[nrf fromtree] net: coap: Add API to send reset message

Add helper API to construct CoAP Reset message.

Signed-off-by: Seppo Takalo <[email protected]>
(cherry picked from commit e96e95b)
(cherry picked from commit eb52d8c)

Author: SeppoTakalo

include/zephyr/net/coap.h

@@ -552,6 +552,21 @@ int coap_packet_init(struct coap_packet *cpkt, uint8_t *data, uint16_t max_len,
 int coap_ack_init(struct coap_packet *cpkt, const struct coap_packet *req,
 		  uint8_t *data, uint16_t max_len, uint8_t code);
 
+/**
+ * @brief Create a new CoAP Reset message for given request.
+ *
+ * This function works like @ref coap_packet_init, filling CoAP header type,
+ * and CoAP header message id fields.
+ *
+ * @param cpkt New packet to be initialized using the storage from @a data.
+ * @param req CoAP request packet that is being acknowledged
+ * @param data Data that will contain a CoAP packet information
+ * @param max_len Maximum allowable length of data
+ *
+ * @return 0 in case of success or negative in case of error.
+ */
+int coap_rst_init(struct coap_packet *cpkt, const struct coap_packet *req,
+		  uint8_t *data, uint16_t max_len);
 /**
  * @brief Returns a randomly generated array of 8 bytes, that can be
  * used as a message's token.

subsys/net/lib/coap/coap.c

@@ -232,6 +232,19 @@ int coap_ack_init(struct coap_packet *cpkt, const struct coap_packet *req,
 				token, code, id);
 }
 
+int coap_rst_init(struct coap_packet *cpkt, const struct coap_packet *req,
+		  uint8_t *data, uint16_t max_len)
+{
+	uint16_t id;
+	uint8_t ver;
+
+	ver = coap_header_get_version(req);
+	id = coap_header_get_id(req);
+
+	return coap_packet_init(cpkt, data, max_len, ver, COAP_TYPE_RESET, 0,
+				NULL, 0, id);
+}
+
 static void option_header_set_delta(uint8_t *opt, uint8_t delta)
 {
 	*opt = (delta & 0xF) << 4;