Support to limit the number of pending outgoing messages

When there's no limitation, the library itself would queue all messages
with QoS > 0 if the messages can not be delivered. This can make the
queue grow to an unreasonable size and lead an out-of-memory issue.
For example, if the user keep publishing messages when the messages
cannot be sent successfully.
Signed-off-by: NChang Yu-heng <mr.changyuheng@gmail.com>

README.rst

@@ -200,6 +200,17 @@ Set the maximum number of messages with QoS>0 that can be part way through their
 
 Defaults to 20. Increasing this value will consume more memory but can increase throughput.
 
+max_queued_messages_set()
+'''''''''''''''''''''''''
+
+::
+
+    max_queued_messages_set(self, queue_size)
+
+Set the maximum number of outgoing messages with QoS>0 that can be pending in the outgoing message queue.
+
+Defaults to 0. 0 means unlimited. When the queue is full, any further outgoing messages would be dropped.
+
 message_retry_set()   
 '''''''''''''''''''
 

src/paho/mqtt/client.py

@@ -128,6 +128,7 @@ MQTT_ERR_AUTH = 11
 MQTT_ERR_ACL_DENIED = 12
 MQTT_ERR_UNKNOWN = 13
 MQTT_ERR_ERRNO = 14
+MQTT_ERR_QUEUE_SIZE = 15
 
 if sys.version_info[0] < 3:
     sockpair_data = "0"
@@ -510,6 +511,7 @@ class Client(object):
         self._in_messages = []
         self._max_inflight_messages = 20
         self._inflight_messages = 0
+        self._max_queued_messages = 0
         self._will = False
         self._will_topic = ""
         self._will_payload = None
@@ -968,6 +970,11 @@ class Client(object):
             message.dup = False
 
             self._out_message_mutex.acquire()
+
+            if self._max_queued_messages > 0 and len(self._out_messages) >= self._max_queued_messages:
+                self._out_message_mutex.release()
+                return (MQTT_ERR_QUEUE_SIZE, local_mid)
+
             self._out_messages.append(message)
             if self._max_inflight_messages == 0 or self._inflight_messages < self._max_inflight_messages:
                 self._inflight_messages = self._inflight_messages+1
@@ -1230,6 +1237,16 @@ class Client(object):
             raise ValueError('Invalid inflight.')
         self._max_inflight_messages = inflight
 
+    def max_queued_messages_set(self, queue_size):
+        """Set the maximum number of messages in the outgoing message queue.
+        0 means unlimited."""
+        if queue_size < 0:
+            raise ValueError('Invalid queue size.')
+        if not isinstance(queue_size, int):
+            raise ValueError('Invalid type of queue size.')
+        self._max_queued_messages = queue_size
+        return self
+
     def message_retry_set(self, retry):
         """Set the timeout in seconds before a message with QoS>0 is retried.
         20 seconds by default."""