sensor: Add availability messages

Home Assistant can mark MQTT-based sensors as "unavailable" when it
receives a special message (`offline`) on a designated topic.  We send
an `online` message when the sensor process starts, and `offline` when
the process shuts down.  Additionally, we set the last will and
testament message to `offline` as well, so that Home Assistant will
still get the notification, even if the sensor does not shut down
cleanly.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "thermostat"
-version = "0.3.0dev1"
+version = "0.3.0dev2"
 description = ""
 authors = ["Dustin C. Hatch <dustin@hatch.name>"]
 

src/thermostat/sensor.py

@@ -3,6 +3,7 @@ import logging
 import os
 import select
 import signal
+import threading
 import time
 from contextlib import closing
 from types import FrameType
@@ -21,6 +22,7 @@ PORT = 8883
 USERNAME = os.environ.get("MQTT_USERNAME", "")
 PASSWORD = os.environ.get("MQTT_PASSWORD", "")
 TOPIC = "homeassistant/sensor/thermostat"
+AVAILABILITY_TOPIC = f"{TOPIC}/availability"
 
 I2CPORT = 1
 SENSOR_ADDR = 0x77
@@ -31,6 +33,7 @@ SENSOR_CONFIG = {
         "device_class": "temperature",
         "name": "Thermostat Temperature",
         "state_topic": TOPIC,
+        "availability_topic": AVAILABILITY_TOPIC,
         "unit_of_measurement": "°C",
         "value_template": r"{{ value_json.temperature }}",
     },
@@ -38,6 +41,7 @@ SENSOR_CONFIG = {
         "device_class": "pressure",
         "name": "Thermostat Pressure",
         "state_topic": TOPIC,
+        "availability_topic": AVAILABILITY_TOPIC,
         "unit_of_measurement": "hPa",
         "value_template": r"{{ value_json.pressure }}",
     },
@@ -45,6 +49,7 @@ SENSOR_CONFIG = {
         "device_class": "humidity",
         "name": "Thermostat Humidity",
         "state_topic": TOPIC,
+        "availability_topic": AVAILABILITY_TOPIC,
         "unit_of_measurement": "%",
         "value_template": r"{{ value_json.humidity }}",
     },
@@ -54,6 +59,7 @@ SENSOR_CONFIG = {
 class Daemon:
     def __init__(self) -> None:
         self.quitpipe = os.pipe()
+        self._ready = threading.Event()
 
     def on_signal(self, signum: int, frame: FrameType) -> None:
         log.debug("Got signal %d at %s", signum, frame)
@@ -65,6 +71,7 @@ class Daemon:
         signal.signal(signal.SIGTERM, self.on_signal)
 
         client = mqtt.Client()
+        client.will_set(AVAILABILITY_TOPIC, "offline")
         client.on_connect = self.on_connect
         client.on_message = self.on_message
         client.on_disconnect = self.on_disconnect
@@ -74,6 +81,7 @@ class Daemon:
         client.loop_start()
         with closing(smbus2.SMBus(I2CPORT)) as bus:
             params = bme280.load_calibration_params(bus, SENSOR_ADDR)
+            self._ready.wait()
             while 1:
                 data = bme280.sample(bus, SENSOR_ADDR, params)
                 values = {
@@ -86,6 +94,7 @@ class Daemon:
                 if self.quitpipe[0] in ready:
                     os.close(self.quitpipe[0])
                     break
+        client.publish(AVAILABILITY_TOPIC, "offline")
         client.disconnect()
         client.loop_stop()
 
@@ -101,6 +110,8 @@ class Daemon:
             client.publish(
                 f"homeassistant/sensor/{key}/config", json.dumps(value)
             )
+        client.publish(AVAILABILITY_TOPIC, "online")
+        self._ready.set()
 
     def on_disconnect(self, client, userdata, rc):
         log.error("Lost connection to MQTT broker")