From 7cbd7725fc3b3e6d953a768d187434e313d27b5d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Korina=20=C5=A0imi=C4=8Devi=C4=87?=
 <korina.simicevic@mireo.com>
Date: Mon, 19 Feb 2024 13:25:20 +0100
Subject: [PATCH] Wait exponentially longer when all hosts have been exhausted
 during reconnect

Summary: related to T13746

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D27995
---
 include/async_mqtt5/impl/reconnect_op.hpp | 29 ++++++++++++++++--
 test/unit/reconnect_op.cpp                | 36 +++++++++++++++++++++++
 2 files changed, 63 insertions(+), 2 deletions(-)
 create mode 100644 test/unit/reconnect_op.cpp

diff --git a/include/async_mqtt5/impl/reconnect_op.hpp b/include/async_mqtt5/impl/reconnect_op.hpp
index 80a4672..ff4780a 100644
--- a/include/async_mqtt5/impl/reconnect_op.hpp
+++ b/include/async_mqtt5/impl/reconnect_op.hpp
@@ -6,15 +6,38 @@
 #include <boost/asio/prepend.hpp>
 #include <boost/asio/ip/tcp.hpp>
 #include <boost/asio/any_completion_handler.hpp>
-
 #include <boost/asio/experimental/parallel_group.hpp>
 
+#include <boost/random/linear_congruential.hpp>
+#include <boost/random/uniform_smallint.hpp>
+
 #include <async_mqtt5/types.hpp>
 #include <async_mqtt5/detail/async_traits.hpp>
 #include <async_mqtt5/impl/connect_op.hpp>
 
 namespace async_mqtt5::detail {
 
+class exponential_backoff {
+	int _curr_exp { 0 };
+
+	static constexpr int _base_mulptilier = 1000;
+	static constexpr int _max_exp = 4;
+
+	// sizeof(_generator) = 8
+	boost::random::rand48 _generator { uint32_t(std::time(0)) };
+	boost::random::uniform_smallint<> _distribution { -500, 500 };
+public:
+	exponential_backoff() = default;
+
+	duration generate() {
+		int exponent = _curr_exp < _max_exp ? _curr_exp++ : _max_exp;
+		int base = 1 << exponent;
+		return std::chrono::milliseconds(
+			base * _base_mulptilier + _distribution(_generator) /* noise */
+		);
+	}
+};
+
 namespace asio = boost::asio;
 
 template <typename Owner>
@@ -31,6 +54,8 @@ class reconnect_op {
 
 	std::unique_ptr<std::string> _buffer_ptr;
 
+	exponential_backoff _generator;
+
 	using endpoint = asio::ip::tcp::endpoint;
 	using epoints = asio::ip::tcp::resolver::results_type;
 
@@ -87,7 +112,7 @@ public:
 	}
 
 	void backoff_and_reconnect() {
-		_owner._connect_timer.expires_from_now(std::chrono::seconds(5));
+		_owner._connect_timer.expires_from_now(_generator.generate());
 		_owner._connect_timer.async_wait(
 			asio::prepend(std::move(*this), on_backoff {})
 		);
diff --git a/test/unit/reconnect_op.cpp b/test/unit/reconnect_op.cpp
new file mode 100644
index 0000000..f37a88c
--- /dev/null
+++ b/test/unit/reconnect_op.cpp
@@ -0,0 +1,36 @@
+#include <boost/test/unit_test.hpp>
+
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/steady_timer.hpp>
+
+#include <async_mqtt5/impl/reconnect_op.hpp>
+
+using namespace async_mqtt5;
+
+BOOST_AUTO_TEST_SUITE(reconnect_op/*, *boost::unit_test::disabled()*/)
+
+BOOST_AUTO_TEST_CASE(exponential_backoff) {
+	using namespace std::chrono_literals;
+
+	detail::exponential_backoff generator;
+
+	auto first_iter = generator.generate();
+	BOOST_TEST((first_iter >= 500ms && first_iter <= 1500ms));
+
+	auto second_iter = generator.generate();
+	BOOST_TEST((second_iter >= 1500ms && first_iter <= 2500ms));
+
+	auto third_iter = generator.generate();
+	BOOST_TEST((third_iter >= 3500ms && third_iter <= 4500ms));
+
+	auto fourth_iter = generator.generate();
+	BOOST_TEST((fourth_iter >= 7500ms && fourth_iter <= 8500ms));
+
+	auto fifth_iter = generator.generate();
+	BOOST_TEST((fifth_iter >= 15500ms && fourth_iter <= 16500ms));
+
+	auto sixth_iter = generator.generate();
+	BOOST_TEST((sixth_iter >= 15500ms && sixth_iter <= 16500ms));
+}
+
+BOOST_AUTO_TEST_SUITE_END();