How to write several tests for a TCPSocket in Minitest - ruby

Question

I try to test a networking application. BUT I might have a knot in my brain.
As far as I know the tests in minitest run parallel. On this assumption I think it is clear that when I allocate a Port in setup() it fails when several tests are run:
RuntimeError: Address already in use - bind(2) for nil port 2000 TCPServer new failed
So what is the best practise to do several tests on a server which listens on a port?
class ServerTest < Minitest::Test
def setup
# -- set env variable
ENV["conf"] = "./tc_gpio.config"
Thread::abort_on_exception = true
#my_thr = Thread.start() do
#server = Server.new
#server.start
assert #server.hi() != nil
end
end
def teardown
Thread.kill(#my_thr) # sends exit() to thr
end
def test_connect_and_disconnect
sleep 1
hostname = 'localhost'
port = 2000
s = TCPSocket.open(hostname, port)
my_s = s.recvmsg()
s.sendmsg(:set.to_s, 0) # Failes since a serialized object is expected
my_s2 = s.recvmsg()
assert_equal( "READY" , my_s[0] )
assert_equal( "eeFIN" , my_s2[0])
end
def test_send_command
# fill command
com = Command.new
com.type = :set
com.device_name = 'pump0'
com.device_address = 'resource0'
com.value = 2
serial_com = YAML::dump(com)
sleep 1
hostname = 'localhost'
port = 2000
s = TCPSocket.open(hostname, port)
my_s = s.recvmsg()
s.sendmsg(serial_com, 0)
my_s2 = s.recvmsg()
assert_equal( "READY" , my_s[0] )
assert_equal( "FIN" , my_s2[0])
end
end

Answer 1

When testing a TCP server in parallel, each instance of the server should be started with a distinct port. That can be done by specifying port number 0 when creating a socket. When port number 0 is given, the socket will be bound to a random unused port:
interface = "0.0.0.0"
port = 0
tcp_server = TCPServer.new(interface, port)
You can find out which port the TCP server socket was bound to:
bound_port = #server_socket.addr[1]
One way to use these facts is to have a server something like this:
class Server
# Create a server instance. If the port is unspecified,
# or 0, then a random ephemeral port is bound to.
def initialize(interface: "127.0.0.1", port: 0)
#server_socket = TCPServer.new(interface, port)
...
end
# Return the port the server socket is bound to.
def bound_port
#server_socket.addr[1]
end
...
end
The test then creates server instances using port 0:
server = Server.new(port: 0)
When making a connection to the server, the test uses the #bound_port accessor to find out what port to connect to:
client = TCPSocket.open("localhost", server.bound_port)
and then carries on normally.