Harden and re-enable DualMode socket tests (#80715)

- UDP tests are moved to a non-parallel collection, since there is no way to prevent parallel UDP tests from intercepting each other's packets. This only increases the execution time by ~2 seconds.
- TCP interferences are prevented by a utility PortBlocker, which creates and binds a "shadow" socket of the opposite address family to prevent sockets in parallel tests to bind to the same port.
- Accept and SendTo tests are refactored to use SocketTestHelperBase<T>, reducing duplicate code.

src/libraries/Common/tests/System/Net/Sockets/SocketTestExtensions.cs

@@ -42,8 +42,16 @@ public static (Socket client, Socket server) CreateConnectedSocketPair(bool ipv6
         {
             IPAddress serverAddress = ipv6 ? IPAddress.IPv6Loopback : IPAddress.Loopback;
 
-            using Socket listener = new Socket(serverAddress.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
-            listener.Bind(new IPEndPoint(serverAddress, 0));
+            // PortBlocker creates a temporary socket of the opposite AddressFamily in the background, so parallel tests won't attempt
+            // to create their listener sockets on the same port, regardless of address family.
+            // This should prevent 'listener' from accepting DualMode connections of unrelated tests.
+            using PortBlocker portBlocker = new PortBlocker(() =>
+            {
+                Socket l = new Socket(serverAddress.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
+                l.BindToAnonymousPort(serverAddress);
+                return l;
+            });
+            Socket listener = portBlocker.MainSocket; // PortBlocker shall dispose this
             listener.Listen(1);
 
             IPEndPoint connectTo = (IPEndPoint)listener.LocalEndPoint;
@@ -84,4 +92,121 @@ public static bool TryConnect(this Socket socket, EndPoint remoteEndpoint, int m
             return false;
         }
     }
+
+    /// <summary>
+    /// A utility to create and bind a socket while blocking it's port for both IPv4 and IPv6
+    /// by also creating and binding a "shadow" socket of the opposite address family.
+    /// </summary>
+    internal class PortBlocker : IDisposable
+    {
+        private const int MaxAttempts = 16;
+        private Socket _shadowSocket;
+        public Socket MainSocket { get; }
+
+        public PortBlocker(Func<Socket> socketFactory)
+        {
+            bool success = false;
+            for (int i = 0; i < MaxAttempts; i++)
+            {
+                MainSocket = socketFactory();
+                if (MainSocket.LocalEndPoint is not IPEndPoint)
+                {
+                    MainSocket.Dispose();
+                    throw new Exception($"{nameof(socketFactory)} is expected create and bind the socket.");
+                }
+
+                IPAddress shadowAddress = MainSocket.AddressFamily == AddressFamily.InterNetwork ?
+                        IPAddress.IPv6Loopback :
+                        IPAddress.Loopback;
+                int port = ((IPEndPoint)MainSocket.LocalEndPoint).Port;
+                IPEndPoint shadowEndPoint = new IPEndPoint(shadowAddress, port);
+
+                try
+                {
+                    _shadowSocket = new Socket(shadowAddress.AddressFamily, MainSocket.SocketType, MainSocket.ProtocolType);
+                    success = TryBindWithoutReuseAddress(_shadowSocket, shadowEndPoint, out _);
+
+                    if (success) break;
+                }
+                catch (SocketException)
+                {
+                    MainSocket.Dispose();
+                    _shadowSocket?.Dispose();
+                }
+            }
+
+            if (!success)
+            {
+                throw new Exception($"Failed to create the 'shadow' (port blocker) socket in {MaxAttempts} attempts.");
+            }
+        }
+
+        public void Dispose()
+        {
+            MainSocket.Dispose();
+            _shadowSocket.Dispose();
+        }
+
+        // Socket.Bind() auto-enables SO_REUSEADDR on Unix to allow Bind() during TIME_WAIT to emulate Windows behavior, see SystemNative_Bind() in 'pal_networking.c'.
+        // To prevent other sockets from succesfully binding to the same port port, we need to avoid this logic when binding the shadow socket.
+        // This method is doing a custom P/Invoke to bind() on Unix to achieve that.
+        private static unsafe bool TryBindWithoutReuseAddress(Socket socket, IPEndPoint endPoint, out int port)
+        {
+            if (PlatformDetection.IsWindows)
+            {
+                try
+                {
+                    socket.Bind(endPoint);
+                }
+                catch (SocketException)
+                {
+                    port = default;
+                    return false;
+                }
+
+                port = ((IPEndPoint)socket.LocalEndPoint).Port;
+                return true;
+            }
+
+            SocketAddress addr = endPoint.Serialize();
+            byte[] data = new byte[addr.Size];
+            for (int i = 0; i < data.Length; i++)
+            {
+                data[i] = addr[i];
+            }
+
+            fixed (byte* dataPtr = data)
+            {
+                int result = bind(socket.SafeHandle, (nint)dataPtr, (uint)data.Length);
+                if (result != 0)
+                {
+                    port = default;
+                    return false;
+                }
+                uint sockLen = (uint)data.Length;
+                result = getsockname(socket.SafeHandle, (nint)dataPtr, (IntPtr)(&sockLen));
+                if (result != 0)
+                {
+                    port = default;
+                    return false;
+                }
+
+                addr = new SocketAddress(endPoint.AddressFamily, (int)sockLen);
+            }
+
+            for (int i = 0; i < data.Length; i++)
+            {
+                addr[i] = data[i];
+            }
+
+            port = ((IPEndPoint)endPoint.Create(addr)).Port;
+            return true;
+
+            [Runtime.InteropServices.DllImport("libc", SetLastError = true)]
+            static extern int bind(SafeSocketHandle socket, IntPtr socketAddress, uint addrLen);
+
+            [Runtime.InteropServices.DllImport("libc", SetLastError = true)]
+            static extern int getsockname(SafeSocketHandle socket, IntPtr socketAddress, IntPtr addrLenPtr);
+        }
+    }
 }

