For the cilantro pesto: In a food processor, combine the cilantro, olive oil, pine nuts, sugar, chile if using, garlic and lemon juice. Process to a paste. Season with salt and loosen with a little olive oil or hot water if necessary. Set aside.
For the linguine: Cook the pasta according to package instructions; drain and set aside.
Rinse the calamari rings, then transfer to paper-towel-lined plate and dry well. The drier the squid, the crispier the end result. Sprinkle with the chile flakes and a generous pinch salt and pepper.
Heat the olive oil in a large frying pan or wok over high heat. When really hot, add the calamari; fry until lightly golden and crispy, 3 to 4 minutes.
Add the drained pasta and 3 or 4 tablespoons pesto to the calamari; toss together.
Serve immediately in warmed bowls drizzled with lemon juice and olive oil and garnished with cilantro.