Add notification feature documentation and unit tests

- Document the --notification flag usage in README with examples
- Add unit tests for NotificationSender

Assisted by: Claude (Anthropic AI)

Signed-off-by: Gunjan Vyas <vyasgun20@gmail.com>

README.md

@@ -210,5 +210,41 @@ This is the same behaviour as [slirp](https://wiki.qemu.org/index.php/Documentat
 3. The tap device receives the packets and injects them in the kernel.
 4. The http server receives the request and send back the response.
 
-### Development
+## Notifications
+
+`gvproxy` can send notifications over a unix socket about hypervisor
+connections, and about network switch connections/disconnections.
+
+These notifications can be enabled with the `--notification unix://$NOTIF_PATH` argument.
+`$NOTIF_PATH` is the path to a listening unix socket.
+`gvproxy` will then send json messages on this socket.
+
+To receive notifications, 2 terminals need to be opened.
+
+### Terminal 1:
+
+```bash
+$ nc -k -U -l /tmp/notification.sock
+```
+
+### Terminal 2:
+
+```bash
+$ gvproxy --notification unix:///tmp/notification.sock
+```
+
+The terminal where `nc` is running will print:
+```json
+{"notification_type":"ready"}
+{"notification_type":"connection_established","mac_address":"5a:94:ef:e4:0c:ee"}
+{"notification_type":"connection_closed","mac_address":"5a:94:ef:e4:0c:ee"}
+```
+
+Notification types:
+- `ready` - sent when gvproxy is ready to accept connections
+- `connection_established` - sent when a VM connects (includes `mac_address`)
+- `connection_closed` - sent when a VM disconnects (includes `mac_address`)
+- `hypervisor_error` - sent on hypervisor errors
+
+## Development
 Developers who want to work on gvisor-tap-vsock should visit the [Development](./DEVELOPMENT.md) document.

pkg/notification/sender_test.go

@@ -0,0 +1,91 @@
+package notification
+
+import (
+	"context"
+	"encoding/json"
+	"net"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/containers/gvisor-tap-vsock/pkg/types"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewNotificationSender_EmptySocket(t *testing.T) {
+	sender := NewNotificationSender("")
+	assert.Nil(t, sender.notificationCh)
+	assert.Empty(t, sender.socket)
+}
+
+func TestNewNotificationSender_NonEmptySocket(t *testing.T) {
+	sender := NewNotificationSender("test.sock")
+	assert.NotNil(t, sender)
+	assert.Equal(t, "test.sock", sender.socket)
+	assert.NotNil(t, sender.notificationCh)
+}
+
+func TestNotificationSender_NilChannel(t *testing.T) {
+	sender := NewNotificationSender("")
+	assert.Nil(t, sender.notificationCh)
+
+	// should not panic
+	sender.Send(types.NotificationMessage{
+		NotificationType: types.ConnectionEstablished,
+		MacAddress:       "5a:94:ef:e4:0c:ee",
+	})
+}
+
+func TestNotificationSender_Success(t *testing.T) {
+	tmpDir := t.TempDir()
+	socketPath := filepath.Join(tmpDir, "test.sock")
+	listener, err := net.Listen("unix", socketPath)
+	assert.NoError(t, err)
+	defer listener.Close()
+
+	expectedNotifications := []types.NotificationMessage{
+		{
+			NotificationType: types.Ready,
+		},
+		{
+			NotificationType: types.ConnectionEstablished,
+			MacAddress:       "5a:94:ef:e4:0c:ee",
+		},
+		{
+			NotificationType: types.HypervisorError,
+		},
+		{
+			NotificationType: types.ConnectionClosed,
+			MacAddress:       "5a:94:ef:e4:0c:ee",
+		},
+	}
+
+	for _, expectedNotification := range expectedNotifications {
+		t.Run(string(expectedNotification.NotificationType), func(t *testing.T) {
+			done := make(chan struct{})
+			go func() {
+				defer close(done)
+				conn, err := listener.Accept()
+				assert.NoError(t, err)
+				assert.NotNil(t, conn)
+				defer conn.Close()
+
+				dec := json.NewDecoder(conn)
+				var notification types.NotificationMessage
+				assert.NoError(t, dec.Decode(&notification))
+				assert.Equal(t, expectedNotification.NotificationType, notification.NotificationType)
+				assert.Equal(t, expectedNotification.MacAddress, notification.MacAddress)
+			}()
+
+			sender := NewNotificationSender(socketPath)
+			go sender.Start(context.Background())
+
+			sender.Send(expectedNotification)
+			select {
+			case <-done:
+			case <-time.After(2 * time.Second):
+				t.Fatal("timeout waiting for notification")
+			}
+		})
+	}
+}