net: mqtt: Add keep alive timeout helper

Add function that returns remaining time until next keep alive message
shall be sent. Such function could be used for instance as a source
for `poll` timeout.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

include/net/mqtt.h

@@ -707,6 +707,18 @@ int mqtt_abort(struct mqtt_client *client);
  */
 int mqtt_live(struct mqtt_client *client);
 
+/**
+ * @brief Helper function to determine when next keep alive message should be
+ *        sent. Can be used for instance as a source for `poll` timeout.
+ *
+ * @param[in] client Client instance for which the procedure is requested.
+ *
+ * @return Time in milliseconds until next keep alive message is expected to
+ *         be sent. Function will return UINT32_MAX if keep alive messages are
+ *         not enabled.
+ */
+u32_t mqtt_keepalive_time_left(const struct mqtt_client *client);
+
 /**
  * @brief Receive an incoming MQTT packet. The registered callback will be
  *        called with the packet content.

subsys/net/lib/mqtt/mqtt.c

@@ -597,6 +597,24 @@ int mqtt_live(struct mqtt_client *client)
 	return 0;
 }
 
+u32_t mqtt_keepalive_time_left(const struct mqtt_client *client)
+{
+	u32_t elapsed_time = mqtt_elapsed_time_in_ms_get(
+					client->internal.last_activity);
+	u32_t keepalive_ms = 1000U * client->keepalive;
+
+	if (client->keepalive == 0) {
+		/* Keep alive not enabled. */
+		return UINT32_MAX;
+	}
+
+	if (keepalive_ms <= elapsed_time) {
+		return 0;
+	}
+
+	return keepalive_ms - elapsed_time;
+}
+
 int mqtt_input(struct mqtt_client *client)
 {
 	int err_code = 0;