Add comprehensive test coverage for WaitGroup fix

Co-authored-by: ignoramous <[email protected]>

Author: Copilot

intra/netstack/waitgroup_test.go

@@ -96,6 +96,78 @@ func TestWaitGroupRaceCondition(t *testing.T) {
 	}
 }
 
+// TestStackTraceScenario tests the specific scenario from the original stack trace
+// where magiclink.Wait() is called during endpoint swapping.
+func TestStackTraceScenario(t *testing.T) {
+	// Create a temp file to simulate a TUN device
+	tmpFile, err := os.CreateTemp("", "test_tun")
+	if err != nil {
+		t.Skip("Cannot create temp file for test")
+	}
+	defer os.Remove(tmpFile.Name())
+	defer tmpFile.Close()
+
+	fd := int(tmpFile.Fd())
+	
+	// Create a magiclink endpoint
+	endpoint, err := NewEndpoint(fd, 1500, &testSink{})
+	if err != nil {
+		t.Fatalf("Failed to create endpoint: %v", err)
+	}
+	defer endpoint.Dispose()
+
+	magicLink, ok := endpoint.(*magiclink)
+	if !ok {
+		t.Fatalf("Expected magiclink, got %T", endpoint)
+	}
+
+	// Simulate the exact scenario from the stack trace:
+	// seamless.go:312>fdbased.go:413 - magiclink.Wait() calls endpoint.Wait()
+	panicked := false
+	done := make(chan struct{})
+	
+	// Start a goroutine that continuously calls Wait() like the tunnel waiter
+	go func() {
+		defer func() {
+			if r := recover(); r != nil {
+				panicked = true
+			}
+			close(done)
+		}()
+		
+		for i := 0; i < 100; i++ {
+			magicLink.Wait()
+			time.Sleep(time.Millisecond)
+		}
+	}()
+	
+	// Concurrently perform rapid endpoint swaps
+	for i := 0; i < 10; i++ {
+		tmpFile2, err := os.CreateTemp("", "test_tun2")
+		if err != nil {
+			continue
+		}
+		fd2 := int(tmpFile2.Fd())
+		
+		// Rapid swap - this should not cause WaitGroup reuse panic
+		magicLink.Swap(fd2, 1500)
+		
+		tmpFile2.Close()
+		os.Remove(tmpFile2.Name())
+		time.Sleep(time.Millisecond * 2)
+	}
+	
+	// Wait for the wait goroutine to complete
+	select {
+	case <-done:
+		if panicked {
+			t.Fatal("WaitGroup reuse panic occurred in stack trace scenario")
+		}
+	case <-time.After(time.Second * 15):
+		t.Fatal("Test timed out")
+	}
+}
+
 // testSink is a simple implementation of io.WriteCloser for testing
 type testSink struct{}