New feature : Aggregated packet sub-topic selection

This new feature allows the user to select the aggregated
sub-topics names for archive and loop packets if needed, this allows to
subscribe to two different topics when both archive and loop bindings
are enabled.
The defaults are to still continue to post on the "loop" topic for both
types in order to not break existing users applications.

Added a bit of documentation for paho-mqtt installation and for the two
new options.

bin/user/mqtt.py

@@ -4,8 +4,9 @@
 Upload data to MQTT server
 
 This service requires the python bindings for mqtt:
-
-   pip install paho-mqtt
+   apt-get install python3-paho-mqtt  # on Debian/Ubuntu based systems if using python3
+   apt-get install python-paho-mqtt   # on Debian/Ubuntu based systems if using python2
+   pip install paho-mqtt              # on any system with pip installed
 
 Minimal configuration:
 
@@ -30,12 +31,12 @@
         ...
         aggregation = individual, aggregate # individual, aggregate, or both
 
-Bind to loop packets or archive records:
+Bind to loop packets or/and archive records:
 
 [StdRestful]
     [[MQTT]]
         ...
-        binding = loop # options are loop or archive
+        binding = loop # options are loop, archive or both
 
 Use the inputs map to customize name, format, or units for any observation:
 
@@ -238,6 +239,14 @@ def __init__(self, engine, config_dict):
         tls: dictionary of TLS parameters used by the Paho client to establish
         a secure connection with the broker.
         Default is None
+
+        agg_topic_loop: Allows to set the destination sub-topic for aggregated
+        loop packets (only used if aggregation contains aggregate)
+        Default is loop
+
+        agg_topic_archive: Allows to set the destination sub-topic for aggregated
+        archive packets (only used if aggregation contains aggregate)
+        Default is loop
         """
         super(MQTT, self).__init__(engine, config_dict)
         loginf("service version is %s" % VERSION)
@@ -261,6 +270,9 @@ def __init__(self, engine, config_dict):
         site_dict.setdefault('qos', 0)
         site_dict.setdefault('aggregation', 'individual,aggregate')
 
+        site_dict.setdefault('agg_topic_loop', 'loop')
+        site_dict.setdefault('agg_topic_archive', 'loop')
+
         usn = site_dict.get('unit_system', None)
         if usn is not None:
             site_dict['unit_system'] = weewx.units.unit_constants[usn]
@@ -378,6 +390,7 @@ def __init__(self, queue, server_url,
                  manager_dict=None, tls=None, qos=0,
                  post_interval=None, stale=None,
                  log_success=True, log_failure=True,
+                 agg_topic_loop='loop', agg_topic_archive='loop',
                  timeout=60, max_tries=3, retry_wait=5,
                  max_backlog=sys.maxsize):
         super(MQTTThread, self).__init__(queue,
@@ -394,6 +407,8 @@ def __init__(self, queue, server_url,
         self.server_url = server_url
         self.client_id = client_id
         self.topic = topic
+        self.agg_topic_loop = agg_topic_loop
+        self.agg_topic_archive = agg_topic_archive
         self.upload_all = True if obs_to_upload.lower() == 'all' else False
         self.append_units_label = append_units_label
         self.tls_dict = {}
@@ -468,8 +483,15 @@ def filter_data(self, record):
 
     def process_record(self, record, dbm):
         import socket
+
+        # Archive packets are easily discriminated by the fact they have an 'interval' property
+        # LOOP packets don't have it
+        aggregation_topic = self.agg_topic_archive \
+            if 'interval' in record else self.agg_topic_loop
+
         if self.augment_record and dbm is not None:
             record = self.get_record(record, dbm)
+
         if self.unit_system is not None:
             record = weewx.units.to_std_system(record, self.unit_system)
         data = self.filter_data(record)
@@ -479,6 +501,7 @@ def process_record(self, record, dbm):
             loginf("skipping upload")
             return
         url = urlparse(self.server_url)
+
         for _count in range(self.max_tries):
             try:
                 client_id = self.client_id
@@ -494,7 +517,8 @@ def process_record(self, record, dbm):
                 mc.connect(url.hostname, url.port)
                 mc.loop_start()
                 if self.aggregation.find('aggregate') >= 0:
-                    tpc = self.topic + '/loop'
+                    tpc = self.topic + '/' + aggregation_topic
+
                     (res, mid) = mc.publish(tpc, json.dumps(data),
                                             retain=self.retain, qos=self.qos)
                     if res != mqtt.MQTT_ERR_SUCCESS: