Avoid socket telemetry errors for pending nonblocking connects

src/libraries/System.Net.Sockets/src/System/Net/Sockets/SocketsTelemetry.cs

@@ -147,9 +147,16 @@ public void AfterConnect(SocketError error, Activity? activity, string? exceptio
             long newCount = Interlocked.Decrement(ref _currentOutgoingConnectAttempts);
             Debug.Assert(newCount >= 0);
 
+            // _currentOutgoingConnectAttempts tracks managed Connect calls. A non-blocking Connect call
+            // can return WouldBlock (Windows) or InProgress (Unix) while the OS connect attempt remains
+            // pending after this method decrements that counter; its eventual outcome is not observable
+            // here. Don't report these pending results as failures.
+            // See https://github.com/dotnet/runtime/issues/129252
+            bool connectPending = error is SocketError.WouldBlock or SocketError.InProgress;
+
             if (activity is not null)
             {
-                if (error != SocketError.Success)
+                if (error != SocketError.Success && !connectPending)
                 {
                     activity.SetStatus(ActivityStatusCode.Error);
                     activity.SetTag("error.type", GetErrorType(error));
@@ -163,7 +170,7 @@ public void AfterConnect(SocketError error, Activity? activity, string? exceptio
                 Debug.Assert(exceptionMessage is null);
                 Interlocked.Increment(ref _outgoingConnectionsEstablished);
             }
-            else
+            else if (!connectPending)
             {
                 ConnectFailed(error, exceptionMessage);
             }

src/libraries/System.Net.Sockets/tests/FunctionalTests/TelemetryTest.cs

@@ -158,6 +158,37 @@ static void VerifyTcpConnectActivity(Activity activity, IPEndPoint remoteEndPoin
             ActivityAssert.HasTag(activity, "network.transport", "tcp");
         }
 
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public async Task Connect_NonBlockingPending_ActivityNotMarkedAsError()
+        {
+            await RemoteExecutor.Invoke(static () =>
+            {
+                using Socket server = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+                server.BindToAnonymousPort(IPAddress.Loopback);
+                server.Listen();
+
+                using Socket client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)
+                {
+                    Blocking = false
+                };
+
+                using ActivityRecorder recorder = new ActivityRecorder(ActivitySourceName, ActivityName);
+
+                // A non-blocking connect reports WouldBlock (Windows) or InProgress (Unix) by design while
+                // the attempt continues in the background. The "socket connect" activity is started and
+                // stopped synchronously inside Connect, so its final state can be asserted immediately.
+                SocketException ex = Assert.Throws<SocketException>(() => client.Connect(server.LocalEndPoint));
+                Assert.True(ex.SocketErrorCode is SocketError.WouldBlock or SocketError.InProgress,
+                    $"Unexpected SocketError: {ex.SocketErrorCode}");
+
+                recorder.VerifyActivityRecorded(1);
+                Activity activity = recorder.LastFinishedActivity;
+                VerifyTcpConnectActivity(activity, (IPEndPoint)server.LocalEndPoint, ipv6: false);
+                Assert.Equal(ActivityStatusCode.Unset, activity.Status);
+                Assert.Null(activity.GetTagItem("error.type"));
+            }).DisposeAsync();
+        }
+
         [OuterLoop("Connection failure takes long on Windows.")]
         [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
         [MemberData(nameof(SocketMethods_WithBools_MemberData))]