Fix stale ACK causing irrecoverable quarantine after transient network disruption

src/core/Akka.Remote.Tests/AckedDeliverySpec.cs

@@ -204,6 +204,33 @@ public void SendBuffer_must_keep_NACKed_messages_in_buffer_if_selective_nacks_ar
             Assert.True(b7.Nacked.Count == 0);
         }
 
+        [Fact]
+        public void SendBuffer_must_ignore_stale_ack_from_previous_association()
+        {
+            // Reproduces GitHub #6414: a fresh send buffer (MaxSeq = -1) receives a stale ACK
+            // from a previous association. This must be a no-op, not a throw.
+            var b0 = new AckedSendBuffer<Sequenced>(10);
+
+            // ACK[0] from stale receiver state - MaxSeq is -1, CumulativeAck is 0
+            var result = b0.Acknowledge(new Ack(new SeqNo(0)));
+            Assert.Same(b0, result); // should return the same instance (no-op)
+        }
+
+        [Fact]
+        public void SendBuffer_must_ignore_stale_ack_when_buffer_has_messages_with_lower_seqnos()
+        {
+            // A buffer with some messages receives a stale ACK referencing a higher sequence
+            // number than anything buffered - should be a no-op.
+            var b0 = new AckedSendBuffer<Sequenced>(10);
+            var msg0 = Msg(0);
+            var msg1 = Msg(1);
+            var b1 = b0.Buffer(msg0).Buffer(msg1); // MaxSeq = 1
+
+            // Stale ACK referencing SeqNo(5) which we never sent
+            var result = b1.Acknowledge(new Ack(new SeqNo(5)));
+            Assert.Same(b1, result); // should return the same instance (no-op)
+        }
+
         [Fact]
         public void SendBuffer_must_throw_exception_if_nonbuffered_sequence_number_is_NACKed()
         {

src/core/Akka.Remote/AckedDelivery.cs

@@ -431,9 +431,13 @@ public AckedSendBuffer(int capacity, SeqNo maxSeq, IImmutableList<T> nacked, IIm
         /// <returns>An updated buffer containing the remaining unacknowledged messages</returns>
         public AckedSendBuffer<T> Acknowledge(Ack ack)
         {
+            // If the ACK references a sequence number higher than anything we've sent,
+            // it's a stale ACK from a previous association. The buffer is empty or has
+            // only messages with lower sequence numbers — there's nothing to acknowledge.
+            // Throwing here would cause an irrecoverable quarantine (see GitHub #6414).
             if (ack.CumulativeAck > MaxSeq)
             {
-                throw new ArgumentException(nameof(ack), $"Highest SEQ so far was {MaxSeq} but cumulative ACK is {ack.CumulativeAck}");
+                return this;
             }
 
             var newNacked = ack.Nacks.Count == 0

src/core/Akka.Remote/Endpoint.cs

@@ -612,6 +612,12 @@ private void Receiving()
                     try
                     {
                         _resendBuffer = _resendBuffer.Acknowledge(ack);
+                        if (ack.CumulativeAck > _resendBuffer.MaxSeq)
+                        {
+                            _log.Warning(
+                                "Ignoring stale ACK [{0}] for send buffer [{1}] - likely from a previous association to [{2}]",
+                                ack, _resendBuffer, _remoteAddress);
+                        }
                     }
                     catch (Exception ex)
                     {