Avoid unnecessary read buffer shifting on every incoming packet

Summary: Fixes #33

Reviewers: ivica

Reviewed By: ivica

Subscribers: korina, miljen

Differential Revision: https://repo.mireo.local/D36258

include/boost/mqtt5/impl/assemble_op.hpp

@@ -97,15 +97,6 @@ public:
 
     template <typename CompletionCondition>
     void perform(CompletionCondition cc) {
-        _read_buff.erase(
-            _read_buff.cbegin(), _data_span.first()
-        );
-        _read_buff.resize(max_recv_size());
-        _data_span = {
-            _read_buff.cbegin(),
-            _read_buff.cbegin() + _data_span.size()
-        };
-
         if (cc(error_code {}, 0) == 0 && _data_span.size()) {
             return asio::post(
                 _svc.get_executor(),
@@ -116,8 +107,11 @@ public:
             );
         }
 
+        prepare_buffer(1);
+
         // Must be evaluated before this is moved
-        auto store_begin = _read_buff.data() + _data_span.size();
+        auto store_begin = _read_buff.data()
+            + std::distance(_read_buff.cbegin(), _data_span.last());
         auto store_size = std::distance(_data_span.last(), _read_buff.cend());
 
         _svc._stream.async_read_some(
@@ -137,7 +131,7 @@ public:
         if (ec == asio::error::try_again) {
             _svc.update_session_state();
             _svc._async_sender.resend();
-            _data_span = { _read_buff.cend(), _read_buff.cend() };
+            _data_span = { _read_buff.cbegin(), _read_buff.cbegin() };
             return perform(std::move(cc));
         }
 
@@ -170,8 +164,10 @@ public:
         )
             return complete(client::error::packet_too_large, 0, {}, {});
 
-        if (std::distance(first, _data_span.last()) < *varlen)
+        if (std::distance(first, _data_span.last()) < *varlen) {
+            prepare_buffer(*varlen - std::distance(first, _data_span.last()));
             return perform(asio::transfer_at_least(1));
+        }
 
         _data_span.remove_prefix(
             std::distance(_data_span.first(), first) + *varlen
@@ -181,6 +177,22 @@ public:
     }
 
 private:
+    void prepare_buffer(std::ptrdiff_t extra_len) {
+        if (std::distance(_data_span.last(), _read_buff.cend()) >= extra_len)
+            return;
+
+        // make room for the packet by erasing bytes we already parsed from the
+        // beginning of the read buffer
+
+        const auto data_span_size = _data_span.size();
+        _read_buff.erase(_read_buff.cbegin(), _data_span.first());
+        _read_buff.resize(max_recv_size());
+        _data_span = {
+            _read_buff.cbegin(),
+            _read_buff.cbegin() + data_span_size
+        };
+    }
+
     uint32_t max_recv_size() const {
         return std::min(
             _svc.connect_property(prop::maximum_packet_size)
@@ -239,7 +251,7 @@ private:
         byte_citer first, byte_citer last
     ) {
         if (ec)
-            _data_span = { _read_buff.cend(), _read_buff.cend() };
+            _data_span = { _read_buff.cbegin(), _read_buff.cbegin() };
         std::move(_handler)(ec, control_code, first, last);
     }
 };

include/boost/mqtt5/impl/client_service.hpp

@@ -276,7 +276,7 @@ private:
         _stream(_executor, _stream_context, _log),
         _replies(_executor),
         _async_sender(*this),
-        _active_span(_read_buff.cend(), _read_buff.cend()),
+        _active_span(_read_buff.cbegin(), _read_buff.cbegin()),
         _rec_channel(_executor, (std::numeric_limits<size_t>::max)()),
         _ping_timer(_executor),
         _sentry_timer(_executor)
@@ -296,7 +296,7 @@ public:
         _stream(ex, _stream_context, _log),
         _replies(ex),
         _async_sender(*this),
-        _active_span(_read_buff.cend(), _read_buff.cend()),
+        _active_span(_read_buff.cbegin(), _read_buff.cbegin()),
         _rec_channel(ex, (std::numeric_limits<size_t>::max)()),
         _ping_timer(ex),
         _sentry_timer(ex)

test/integration/read_message.cpp

@@ -12,6 +12,7 @@
 #include <boost/asio/detached.hpp>
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/steady_timer.hpp>
+#include <boost/test/data/test_case.hpp>
 #include <boost/test/unit_test.hpp>
 
