Answer the final transfer NOTIFY before hanging up the original leg

Network/SipClient.cs

@@ -35,6 +35,12 @@
     private const int DefaultRegistrationExpirySeconds = 60;
     private const int BlindTransferTimeoutSeconds = 10;
 
+    /// <summary>
+    /// How long <see cref="BlindTransferAsync"/> waits, after the REFER is accepted, for the final
+    /// transfer-progress NOTIFY before it hangs up.
+    /// </summary>
+    internal static readonly TimeSpan TransferNotifyWait = TimeSpan.FromSeconds(5);
+
     /// <summary>How long <see cref="Shutdown"/> waits for the registrar to confirm the unregister.</summary>
     internal static readonly TimeSpan UnregisterTimeout = TimeSpan.FromSeconds(3);
 
@@ -76,8 +82,8 @@
     public event Action<SipClient>? CallAnswer;
     public event Action<SipClient>? CallEnded;
     public event Action<SipClient, string>? StatusMessage;
     public event Action<SipClient>? RemotePutOnHold;
     public event Action<SipClient>? RemoteTookOffHold;
     public event Action<SipClient, Exception>? ErrorOccurred;
     public event Action<SipClient>? RegistrationStatusChanged;
     public event Action<SipClient, TimeSpan>? CallDurationUpdated;
@@ -139,7 +145,7 @@
     public string Username => _sipUsername;
     public string Server => _sipServer;
 
     public SipClient(
         SIPTransport sipTransport,
         SipConfig sipSettings,
         int registrationExpirySeconds = DefaultRegistrationExpirySeconds,
@@ -654,6 +660,22 @@
             return false;
         }
 
+        // The transferee (Asterisk here) reports progress with in-dialog NOTIFYs carrying a sipfrag
+        // ("SIP/2.0 100 Trying" ... "SIP/2.0 200 OK"). SIPUserAgent answers them while the dialog
+        // exists; hanging up before they arrive leaves them unanswered and the PBX retransmits them.
+        // Subscribe before sending the REFER: they can arrive right behind the 202.
+        var finalNotify = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);
+        void OnTransferNotify(string sipfrag)
+        {
+            string statusLine = sipfrag.Split('\n')[0].Trim();
+            if (statusLine.StartsWith("SIP/2.0 ") && statusLine.Length >= 11
+                && int.TryParse(statusLine.AsSpan(8, 3), out int code) && code >= 200)
+            {
+                finalNotify.TrySetResult(statusLine);
+            }
+        }
+        _userAgent.OnTransferNotify += OnTransferNotify;
+
         try
         {
             sipUri = SipUriNormalizer.Normalize(sipUri, _sipServer);
@@ -679,6 +701,13 @@
                 StatusMessage?.Invoke(this, $"Blind transfer to {sipUri} succeeded.");
                 Log.Information($"Blind transfer to {sipUri} succeeded");
 
+                // Let the final progress NOTIFY arrive (and be answered in the dialog) first. Not
+                // every transferee sends one, so the wait is bounded.
+                var first = await Task.WhenAny(finalNotify.Task, Task.Delay(TransferNotifyWait)).ConfigureAwait(false);
+                Log.Information("Transfer progress: {Outcome}", first == finalNotify.Task
+                    ? finalNotify.Task.Result
+                    : $"no final NOTIFY within {TransferNotifyWait.TotalSeconds:0} s");
+
                 // The REFER was accepted, so the transferee now belongs to the transfer target.
                 // PBXs such as Asterisk take this leg out of the bridge but leave it up; end it
                 // here (BYE), which raises CallEnded as for any hangup.
@@ -710,6 +739,10 @@
             Log.Error(ex, msg);
             return false;
         }
+        finally
+        {
+            _userAgent.OnTransferNotify -= OnTransferNotify;
+        }
     }
 
