[20729] Allow processing of AckNack submessages with count == 0

include/fastdds/rtps/writer/ReaderProxy.h

@@ -307,9 +307,10 @@ class ReaderProxy
     bool check_and_set_acknack_count(
             uint32_t acknack_count)
     {
-        if (last_acknack_count_ < acknack_count)
+        if (acknack_count >= next_expected_acknack_count_)
         {
-            last_acknack_count_ = acknack_count;
+            next_expected_acknack_count_ = acknack_count;
+            ++next_expected_acknack_count_;
             return true;
         }
 
@@ -442,8 +443,8 @@ class ReaderProxy
     TimedEvent* initial_heartbeat_event_;
     //! Are timed events enabled?
     std::atomic_bool timers_enabled_;
-    //! Last ack/nack count
-    uint32_t last_acknack_count_;
+    //! Next expected ack/nack count
+    uint32_t next_expected_acknack_count_;
     //! Last  NACKFRAG count.
     uint32_t last_nackfrag_count_;
 

src/cpp/rtps/writer/ReaderProxy.cpp

@@ -57,7 +57,7 @@ ReaderProxy::ReaderProxy(
     , nack_supression_event_(nullptr)
     , initial_heartbeat_event_(nullptr)
     , timers_enabled_(false)
-    , last_acknack_count_(0)
+    , next_expected_acknack_count_(0)
     , last_nackfrag_count_(0)
 {
     nack_supression_event_ = new TimedEvent(writer_->getRTPSParticipant()->getEventResource(),
@@ -166,7 +166,7 @@ void ReaderProxy::stop()
     disable_timers();
 
     changes_for_reader_.clear();
-    last_acknack_count_ = 0;
+    next_expected_acknack_count_ = 0;
     last_nackfrag_count_ = 0;
     changes_low_mark_ = SequenceNumber_t();
 }

test/unittest/rtps/writer/ReaderProxyTests.cpp

@@ -382,6 +382,38 @@ TEST(ReaderProxyTests, has_been_delivered_test)
     expect_result({0, 3}, false, false);
 }
 
+// Test expectations regarding acknack count.
+// Serves as a regression test for redmine issue #20729.
+TEST(ReaderProxyTests, acknack_count)
+{
+    StatefulWriter writer_mock;
+    WriterTimes w_times;
+    RemoteLocatorsAllocationAttributes alloc;
+    ReaderProxy rproxy(w_times, alloc, &writer_mock);
+
+    ReaderProxyData reader_attributes(0, 0);
+    reader_attributes.m_qos.m_reliability.kind = RELIABLE_RELIABILITY_QOS;
+    rproxy.start(reader_attributes);
+
+    // Check that the initial acknack count is 0.
+    EXPECT_TRUE(rproxy.check_and_set_acknack_count(0u));
+    // Check that it is not accepted twice.
+    EXPECT_FALSE(rproxy.check_and_set_acknack_count(0u));
+    // Check that it is accepted if it is incremented.
+    EXPECT_TRUE(rproxy.check_and_set_acknack_count(1u));
+    // Check that it is not accepted twice.
+    EXPECT_FALSE(rproxy.check_and_set_acknack_count(1u));
+    // Check that it is not accepted if it is decremented.
+    EXPECT_FALSE(rproxy.check_and_set_acknack_count(0u));
+    // Check that it is accepted if it has a big increment.
+    EXPECT_TRUE(rproxy.check_and_set_acknack_count(100u));
+    // Check that previous values are rejected.
+    for (uint32_t i = 0; i <= 100u; ++i)
+    {
+        EXPECT_FALSE(rproxy.check_and_set_acknack_count(i));
+    }
+}
+
 } // namespace rtps
 } // namespace fastrtps
 } // namespace eprosima