class Rack::Session::Pool

def delete_session(req, session_id, options)
  @mutex.synchronize do
    @pool.delete(session_id.public_id)
    @pool.delete(session_id.private_id)
    generate_sid(use_mutex: false) unless options[:drop]
  end
end

def find_session(req, sid)
  @mutex.synchronize do
    unless sid and session = get_session_with_fallback(sid)
      sid, session = generate_sid(use_mutex: false), {}
      @pool.store sid.private_id, session
    end
    [sid, session]
  end
end

def generate_sid(*args, use_mutex: true)
  loop do
    sid = super(*args)
    break sid unless use_mutex ? @mutex.synchronize { @pool.key? sid.private_id } : @pool.key?(sid.private_id)
  end
end

def get_session_with_fallback(sid)
  @pool[sid.private_id] || (@pool[sid.public_id] if @allow_fallback)
end

def initialize(app, options = {})
  super
  @pool = Hash.new
  @mutex = Mutex.new
  @allow_fallback = @default_options.delete(:allow_fallback)
end

def write_session(req, session_id, new_session, options)
  @mutex.synchronize do
    @pool.store session_id.private_id, new_session
    session_id
  end
end