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

@@ -195,6 +195,28 @@ await RetryHelper.ExecuteAsync(async () =>
                 }
             }, maxAttempts: 10, retryWhen: e => e is XunitException);
         }
+
+        [OuterLoop("Connection failure takes long on Windows.")]
+        [Fact]
+        public async Task Connect_WithoutListener_ThrowSocketExceptionWithAppropriateInfo()
+        {
+            using PortBlocker portBlocker = new PortBlocker(() =>
+            {
+                Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+                socket.BindToAnonymousPort(IPAddress.Loopback);
+                return socket;
+            });
+            Socket a = portBlocker.MainSocket;
+            using Socket b = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+
+            SocketException ex = await Assert.ThrowsAsync<SocketException>(() => ConnectAsync(b, a.LocalEndPoint));
+            Assert.Contains(Marshal.GetPInvokeErrorMessage(ex.NativeErrorCode), ex.Message);
+
+            if (UsesSync)
+            {
+                Assert.Contains(a.LocalEndPoint.ToString(), ex.Message);
+            }
+        }
     }
 
     public sealed class ConnectSync : Connect<SocketHelperArraySync>
@@ -215,29 +237,6 @@ public ConnectApm(ITestOutputHelper output) : base(output) {}
     public sealed class ConnectTask : Connect<SocketHelperTask>
     {
         public ConnectTask(ITestOutputHelper output) : base(output) {}
-
-        [OuterLoop]
-        [Fact]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/79820", TestPlatforms.Linux | TestPlatforms.Android)]
-        public static void Connect_ThrowSocketException_Success()
-        {
-            using (Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
-            {
-                int anonymousPort = socket.BindToAnonymousPort(IPAddress.Loopback);
-                IPEndPoint ep = new IPEndPoint(IPAddress.Loopback, anonymousPort);
-                Assert.ThrowsAsync<SocketException>(() => socket.ConnectAsync(ep));
-                try
-                {
-                    socket.Connect(ep);
-                    Assert.Fail("Socket Connect should throw SocketException in this case.");
-                }
-                catch (SocketException ex)
-                {
-                    Assert.Contains(Marshal.GetPInvokeErrorMessage(ex.NativeErrorCode), ex.Message);
-                    Assert.Contains(ep.ToString(), ex.Message);
-                }
-            }
-        }
     }
 
     public sealed class ConnectEap : Connect<SocketHelperEap>