When reconnecting redis, a callback data can be duplicated.

[dobeeisfree]

problem context

0. a client have two database index, so need to re_select()

the problem is here.

1. when redis sentinel try to reconnect, called connection_disconnection_handler(...)

cpp_redis/sources/core/client.cpp

Line 330 in d1fc9c7

client::connection_disconnection_handler(network::redis_connection &) {

cpp_redis/sources/core/client.cpp

Lines 433 to 482 in d1fc9c7

	void
	client::reconnect() {
	/**
	* increase the number of attempts to reconnect
	*/
	++m_current_reconnect_attempts;
	/**
	* We rely on the sentinel to tell us which redis server is currently the master.
	*/
	if (!m_master_name.empty() &&
	!m_sentinel.get_master_addr_by_name(m_master_name, m_redis_server, m_redis_port, true)) {
	if (m_connect_callback) {
	m_connect_callback(m_redis_server, m_redis_port, connect_state::lookup_failed);
	}
	return;
	}
	/**
	* Try catch block because the redis client throws an error if connection cannot be made.
	*/
	try {
	connect(m_redis_server, m_redis_port, m_connect_callback, m_connect_timeout_ms, m_max_reconnects,
	m_reconnect_interval_ms);
	}
	catch (...) {
	}
	if (!is_connected()) {
	if (m_connect_callback) {
	m_connect_callback(m_redis_server, m_redis_port, connect_state::failed);
	}
	return;
	}
	/**
	* notify end
	*/
	if (m_connect_callback) {
	m_connect_callback(m_redis_server, m_redis_port, connect_state::ok);
	}
	__CPP_REDIS_LOG(info, "client reconnected ok");
	re_auth();
	re_select();
	resend_failed_commands();
	try_commit();
	}

2. That can be successful, so internal m_commands pushed command_request by re_select() and m_buffer builded (by m_client.send(redis_cmd); )

cpp_redis/sources/core/client.cpp

Lines 1949 to 1958 in d1fc9c7

	client::unprotected_select(int index, const reply_callback_t &reply_callback) {
	/**
	* save the index of the database for reconnect attempts.
	*/
	m_database_index = index;
	/**
	* save command in the pipeline
	*/
	unprotected_send({"SELECT", std::to_string(index)}, reply_callback);
	}

cpp_redis/sources/core/client.cpp

Lines 194 to 198 in d1fc9c7

	void
	client::unprotected_send(const std::vector<std::string> &redis_cmd, const reply_callback_t &callback) {
	m_client.send(redis_cmd);
	m_commands.push({redis_cmd, callback});
	}

3. call next function resend_failed_commands()

m_commands is not empty, so ...

cpp_redis/sources/core/client.cpp

Lines 308 to 327 in d1fc9c7

	void
	client::resend_failed_commands() {
	if (m_commands.empty()) {
	return;
	}
	/**
	* dequeue commands and move them to a local variable
	*/
	std::queue<command_request> commands = std::move(m_commands);
	while (!commands.empty()) {
	/**
	* Reissue the pending command and its callback.
	*/
	unprotected_send(commands.front().command, commands.front().callback);
	commands.pop();
	}
	}

m_commands, this variable will still have the previously select(call by unprotected_select()) command

even if commands.pop(), this variable m_commandspushed again (by unprotected_send)

also, m_buffer have duplicate data..

"SELECT" cmd sended twice, so callback data can be weired.. (even if reconnecting successful) 🙀

4. after try_commit() end of reconnect(), some commands request & response can pushed

but reply is not a current callback, the callback is previous old data when reconnecting (by re_select functions called)

solving problem

I suggest this code:

re_auth(); // => m_commands pushed
resend_failed_commands(); // build buffer from old command and m_commands pushed
re_select(); // build buffer from this new one!
try_commit();

original code:

cpp_redis/sources/core/client.cpp

Lines 478 to 481 in d1fc9c7

	re_auth();
	re_select();
	resend_failed_commands();
	try_commit();