ismaster failure causes removal of a server from the cluster, and in handling the remove, the server is removed from the original seed addresses. If the cluster server array becomes empty, it does not ever add a server back after recovery, at least in the test environment.

The original seed list of address should at least be preserved. This can be done via the following.

module Mongo
  class Cluster
    def remove(address)
      removed_servers = @servers.reject!{ |server| server.address.seed == address }
      removed_servers.each{ |server| server.disconnect! } if removed_servers
    end
  end
end

The line removed from the #remove method is:

      addresses.reject!{ |addr| addr == address }