Debugging with AI

Production panic happening for 2 days. Only under load — can't reproduce locally.

Full stack trace:

panic: runtime error: invalid memory address or nil pointer dereference
goroutine 47 [running]:
main.(*OrderService).ProcessOrder(...)
    /app/service/orders.go:89 +0x2a4

Code (orders.go:85-95):

func (s *OrderService) ProcessOrder(ctx context.Context, orderID string) (*Receipt, error) {
    order, err := s.repo.FindOrder(ctx, orderID)
    if err != nil {
        return nil, err
    }
    charge, _ := s.payments.ChargeCard(ctx, order.Total, order.CustomerID)
    receipt := &Receipt{
        OrderID:  order.ID,
        ChargeID: charge.ID,
    }
    return receipt, nil
}

Expected: charge card, return receipt Actual: panics at line 89 during high load

Already tried:

• Added logs — order is non-nil when function is called
• CustomerID is validated at the handler level
• The payment processor returns rate limit errors under load

Hypothesis: charge, _ := ... is the issue but I'm not sure how to handle partial failures safely.

This Python function calculates percentage change but returns wrong results for specific inputs.

def percent_change(old_val, new_val):
    return ((new_val - old_val) / old_val) * 100

Test cases:

The only failing case is old_val=0. I need to:

1. Handle it gracefully (return None when old_val is 0)
2. Understand why it's not caught by the try/except I have elsewhere

Also: are there any other edge cases I haven't tested that could cause wrong results (not errors, just wrong math)?