     /// <summary>

tests/SipBotLib.Tests/BlindTransferTests.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.Net;
 using SIPSorcery.Media;
 using SIPSorcery.SIP;
@@ -33,6 +34,11 @@
     private readonly TaskCompletionSource<bool> _byeAtCallee = new(TaskCreationOptions.RunContinuationsAsynchronously);
     private readonly TaskCompletionSource<bool> _callerEnded = new(TaskCreationOptions.RunContinuationsAsynchronously);
     private bool _acceptTransfer = true;
+    private bool _sendFinalNotify;
+    private readonly Stopwatch _clock = Stopwatch.StartNew();
+    private readonly TaskCompletionSource<(SIPResponseStatusCodesEnum Status, TimeSpan At)> _notifyAnswered =
+        new(TaskCreationOptions.RunContinuationsAsynchronously);
+    private TimeSpan _byeAt;
 
     public BlindTransferTests()
     {
@@ -56,11 +62,16 @@
         // so OnCallHungup is not a reliable signal here.
         _calleeTransport.SIPTransportRequestReceived += (_, _, req) =>
         {
-            if (req.Method == SIPMethodsEnum.BYE)
-                _byeAtCallee.TrySetResult(true);
+            if (req.Method == SIPMethodsEnum.BYE && _byeAtCallee.TrySetResult(true))
+                _byeAt = _clock.Elapsed;
             return Task.CompletedTask;
         };
-        _callee.OnTransferRequested += (_, _) => _acceptTransfer;
+        _callee.OnTransferRequested += (_, _) =>
+        {
+            if (_acceptTransfer && _sendFinalNotify)
+                _ = SendFinalReferNotifyAsync();
+            return _acceptTransfer;
+        };
 
         var config = new SipConfig { Server = "127.0.0.1", Username = "101", Password = "x" };
         _caller = new SipClient(_callerTransport, config, enableAutoReconnection: false, enableHealthMonitoring: false);
@@ -75,6 +86,27 @@
         _calleeTransport.Shutdown();
     }
 
+    /// <summary>
+    /// What Asterisk does after accepting a REFER: an in-dialog NOTIFY (Event: refer) whose
+    /// sipfrag reports the outcome. SIPSorcery's own transferee does not send one.
+    /// </summary>
+    private async Task SendFinalReferNotifyAsync()
+    {
+        await Task.Delay(50); // let the 202 go out first
+        var notify = _callee.Dialogue.GetInDialogRequest(SIPMethodsEnum.NOTIFY);
+        notify.Header.Event = "refer";
+        notify.Header.SubscriptionState = "terminated;reason=noresource";
+        notify.Header.ContentType = "message/sipfrag;version=2.0";
+        notify.Body = "SIP/2.0 200 OK\r\n";
+        var tx = new SIPNonInviteTransaction(_calleeTransport, notify, null);
+        tx.NonInviteTransactionFinalResponseReceived += (_, _, _, resp) =>
+        {
+            _notifyAnswered.TrySetResult((resp.Status, _clock.Elapsed));
+            return Task.FromResult(System.Net.Sockets.SocketError.Success);
+        };
+        tx.SendRequest();
+    }
+
     private static async Task<bool> Within(Task task, TimeSpan timeout) =>
         await Task.WhenAny(task, Task.Delay(timeout)) == task;
 
@@ -99,6 +131,26 @@
         Assert.True(await Within(_byeAtCallee.Task, TimeSpan.FromSeconds(5)), "the transferee never received a BYE");
     }
 
+    [Fact]
+    public async Task Final_progress_NOTIFY_is_answered_before_the_BYE()
+    {
+        _sendFinalNotify = true;
+        await ConnectAsync();
+
+        var elapsed = Stopwatch.StartNew();
+        bool transferred = await _caller.BlindTransferAsync(TransferTarget, TimeSpan.FromSeconds(5));
+        elapsed.Stop();
+
+        Assert.True(transferred);
+        Assert.True(await Within(_notifyAnswered.Task, TimeSpan.FromSeconds(5)), "the NOTIFY got no final response");
+        var (status, answeredAt) = _notifyAnswered.Task.Result;
+        Assert.Equal(SIPResponseStatusCodesEnum.Ok, status);
+        Assert.True(await Within(_byeAtCallee.Task, TimeSpan.FromSeconds(5)), "the transferee never received a BYE");
+        Assert.True(_byeAt >= answeredAt, $"BYE at {_byeAt} came before the NOTIFY was answered at {answeredAt}");
+        Assert.True(elapsed.Elapsed < TimeSpan.FromSeconds(3), $"BlindTransferAsync took {elapsed.Elapsed}; it should not wait out the cap");
+        Assert.True(_callerEnded.Task.IsCompleted);
+    }
+
     [Fact]
     public async Task Declined_transfer_leaves_the_call_up()
     {