[nrf fromtree] net: mqtt: Add support for MQTT 5.0 topic aliases

Add support for handling topic aliases received from the broker.
The MQTT client implementation will store received topic for further use
in case consecutive PUBLISH received from the broker contains no topic
but alias only.

Signed-off-by: Robert Lubos <[email protected]>
(cherry picked from commit e9da3b3)

Author: rlubos

include/zephyr/net/mqtt.h

@@ -242,6 +242,15 @@ struct mqtt_binstr {
 	uint32_t len;              /**< Length of binary stream. */
 };
 
+/** @brief Abstracts aliased topic. */
+struct mqtt_topic_alias {
+	/** UTF-8 encoded topic name. */
+	uint8_t topic_buf[CONFIG_MQTT_TOPIC_ALIAS_STRING_MAX];
+
+	/** Topic name size. */
+	uint16_t topic_size;
+};
+
 /** @brief Abstracts MQTT UTF-8 encoded topic that can be subscribed
  *         to or published.
  */
@@ -866,6 +875,11 @@ struct mqtt_internal {
 
 	/** Internal. Remaining payload length to read. */
 	uint32_t remaining_payload;
+
+#if defined(CONFIG_MQTT_VERSION_5_0)
+	/** Internal. MQTT 5.0 topic alias mapping. */
+	struct mqtt_topic_alias topic_aliases[CONFIG_MQTT_TOPIC_ALIAS_MAX];
+#endif /* CONFIG_MQTT_VERSION_5_0 */
 };
 
 /**

subsys/net/lib/mqtt/mqtt_decoder.c

@@ -749,6 +749,56 @@ static int publish_properties_decode(struct buf_ctx *buf,
 
 	return properties_decode(prop, ARRAY_SIZE(prop), buf);
 }
+
+static int publish_topic_alias_check(struct mqtt_client *client,
+				     struct mqtt_publish_param *param)
+{
+	uint16_t alias = param->prop.topic_alias;
+	struct mqtt_utf8 *topic_str = &param->message.topic.topic;
+	struct mqtt_topic_alias *topic_alias;
+
+	/* Topic alias must not exceed configured CONFIG_MQTT_TOPIC_ALIAS_MAX */
+	if (alias > CONFIG_MQTT_TOPIC_ALIAS_MAX) {
+		return -EBADMSG;
+	}
+
+	if (topic_str->size == 0) {
+		/* In case there's no topic, topic alias must be set */
+		if (alias == 0) {
+			return -EBADMSG;
+		}
+
+		/* Topic alias number corresponds to the aliases array index. */
+		topic_alias = &client->internal.topic_aliases[alias - 1];
+
+		/* In case topic alias has not been configured yet, report an error. */
+		if (topic_alias->topic_size == 0) {
+			return -EBADMSG;
+		}
+
+		topic_str->utf8 = topic_alias->topic_buf;
+		topic_str->size = topic_alias->topic_size;
+
+		return 0;
+	}
+
+	/* If topic is present and topic alias is set, configure the alias locally. */
+	if (alias > 0) {
+		topic_alias = &client->internal.topic_aliases[alias - 1];
+
+		if (topic_str->size > ARRAY_SIZE(topic_alias->topic_buf)) {
+			NET_ERR("Topic too long (%d bytes) to store, increase "
+				"CONFIG_MQTT_TOPIC_ALIAS_STRING_MAX",
+				topic_str->size);
+			return -EBADMSG;
+		}
+
+		memcpy(topic_alias->topic_buf, topic_str->utf8, topic_str->size);
+		topic_alias->topic_size = topic_str->size;
+	}
+
+	return 0;
+}
 #else
 static int publish_properties_decode(struct buf_ctx *buf,
 				     struct mqtt_publish_param *param)
@@ -758,9 +808,18 @@ static int publish_properties_decode(struct buf_ctx *buf,
 
 	return -ENOTSUP;
 }
+
+static int publish_topic_alias_check(struct mqtt_client *client,
+				     struct mqtt_publish_param *param)
+{
+	ARG_UNUSED(client);
+	ARG_UNUSED(param);
+
+	return -ENOTSUP;
+}
 #endif /* CONFIG_MQTT_VERSION_5_0 */
 
-int publish_decode(const struct mqtt_client *client, uint8_t flags,
+int publish_decode(struct mqtt_client *client, uint8_t flags,
 		   uint32_t var_length, struct buf_ctx *buf,
 		   struct mqtt_publish_param *param)
 {
@@ -795,6 +854,11 @@ int publish_decode(const struct mqtt_client *client, uint8_t flags,
 
 		/* Add parsed properties length */
 		var_header_length += err_code;
+
+		err_code = publish_topic_alias_check(client, param);
+		if (err_code < 0) {
+			return err_code;
+		}
 	}
 
 	if (var_length < var_header_length) {

subsys/net/lib/mqtt/mqtt_internal.h

@@ -387,7 +387,7 @@ int connect_ack_decode(const struct mqtt_client *client, struct buf_ctx *buf,
 
 /**@brief Decode MQTT Publish packet.
  *
- * @param[in] MQTT client for which packet is decoded.
+ * @param[inout] MQTT client for which packet is decoded.
  * @param[in] flags Byte containing message type and flags.
  * @param[in] var_length Length of the variable part of the message.
  * @param[inout] buf A pointer to the buf_ctx structure containing current
@@ -396,7 +396,7 @@ int connect_ack_decode(const struct mqtt_client *client, struct buf_ctx *buf,
  *
  * @return 0 if the procedure is successful, an error code otherwise.
  */
-int publish_decode(const struct mqtt_client *client, uint8_t flags,
+int publish_decode(struct mqtt_client *client, uint8_t flags,
 		   uint32_t var_length, struct buf_ctx *buf,
 		   struct mqtt_publish_param *param);