无缝重启unicorn时，不丢失长连接

USR2重启unicorn时没管redis就会碰到：

kill -USR2 `cat /tmp/ting.pid`

Redis::InheritedError (Tried to use a connection from a child process without reconnecting. You need to reconnect to Redis after forking.)

在有请求过来redis感觉到连接失效时，会聪明的抛这个错同时重连。

http://unicorn.bogomips.org/SIGNALS.html

编辑unicorn配置文件，在after_fork里重连redis，如果用了ConnectionPool就用#checkout取得当前连接重连。数据库连接也同样。

http://unicorn.bogomips.org/examples/unicorn.conf.rb

preload_app true
GC.respond_to?(:copy_on_write_friendly=) and
  GC.copy_on_write_friendly = true

before_fork do |server, worker|
  # the following is highly recomended for Rails + "preload_app true"
  # as there's no need for the master process to hold a connection
  defined?(ActiveRecord::Base) and
    ActiveRecord::Base.connection.disconnect!

  # The following is only recommended for memory/DB-constrained
  # installations.  It is not needed if your system can house
  # twice as many worker_processes as you have configured.
  
  # This allows a new master process to incrementally
  # phase out the old master process with SIGTTOU to avoid a
  # thundering herd (especially in the "preload_app false" case)
  # when doing a transparent upgrade.  The last worker spawned
  # will then kill off the old master process with a SIGQUIT.
  old_pid = "#{server.config[:pid]}.oldbin"
  if old_pid != server.pid
    begin
      sig = (worker.nr + 1) >= server.worker_processes ? :QUIT : :TTOU
      Process.kill(sig, File.read(old_pid).to_i)
    rescue Errno::ENOENT, Errno::ESRCH
    end
  end
  
  # Throttle the master from forking too quickly by sleeping.  Due
  # to the implementation of standard Unix signal handlers, this
  # helps (but does not completely) prevent identical, repeated signals
  # from being lost when the receiving process is busy.
  sleep 1
end

after_fork do |server, worker|
  # the following is *required* for Rails + "preload_app true",
  defined?(ActiveRecord::Base) and
    ActiveRecord::Base.establish_connection

  # if preload_app is true, then you may also want to check and
  # restart any other shared sockets/descriptors such as Memcached,
  # and Redis.  TokyoCabinet file handles are safe to reuse
  # between any number of forked children (assuming your kernel
  # correctly implements pread()/pwrite() system calls)
  defined?(CREDIS) and
    CREDIS.checkout.client.reconnect

  defined?(CMSREDIS) and
    CMSREDIS.client.reconnect
end