allow mqtt_topic to be set from the channel config

This backwards-compatible change allows one to set the mqtt topic from
the channel configurtation. Currently, the meter values are published
via an mqtt topic that is set to the channel number, which changes
whenever a new channel is defined or and old deleted. And it's totally
not mnemonic.

This addresses issue #420 from 2020.

Author: Marc Haber

include/Channel.hpp

@@ -74,6 +74,7 @@ class Channel {
 
 	const char *name() const { return _name.c_str(); }
 	std::list<Option> &options() { return _options; }
+	const char *mqttTopic() const { return _mqttTopic.c_str(); }
 
 	ReadingIdentifier::Ptr identifier() {
 		if (_identifier.use_count() < 1)
@@ -116,6 +117,7 @@ class Channel {
 	std::string _name; // name of the channel
 	std::list<Option> _options;
 
+	std::string _mqttTopic;
 	Buffer::Ptr _buffer; // circular queue to buffer readings
 
 	ReadingIdentifier::Ptr _identifier; // channel identifier (OBIS, string)

src/Channel.cpp

@@ -86,6 +86,11 @@ Channel::Channel(const std::list<Option> &pOptions, const std::string apiProtoco
 		throw;
 	}
 
+	try {
+		_mqttTopic = optlist.lookup_string(pOptions, "mqtt_topic");
+	} catch (vz::OptionNotFoundException &e) {
+		// optional, defaults to empty string
+	}
 	pthread_cond_init(&condition, NULL); // initialize thread syncronization helpers
 }
 

src/mqtt.cpp

@@ -222,7 +222,9 @@ MqttClient::~MqttClient() {
 void MqttClient::ChannelEntry::generateNames(const std::string &prefix, Channel &ch) {
 	_announceValues.clear();
 	_fullTopicRaw = prefix;
-	_fullTopicRaw += ch.name(); // todo this converts from std::string to const char and back...
+	// Use configured mqtt_topic if set, otherwise fall back to ch.name()
+	const char *topicSegment = (ch.mqttTopic()[0] != '\0') ? ch.mqttTopic() : ch.name();
+	_fullTopicRaw += topicSegment;
 	_fullTopicRaw += '/';
 	if (ch.identifier()) {
 		char unparseBuf[200];