class Bundler::Persistent::Net::HTTP::Persistent::Pool

def checkin net_http_args
  stack = Thread.current[@key][net_http_args] ||= []
  raise Bundler::ConnectionPool::Error, 'no connections are checked out' if
    stack.empty?
  conn = stack.pop
  if stack.empty?
    @available.push conn, connection_args: net_http_args
    Thread.current[@key].delete(net_http_args)
    Thread.current[@key] = nil if Thread.current[@key].empty?
  end
  nil
end