Nodes parameter of "sample_infohashes" reply

[mattie20]

I was wondering the amount of nodes libtorrent sends when it receives a sample_infohashes query. I can't seem to find it in the commit 219a8e7 the parameter defined in BEP 51 as "nodes". I thought it might be put in the dictionary empty or just left out all-together, but I have had a difficult time trying to track it down. I got as far as node.cpp and the corresponding function get_infohashes_sample() in dht_storage.cpp.

[arvidn]

the handler for that message is implemented here: https://github.com/arvidn/libtorrent/blob/master/src/kademlia/dht_storage.cpp#L457-L470

It looks like it's capped at 20 info-hashes

[mattie20]

"nodes" is a misnomer in that it is an array of peers that are sent along with the infohashes. I dug a little deeper following how the sample_infohash query is parsed. I originally thought that write_nodes_entries() put the packet in the sending queue, but it seems to add the additional "nodes" entry. The Number of nodes used seems to be the default value for the find_node function ( the default for count is 0 ). In the function when 0 it defaults to the bucket size. The bucket size is then some power of 2 between 0 and 256. So, the number of the peers included in the "nodes" parameter is between 0 and 256. Does this seem to be correct?

libtorrent/src/kademlia/node.cpp

Lines 1159 to 1182 in e5fe95f

	else if (query == "sample_infohashes")
	{
	static key_desc_t const msg_desc[] = {
	{"target", bdecode_node::string_t, 20, 0},
	{"want", bdecode_node::list_t, 0, key_desc_t::optional},
	};
	bdecode_node msg_keys[2];
	if (!verify_message(arg_ent, msg_desc, msg_keys, error_string))
	{
	m_counters.inc_stats_counter(counters::dht_invalid_sample_infohashes);
	incoming_error(e, error_string);
	return;
	}
	m_counters.inc_stats_counter(counters::dht_sample_infohashes_in);
	sha1_hash const target(msg_keys[0].string_ptr());
	// TODO: keep the returned value to pass as a limit
	// to write_nodes_entries when implemented
	m_storage.get_infohashes_sample(reply);
	write_nodes_entries(target, msg_keys[1], reply);
	}

libtorrent/src/kademlia/node.cpp

Lines 1186 to 1193 in 60eead7

	// if no wants entry was specified, include a nodes
	// entry based on the protocol the request came in with
	if (want.type() != bdecode_node::list_t)
	{
	std::vector<node_entry> const n = m_table.find_node(info_hash, {});
	r[protocol_nodes_key()] = write_nodes_entry(n);
	return;
	}

libtorrent/src/kademlia/routing_table.cpp

Lines 1103 to 1107 in e5fe95f

	std::vector<node_entry> routing_table::find_node(node_id const& target
	, find_nodes_flags_t const options, int count)
	{
	std::vector<node_entry> l;
	if (count == 0) count = m_bucket_size;