Add support and tests for Unix domain socket connections

This commit introduces functionality to handle Unix domain socket (UDS) connections. It includes updates to the client and test suite to validate UDS support and extends the test server to support Unix sockets. These changes ensure compatibility for environments requiring UDS communication.

client_test.go

@@ -780,6 +780,19 @@ func TestNewClient(t *testing.T) {
 			})
 		}
 	})
+	t.Run("NewClient on Unix Domain Socket", func(t *testing.T) {
+		client, err := NewClient("unix:///tmp/mail.sock")
+		if err != nil {
+			t.Fatalf("failed to create new client: %s", err)
+		}
+		if !client.useUnixSocket {
+			t.Error("Expected useUnixSocket flag to be set to true")
+		}
+		if !strings.EqualFold(client.host, "/tmp/mail.sock") {
+			t.Errorf("expected host to be set to unix socket path, expected: %s, got: %s", "/tmp/mail.sock",
+				client.host)
+		}
+	})
 }
 
 func TestClient_TLSPolicy(t *testing.T) {
@@ -2816,6 +2829,64 @@ func TestClient_DialToSMTPClientWithContext(t *testing.T) {
 			t.Fatal("expected connection to fake to fail")
 		}
 	})
+	t.Run("dial to Unix domain socket", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		PortAdder.Add(1)
+		serverPort := int(TestServerPortBase + PortAdder.Load())
+		featureSet := "250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+		props := &serverProps{
+			FeatureSet: featureSet,
+			ListenPort: serverPort,
+			UnixSocket: true,
+		}
+		go func() {
+			if err := simpleSMTPServer(ctx, t, props); err != nil {
+				t.Errorf("failed to start test server: %s", err)
+				return
+			}
+		}()
+		time.Sleep(time.Millisecond * 30)
+
+		ctxDial, cancelDial := context.WithTimeout(ctx, time.Millisecond*500)
+		t.Cleanup(cancelDial)
+		t.Cleanup(func() {
+			if err := os.RemoveAll(props.UnixSocketPath); err != nil {
+				t.Errorf("failed to remove unix socket: %s", err)
+			}
+		})
+
+		client, err := NewClient("unix://"+props.UnixSocketPath+"/server.sock", WithTLSPolicy(NoTLS))
+		if err != nil {
+			t.Fatalf("failed to create new client: %s", err)
+		}
+		smtpClient, err := client.DialToSMTPClientWithContext(ctxDial)
+		if err != nil {
+			var netErr net.Error
+			if errors.As(err, &netErr) && netErr.Timeout() {
+				t.Skip("failed to connect to the test server due to timeout")
+			}
+			t.Fatalf("failed to connect to test server: %s", err)
+		}
+		t.Cleanup(func() {
+			if err := client.CloseWithSMTPClient(smtpClient); err != nil {
+				var netErr net.Error
+				if errors.As(err, &netErr) && netErr.Timeout() {
+					t.Skip("failed to close the test server connection due to timeout")
+				}
+				t.Errorf("failed to close client: %s", err)
+			}
+		})
+		if smtpClient == nil {
+			t.Fatal("expected SMTP client, got nil")
+		}
+		if !smtpClient.HasConnection() {
+			t.Fatal("expected connection on smtp client")
+		}
+		if ok, _ := smtpClient.Extension("DSN"); !ok {
+			t.Error("expected DSN extension but it was not found")
+		}
+	})
 }
 
 func TestClient_sendSingleMsg(t *testing.T) {
@@ -3837,6 +3908,8 @@ type serverProps struct {
 	SSLListener     bool
 	IsTLS           bool
 	SupportDSN      bool
+	UnixSocket      bool
+	UnixSocketPath  string
 }
 
 // simpleSMTPServer starts a simple TCP server that resonds to SMTP commands.
@@ -3850,18 +3923,23 @@ func simpleSMTPServer(ctx context.Context, t *testing.T, props *serverProps) err
 
 	var listener net.Listener
 	var err error
-	if props.SSLListener {
-		keypair, err := tls.X509KeyPair(localhostCert, localhostKey)
-		if err != nil {
-			return fmt.Errorf("failed to read TLS keypair: %w", err)
+	switch {
+	case props.UnixSocket:
+		path, perr := os.MkdirTemp("", "go-mail-server-*")
+		if perr != nil {
+			return fmt.Errorf("failed to create temp directory: %w", perr)
+		}
+		listener, err = net.Listen("unix", path+"/server.sock")
+		props.UnixSocketPath = path
+	case props.SSLListener:
+		keypair, kerr := tls.X509KeyPair(localhostCert, localhostKey)
+		if kerr != nil {
+			return fmt.Errorf("failed to read TLS keypair: %w", kerr)
 		}
 		tlsConfig := &tls.Config{Certificates: []tls.Certificate{keypair}}
 		listener, err = tls.Listen(TestServerProto, fmt.Sprintf("%s:%d", TestServerAddr, props.ListenPort),
 			tlsConfig)
-		if err != nil {
-			t.Fatalf("failed to create TLS listener: %s", err)
-		}
-	} else {
+	default:
 		listener, err = net.Listen(TestServerProto, fmt.Sprintf("%s:%d", TestServerAddr, props.ListenPort))
 	}
 	if err != nil {