 #include <chrono>
@@ -224,7 +225,7 @@ BOOST_FIXTURE_TEST_CASE(receive_disconnect_while_reconnecting, shared_test_data)
 template <typename VerifyFun>
 void run_receive_test(
     test::msg_exchange broker_side, int num_of_receives,
-    VerifyFun&& verify_fun
+    size_t max_packet_size, VerifyFun&& verify_fun
 ) {
     const int expected_handlers_called = num_of_receives;
     int handlers_called = 0;
@@ -238,6 +239,7 @@ void run_receive_test(
     using client_type = mqtt_client<test::test_stream>;
     client_type c(executor);
     c.brokers("127.0.0.1,127.0.0.1") // to avoid reconnect backoff
+        .connect_property(prop::maximum_packet_size, max_packet_size)
         .async_run(asio::detached);
 
     for (int i = 0; i < num_of_receives; ++i)
@@ -277,7 +279,47 @@ BOOST_FIXTURE_TEST_CASE(receive_byte_by_byte, shared_test_data) {
         BOOST_TEST(payload == payload_);
     };
 
-    run_receive_test(std::move(broker_side), 1, std::move(verify_fun));
+    run_receive_test(std::move(broker_side), 1, 65'536, std::move(verify_fun));
+}
+
+namespace bdata = boost::unit_test::data;
+
+BOOST_DATA_TEST_CASE_F(
+    shared_test_data, receive_with_different_max_packet_sizes,
+    bdata::make(5, 500, 50'000) * bdata::make(1, 2) * bdata::xrange(-1, 5),
+    publish_payload_size, max_packet_size_multiplier, max_packet_size_offset
+) {
+    auto payload1 = std::string(publish_payload_size, 'a');
+    auto publish1 = encoders::encode_publish(
+        0, topic, payload1, qos_e::at_most_once, retain_e::no, dup_e::no, {}
+    );
+
+    const uint32_t max_packet_size = std::max(
+        publish1.size(),
+        publish1.size() * max_packet_size_multiplier + max_packet_size_offset
+    );
+    connect_props cprops;
+    cprops[prop::maximum_packet_size] = max_packet_size;
+    auto connect1 = encoders::encode_connect(
+        "", std::nullopt, std::nullopt, 60, false, cprops, std::nullopt
+    );
+
+    test::msg_exchange broker_side;
+    broker_side
+        .expect(connect1)
+            .complete_with(success, after(1ms))
+            .reply_with(connack, after(2ms))
+        .send(publish1, publish1, publish1, publish1, publish1, after(100ms));
+
+    auto verify_fun = [&](
+        error_code ec, std::string topic_, std::string payload_, publish_props
+    ) {
+        BOOST_TEST(!ec);
+        BOOST_TEST(topic == topic_);
+        BOOST_TEST(payload1 == payload_);
+    };
+
+    run_receive_test(std::move(broker_side), 5, max_packet_size, std::move(verify_fun));
 }
 
 BOOST_FIXTURE_TEST_CASE(receive_multiple_packets_at_once, shared_test_data) {
@@ -290,13 +332,13 @@ BOOST_FIXTURE_TEST_CASE(receive_multiple_packets_at_once, shared_test_data) {
 
     auto verify_fun = [&](
         error_code ec, std::string topic_, std::string payload_, publish_props
-        ) {
-            BOOST_TEST(!ec);
-            BOOST_TEST(topic == topic_);
-            BOOST_TEST(payload == payload_);
-        };
+    ) {
+        BOOST_TEST(!ec);
+        BOOST_TEST(topic == topic_);
+        BOOST_TEST(payload == payload_);
+    };
 
-    run_receive_test(std::move(broker_side), 5, std::move(verify_fun));
+    run_receive_test(std::move(broker_side), 5, 65'536, std::move(verify_fun));
 }
 
 BOOST_FIXTURE_TEST_CASE(receive_multiple_packets_with_malformed, shared_test_data) {
@@ -329,7 +371,7 @@ BOOST_FIXTURE_TEST_CASE(receive_multiple_packets_with_malformed, shared_test_dat
         BOOST_TEST(payload == payload_);
     };
 
-    run_receive_test(std::move(broker_side), 3, std::move(verify_fun));
+    run_receive_test(std::move(broker_side), 3, 65'536, std::move(verify_fun));
 }
 
 BOOST_AUTO_TEST_SUITE_END();