net: lwm2m: empty CBOR array support

Makes possible to write an empty CBOR array if there are no SenML CBOR
records that needs to be written. This came up when trying to delete a
portfolio object instance.

Signed-off-by: Veijo Pesonen <[email protected]>

Author: Veijo Pesonen

subsys/net/lib/lwm2m/lwm2m_rw_senml_cbor.c

@@ -141,13 +141,29 @@ static int put_basename(struct lwm2m_output_context *out, struct lwm2m_obj_path
 	return 0;
 }
 
+static int put_empty_array(struct lwm2m_output_context *out)
+{
+	int len = 1;
+
+	memset(CPKT_BUF_W_PTR(out->out_cpkt), 0x80, len); /* 80 # array(0) */
+	out->out_cpkt->offset += len;
+
+	return len;
+}
+
 static int put_end(struct lwm2m_output_context *out, struct lwm2m_obj_path *path)
 {
 	size_t len;
+	struct lwm2m_senml *input = &(LWM2M_OFD_CBOR(out)->input);
+
+	if (!input->_lwm2m_senml__record_count) {
+		len = put_empty_array(out);
+
+		return len;
+	}
 
 	uint_fast8_t ret =
-		cbor_encode_lwm2m_senml(CPKT_BUF_W_REGION(out->out_cpkt),
-					(struct lwm2m_senml *)&(LWM2M_OFD_CBOR(out)->input), &len);
+		cbor_encode_lwm2m_senml(CPKT_BUF_W_REGION(out->out_cpkt), input, &len);
 
 	if (ret != ZCBOR_SUCCESS) {
 		LOG_ERR("unable to encode senml cbor msg");