Fix race conditions in networking test suite

tfw/internals/networking/test_networking.py

@@ -39,9 +39,12 @@ def zmq_connector(_listener_and_connector):
 
 
 def run_ioloop_once():
-    # hack: we have to wait for the messages to get through
+    # Hack: we have to wait for the messages to get through
     # the network stack of the OS while the IOLoop is waiting
-    # for them via select/epoll/kqueue
+    # for them via select/epoll/kqueue.
+    # This is an inherent race condition, but solving this
+    # problem properly would make the test code difficult
+    # to understand, so we use this half measure.
     IOLoop.current().call_later(0.1, IOLoop.current().stop)
     IOLoop.current().start()
 
@@ -67,7 +70,7 @@ def test_messages():
 def wait_until_subscriber_connects(listener, connector):
     # Warning: you are better off without comprehending how this works
     # Reference: ZMQ PUB-SUB slow joiner problem
-
+    connector.subscribe('-', '_')
     # Wait until something can go through the connection
     dummy = {'key': '-'}
     while True:
@@ -82,6 +85,7 @@ def wait_until_subscriber_connects(listener, connector):
         with suppress(IOError):
             if connector.recv_message(block=False) == sentinel:
                 break
+    connector.unsubscribe('-', '_')
 
 
 def test_server_downlink(zmq_listener, zmq_connector, test_messages):
@@ -158,8 +162,10 @@ def test_connector_preserves_intent(zmq_listener, zmq_connector):
 
 
 def test_server_uplink(zmq_listener, zmq_connector, test_messages):
-    messages = []
     zmq_connector.subscribe('')
+    wait_until_subscriber_connects(zmq_listener, zmq_connector)
+
+    messages = []
     zmq_connector.register_callback(messages.append)
 
     for message in test_messages:
@@ -175,8 +181,10 @@ def test_connector_downlink_subscribe(zmq_listener, zmq_connector):
     key2_messages = [{'key': '2', 'data': i} for i in range(randint(128, 256))]
     all_messages = key1_messages + key2_messages
 
-    messages = []
     zmq_connector.subscribe('1')
+    wait_until_subscriber_connects(zmq_listener, zmq_connector)
+
+    messages = []
     zmq_connector.register_callback(messages.append)
 
     for message in all_messages:
@@ -197,6 +205,7 @@ def test_listener_sync_recv(zmq_listener, zmq_connector, test_messages):
 def test_connector_sync_recv(zmq_listener, zmq_connector, test_messages):
     zmq_connector.subscribe('')
     wait_until_subscriber_connects(zmq_listener, zmq_connector)
+
     for message in test_messages:
         zmq_listener.send_message(message)
         assert zmq_connector.recv_message() == message