Mongo::Cluster::Mode::Standalone.servers erroneously returns [nil] if the configuration is empty, for example, before cluster.servers.scan! or when the server is down and cluster.servers.scan! is run.

The current code is:

module Mongo
  class Cluster
    module Mode
      class Standalone
        def self.servers(servers, name = nil)
          [ servers.detect{ |server| server.standalone? } ]
        end
      end
    end
  end
end

Enumerable#detect returns nil if there is no match, which causes other complicated errors up the call chain, for example in Mongo::ServerPreference::Selectable#primary. It smells like a definite bug since Enumerable#select would be better, although other issues still exist up the chain.

The following is a possible improvement, with a check for now to catch other errors that have occurred, for example with a standalone client connecting to a single member of a replica set.

module Mongo
  class Cluster
    module Mode
      class Standalone
        def self.servers(servers, name = nil)
          raise "#{self.name}.#{__method__}: only one server expected, servers: #{servers.inspect}" if servers.size != 1
          servers
        end
      end
    end
  end

An alternative would be to somehow make Mongo::Server::Description#standalone mode aware.