fix(System.Net.Sockets): avoid errors for pending nonblocking connects

Non-blocking Socket.Connect can return WouldBlock on Windows or InProgress on Unix while the OS connect attempt remains pending. Treat those results as pending telemetry states instead of failures.

Leave the emitted connect Activity status unset, omit error.type, and avoid emitting ConnectFailed for these pending results. Add a regression test for the real synchronous Connect path so the pending result is verified through the public socket API.

Author: tulior

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))]