net: lwm2m: Store the actual resource size in the resource instance

So far, the resource instance structure kept only the information about
the buffer length provided to the resource (in the `data_len` field).
While this approach might be enough for integer resources, where the
actual data size is fixed, it did not work for opaque resources. It is
impossible to determine the actual opaque resource length after it's
been written into.

Fix this, by replacing the current `data_len` field of the
`lwm2m_engine_res_inst` with `max_data_len`, indicating the buffer
size, and making the `data_len` field to hold the actual data size of
the resource.

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

subsys/net/lib/lwm2m/lwm2m_engine.c

@@ -1361,6 +1361,7 @@ int lwm2m_engine_set_res_data(char *pathstr, void *data_ptr, uint16_t data_len,
 	/* assign data elements */
 	res_inst->data_ptr = data_ptr;
 	res_inst->data_len = data_len;
+	res_inst->max_data_len = data_len;
 	res_inst->data_flags = data_flags;
 
 	return ret;
@@ -1374,7 +1375,7 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 	struct lwm2m_engine_res *res = NULL;
 	struct lwm2m_engine_res_inst *res_inst = NULL;
 	void *data_ptr = NULL;
-	size_t data_len = 0;
+	size_t max_data_len = 0;
 	int ret = 0;
 	bool changed = false;
 
@@ -1411,13 +1412,13 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 
 	/* setup initial data elements */
 	data_ptr = res_inst->data_ptr;
-	data_len = res_inst->data_len;
+	max_data_len = res_inst->max_data_len;
 
 	/* allow user to override data elements via callback */
 	if (res->pre_write_cb) {
 		data_ptr = res->pre_write_cb(obj_inst->obj_inst_id,
 					     res->res_id, res_inst->res_inst_id,
-					     &data_len);
+					     &max_data_len);
 	}
 
 	if (!data_ptr) {
@@ -1428,7 +1429,7 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 	}
 
 	/* check length (note: we add 1 to string length for NULL pad) */
-	if (len > res_inst->data_len -
+	if (len > res_inst->max_data_len -
 		(obj_field->data_type == LWM2M_RES_TYPE_STRING ? 1 : 0)) {
 		LOG_ERR("length %u is too long for res instance %d data",
 			len, path.res_id);
@@ -1512,6 +1513,8 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 
 	}
 
+	res_inst->data_len = len;
+
 	if (res->post_write_cb) {
 		ret = res->post_write_cb(obj_inst->obj_inst_id,
 					 res->res_id, res_inst->res_inst_id,
@@ -1951,6 +1954,7 @@ int lwm2m_engine_delete_res_inst(char *pathstr)
 	}
 
 	res_inst->data_ptr = NULL;
+	res_inst->max_data_len = 0U;
 	res_inst->data_len = 0U;
 	res_inst->res_inst_id = RES_INSTANCE_NOT_CREATED;
 
@@ -2273,7 +2277,7 @@ static int lwm2m_write_handler_opaque(struct lwm2m_engine_obj_inst *obj_inst,
 		in->block_ctx->opaque = opaque_ctx;
 	}
 
-	return ret;
+	return opaque_ctx.len;
 }
 
 /* This function is exposed for the content format writers */
@@ -2302,7 +2306,7 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst,
 
 	/* setup initial data elements */
 	data_ptr = res_inst->data_ptr;
-	data_len = res_inst->data_len;
+	data_len = res_inst->max_data_len;
 
 	/* allow user to override data elements via callback */
 	if (res->pre_write_cb) {
@@ -2334,6 +2338,7 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst,
 				return ret;
 			}
 
+			len = ret;
 			break;
 
 		case LWM2M_RES_TYPE_STRING:
@@ -2421,6 +2426,8 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst,
 		return -ENOENT;
 	}
 
+	res_inst->data_len = len;
+
 	if (res->post_write_cb &&
 	    obj_field->data_type != LWM2M_RES_TYPE_OPAQUE) {
 		ret = res->post_write_cb(obj_inst->obj_inst_id,

subsys/net/lib/lwm2m/lwm2m_obj_firmware_pull.c

@@ -314,7 +314,7 @@ do_firmware_transfer_reply_cb(const struct coap_packet *response,
 
 		/* get buffer data */
 		write_buf = res->res_instances->data_ptr;
-		write_buflen = res->res_instances->data_len;
+		write_buflen = res->res_instances->max_data_len;
 
 		/* check for user override to buffer */
 		if (res->pre_write_cb) {

subsys/net/lib/lwm2m/lwm2m_object.h

@@ -214,6 +214,8 @@ struct lwm2m_engine_obj {
 			for (int _i = 0; _i < _ri_count; _i++) { \
 				_ri_ptr[_ri_idx + _i].data_ptr = \
 						(_data_ptr + _i); \
+				_ri_ptr[_ri_idx + _i].max_data_len = \
+						_data_len; \
 				_ri_ptr[_ri_idx + _i].data_len = \
 						_data_len; \
 				if (_ri_create) { \
@@ -233,6 +235,7 @@ struct lwm2m_engine_obj {
 		if (_ri_count > 0) { \
 			for (int _i = 0; _i < _ri_count; _i++) { \
 				_ri_ptr[_ri_idx + _i].data_ptr = NULL; \
+				_ri_ptr[_ri_idx + _i].max_data_len = 0; \
 				_ri_ptr[_ri_idx + _i].data_len = 0; \
 				if (_ri_create) { \
 					_ri_ptr[_ri_idx + _i].res_inst_id = \
@@ -323,6 +326,7 @@ struct lwm2m_attr {
 
 struct lwm2m_engine_res_inst {
 	void  *data_ptr;
+	uint16_t max_data_len;
 	uint16_t data_len;
 	uint16_t res_inst_id; /* 65535 == not "created" */
 	uint8_t  data_flags;