Fix wait_for_publish that could hang for QoS=0 message

ChangeLog.txt

@@ -5,6 +5,8 @@ v2.0.0 - 2023-xx-xx
   Minimum tested version is Python 3.7
 - Add on_pre_connect() callback, which is called immediately before a
   connection attempt is made.
+- Fix wait_for_publish that could hang with QoS == 0 message on reconnection
+  or publish during connection. Closes #549.
 
 
 v1.6.1 - 2021-10-21

src/paho/mqtt/client.py

@@ -349,6 +349,10 @@ def timed_out():
             while not self._published and not timed_out():
                 self._condition.wait(timeout_tenth)
 
+        if self.rc > 0:
+            raise RuntimeError(f'Message publish failed: {error_string(self.rc)}')
+
+
     def is_published(self):
         """Returns True if the message associated with this object has been
         published, else returns False."""
@@ -1018,8 +1022,16 @@ def reconnect(self):
             "to_process": 0,
             "pos": 0}
 
+        # Before dropping all out_packet, ensure any QoS == 0 message info get
+        # marked as MQTT_ERR_CONN_LOST or the wait_for_publish() could hang forever
+        old_queue = self._out_packet
         self._out_packet = collections.deque()
 
+        for pkt in old_queue:
+            if pkt["command"] & 0xF0 == PUBLISH and pkt["qos"] == 0:
+                pkt["info"].rc = MQTT_ERR_CONN_LOST
+                pkt["info"]._set_as_published()
+
         with self._msgtime_mutex:
             self._last_msg_in = time_func()
             self._last_msg_out = time_func()