conn: Return non-zero delay when in CONNECTING state

If connection is in state CONNECTING and there is data ready to be sent
using value 0 for the delay results in busy loop because Sender calls
select with the value returned from here and then just keeps on calling
it again and again until connection completes or fails. In some cases
this can take a long time and if the connection failed it will just be
retried and the busy loop continues.

This return value was changed from 999999999 to 0 in commit 16c13f9 and
it doesn't look like it needs to be exactly zero for other logic to work
correctly.

Fixes #1907

Author: rikonen

kafka/conn.py

@@ -107,6 +107,10 @@ class BrokerConnection(object):
             server-side log entries that correspond to this client. Also
             submitted to GroupCoordinator for logging with respect to
             consumer group administration. Default: 'kafka-python-{version}'
+        connecting_delay_ms (int): Time to wait before sending data when a
+            connection is in state CONNECTING. Small but non-zero timeout is
+            recommended to avoid busy loop as well as to avoid unnecessary
+            delays while connecting. Default: 100
         reconnect_backoff_ms (int): The amount of time in milliseconds to
             wait before attempting to reconnect to a given host.
             Default: 50.
@@ -192,6 +196,7 @@ class BrokerConnection(object):
 
     DEFAULT_CONFIG = {
         'client_id': 'kafka-python-' + __version__,
+        'connecting_delay_ms': 100,
         'node_id': 0,
         'request_timeout_ms': 30000,
         'reconnect_backoff_ms': 50,
@@ -769,15 +774,16 @@ def connection_delay(self):
         """
         Return the number of milliseconds to wait, based on the connection
         state, before attempting to send data. When disconnected, this respects
-        the reconnect backoff time. When connecting, returns 0 to allow
-        non-blocking connect to finish. When connected, returns a very large
-        number to handle slow/stalled connections.
+        the reconnect backoff time. When connecting, returns value defined by
+        connecting_delay_ms config value -- typically something less than a
+        second -- to allow non-blocking connect to finish. When connected,
+        returns a very large number to handle slow/stalled connections.
         """
         time_waited = time.time() - (self.last_attempt or 0)
         if self.state is ConnectionStates.DISCONNECTED:
             return max(self._reconnect_backoff - time_waited, 0) * 1000
         elif self.connecting():
-            return 0
+            return self.config['connecting_delay_ms']
         else:
             return float('inf')