Add MQTT backup reporting and trigger listener

lib/mqtt.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+mqtt_common_args() {
+  MQTT_ARGS=(-h "$MQTT_HOST" -p "$MQTT_PORT")
+  [[ -n "$MQTT_USERNAME" ]] && MQTT_ARGS+=(-u "$MQTT_USERNAME")
+  if [[ -n "$MQTT_PASSWORD_FILE" ]]; then
+    [[ -r "$MQTT_PASSWORD_FILE" ]] || die "MQTT password file is not readable: $MQTT_PASSWORD_FILE"
+    MQTT_ARGS+=(-P "$(<"$MQTT_PASSWORD_FILE")")
+  fi
+}
+
+mqtt_publish() {
+  local topic="$1" payload="$2" retain="${3:-0}"
+  [[ -n "$MQTT_HOST" ]] || die "MQTT_HOST is not configured"
+  require_command mosquitto_pub
+  mqtt_common_args
+  (( retain )) && MQTT_ARGS+=(-r)
+  mosquitto_pub "${MQTT_ARGS[@]}" -t "$topic" -m "$payload"
+}