fix: handle unmatched 487 response on delayed call cancellation (#1542)

* fix: handle unmatched 487 response on delayed call cancellation

- Add 'CancelledAt' property to SIPTransaction to track cancellation time.
- Modify SIPTransactionEngine to use 'CancelledAt' instead of 'Created'
  when expiring transactions in the Cancelled state.

This resolves an issue where cancelling an INVITE after 32 seconds
resulted in the transaction being immediately garbage collected,
leaving the final 487 response orphaned and triggering a generic
SIPTransportResponseReceived event instead of closing gracefully.

Assisted-by: GPT-5.3-Codex

* fix: encapsulate CancelledAt property and update unit tests for transaction cancellation

Assisted-by: GPT-5.4

Author: shokoban

src/SIPSorcery/core/SIPTransactions/SIPTransaction.cs

@@ -138,6 +138,12 @@ public string TransactionId
         /// </summary>
         public DateTime TimedOutAt;
 
+        /// <summary>
+        /// For cancelled INVITEs this is the time they entered the cancelled state and is used to
+        /// calculate expiry after T6. If unset, Created is used as a fallback by the transaction engine.
+        /// </summary>
+        public DateTime CancelledAt { get; private set; }
+
         protected string m_branchId;
         public string BranchId
         {
@@ -342,6 +348,11 @@ protected void UpdateTransactionState(SIPTransactionStatesEnum transactionState)
                 transactionState == SIPTransactionStatesEnum.Terminated ||
                 transactionState == SIPTransactionStatesEnum.Cancelled)
             {
+                if (transactionState == SIPTransactionStatesEnum.Cancelled && CancelledAt == DateTime.MinValue)
+                {
+                    CancelledAt = DateTime.Now;
+                }
+
                 DeliveryPending = false;
             }
             else if (transactionState == SIPTransactionStatesEnum.Completed)

src/SIPSorcery/core/SIPTransactions/SIPTransactionEngine.cs

@@ -689,6 +689,15 @@ private void RemoveExpiredTransactions()
                                 expiredTransactionIds.Add(transaction.TransactionId);
                             }
                         }
+                        else if (transaction.TransactionState == SIPTransactionStatesEnum.Cancelled)
+                        {
+                            var cancelledAt = (transaction.CancelledAt == DateTime.MinValue) ? transaction.Created : transaction.CancelledAt;
+
+                            if (now.Subtract(cancelledAt).TotalMilliseconds >= m_t6)
+                            {
+                                expiredTransactionIds.Add(transaction.TransactionId);
+                            }
+                        }
                         else if (now.Subtract(transaction.Created).TotalMilliseconds >= m_t6)
                         {
                             //logger.LogDebug("INVITE transaction (" + transaction.TransactionId + ") " + transaction.TransactionRequestURI.ToString() + " in " + transaction.TransactionState + " has been alive for " + DateTime.Now.Subtract(transaction.Created).TotalSeconds.ToString("0") + ".");

test/unit/core/SIPTransactions/SIPTransactionEngineUnitTest.cs

@@ -14,6 +14,7 @@
 using System.Linq;
 using System.Net;
 using System.Net.Sockets;
+using System.Reflection;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Logging;
 using Xunit;
@@ -222,6 +223,56 @@ public void AckRecognitionIIUnitTest()
             Assert.True(matchingTransaction.TransactionId == serverTransaction.TransactionId, "ACK transaction did not match INVITE transaction.");
         }
 
+        [Fact]
+        public void CancelledInviteWithoutCancelledAt_ExpiresUsingCreatedFallback()
+        {
+            logger.LogDebug("--> {MethodName}", System.Reflection.MethodBase.GetCurrentMethod().Name);
+            logger.BeginScope(System.Reflection.MethodBase.GetCurrentMethod().Name);
+
+            SIPTransport sipTransport = new SIPTransport();
+            SIPTransactionEngine engine = sipTransport.m_transactionEngine;
+
+            SIPRequest inviteRequest = GetDummyINVITERequest(SIPURI.ParseSIPURI("sip:dummy@127.0.0.1:12014"));
+            var tx = new UACInviteTransaction(sipTransport, inviteRequest, null);
+            engine.AddTransaction(tx);
+
+            tx.Created = DateTime.Now.AddMilliseconds(-(SIPTimings.T6 + 1000));
+
+            var stateField = typeof(SIPTransaction).GetField("m_transactionState", BindingFlags.Instance | BindingFlags.NonPublic);
+            stateField.SetValue(tx, SIPTransactionStatesEnum.Cancelled);
+
+            var removeExpiredMethod = typeof(SIPTransactionEngine).GetMethod("RemoveExpiredTransactions", BindingFlags.Instance | BindingFlags.NonPublic);
+            removeExpiredMethod.Invoke(engine, null);
+
+            Assert.Null(engine.GetTransaction(inviteRequest));
+
+            sipTransport.Shutdown();
+        }
+
+        [Fact]
+        public void CancelledInviteWithRecentCancelledAt_IsNotExpired()
+        {
+            logger.LogDebug("--> {MethodName}", System.Reflection.MethodBase.GetCurrentMethod().Name);
+            logger.BeginScope(System.Reflection.MethodBase.GetCurrentMethod().Name);
+
+            SIPTransport sipTransport = new SIPTransport();
+            SIPTransactionEngine engine = sipTransport.m_transactionEngine;
+
+            SIPRequest inviteRequest = GetDummyINVITERequest(SIPURI.ParseSIPURI("sip:dummy@127.0.0.1:12014"));
+            var tx = new UACInviteTransaction(sipTransport, inviteRequest, null);
+            engine.AddTransaction(tx);
+
+            tx.Created = DateTime.Now.AddMilliseconds(-(SIPTimings.T6 * 2));
+            tx.CancelCall();
+
+            var removeExpiredMethod = typeof(SIPTransactionEngine).GetMethod("RemoveExpiredTransactions", BindingFlags.Instance | BindingFlags.NonPublic);
+            removeExpiredMethod.Invoke(engine, null);
+
+            Assert.NotNull(engine.GetTransaction(inviteRequest));
+
+            sipTransport.Shutdown();
+        }
+
         private SIPRequest GetDummyINVITERequest(SIPURI dummyURI)
         {
             string dummyFrom = "<sip:unittest@mysipswitch.com>";