Public demo diagnosis

Summary

Nil pointer dereference in the Cart.Total method.

Root cause

The crash occurred in the 'Total' method of the 'ain.Cart' struct. The stack trace indicates that the receiver for the method (the pointer to the Cart struct) was nil (0x0) when the method was called. This typically happens when a pointer to a struct is not initialized before invoking its methods, or when a function returning a pointer returns 'nil' upon error, and the caller fails to check the error before accessing the receiver.

File / line

/app/billing/charge.go:114

Suggested fix

```go
func (c *Cart) Total() float64 {
	if c == nil {
		return 0.0
	}
	//... existing logic
}
```

Prevention

1. Implement strict nil-receiver checks for critical business logic methods. 2. Enforce linting rules (e.g., errcheck, staticcheck) to ensure error values from factory functions are handled before dereferencing. 3. Use unit tests to verify behavior when pointers are nil to ensure graceful failure rather than panics.