/unicorn/http_server.rb:206:in 'pid=': Already running on PID:19216 (or pid=/srv/ourapp/shared/pids/unicorn.pid is stale) (ArgumentError)
 from /srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/lib/unicorn/http_server.rb:134:in 'start'
 from /srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/bin/unicorn:126:in '<top (required)>'
 from /srv/ourapp/shared/bundle/ruby/2.2.0/bin/unicorn:23:in 'load'
 from /srv/ourapp/shared/bundle/ruby/2.2.0/bin/unicorn:23:in '<main>'
E, [2015-03-11T23:47:36.519549 #19216] ERROR -- : reaped #<Process::Status: pid 6848 exit 1> exec()-ed
E, [2015-03-11T23:47:36.520296 #19216] ERROR -- : master loop error: Already running on PID:19216 (or pid=/srv/ourapp/shared/pids/unicorn.pid is stale) (ArgumentError)
E, [2015-03-11T23:47:36.520496 #19216] ERROR -- : /srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/lib/unicorn/http_server.rb:206:in 'pid='
E, [2015-03-11T23:47:36.520650 #19216] ERROR -- : /srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/lib/unicorn/http_server.rb:404:in 'reap_all_workers'
E, [2015-03-11T23:47:36.520790 #19216] ERROR -- : /srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/lib/unicorn/http_server.rb:279:in 'join'
E, [2015-03-11T23:47:36.520928 #19216] ERROR -- : /srv/ourapp/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.1/bin/unicorn:126:in '<top (required)>'
E, [2015-03-11T23:47:36.521115 #19216] ERROR -- : /srv/ourapp/shared/bundle/ruby/2.2.0/bin/unicorn:23:in 'load'
E, [2015-03-11T23:47:36.521254 #19216] ERROR -- : /srv/ourapp/shared/bundle/ruby/2.2.0/bin/unicorn:23:in '<main>’

E quando eu verifico, de fato, agora há unicorn.pid e unicorn.pid.oldbin, ambos contendo 19216.

O que poderia causar essa situação?

Este é o nosso unicorn.rb FWIW:

# Set your full path to application.
app_path = "/srv/ourapp/current"

# Set unicorn options
worker_processes 3
preload_app true
timeout 30
listen "/srv/ourapp/shared/sockets/unicorn.sock", :backlog => 64

# Spawn unicorn master worker for user deploy (group: deploy)
user 'deploy', 'deploy'

# Fill path to your app
working_directory app_path

# Should be 'production' by default, otherwise use other env
rails_env = ENV['RAILS_ENV'] || 'production'

# Log everything to one file
stderr_path "/srv/ourapp/shared/log/unicorn.log"
stdout_path "/srv/ourapp/shared/log/unicorn.log"

# Set master PID location
pid "/srv/ourapp/shared/pids/unicorn.pid"

before_exec do |server|
 ENV["BUNDLE_GEMFILE"] = "#{app_path}/Gemfile"
end

before_fork do |server, worker|
 ActiveRecord::Base.connection.disconnect!

 sleep 10

 old_pid = "#{server.config[:pid]}.oldbin"
 if File.exists?(old_pid) && server.pid != old_pid
   begin
     Process.kill("QUIT", File.read(old_pid).to_i)
   rescue Errno::ENOENT, Errno::ESRCH
     # someone else did our job for us
   end
 end
end

after_fork do |server, worker|
 ActiveRecord::Base.establish_connection

 Sidekiq.configure_client do |config|
   config.redis = { namespace: 'sidekiq' }
 end
end