handler.go

package handler

type (
	options struct {
		MaxPayload int64
	}
	// HandlerOptions is a function that configures the handler.
	HandlerOptions func(*options)
)

// WithMaxPayload sets the maximum payload size for the handler.
func WithMaxPayload(max int64) HandlerOptions {
	return func(o *options) {
		o.MaxPayload = max
	}
}

// MakeHandler returns an http.HandlerFunc that decodes the request body into
// the provided type, calls the provided handler, and encodes the result as JSON.
func MakeHandler[R any, S any](handle func(context.Context, R) (S, error), opts ...HandlerOptions) http.HandlerFunc {
	opt := &options{MaxPayload: 0}
	for _, o := range opts {
		o(opt)
	}
	return func(w http.ResponseWriter, r *http.Request) {
		if m := opt.MaxPayload; m > 0 && r.ContentLength > m {
			log.Printf("Payload too large: %d, limited to %d", r.ContentLength, m)
			http.Error(w, "Payload too large", http.StatusRequestEntityTooLarge)
			return
		}
		var p R
		if err := json.NewDecoder(r.Body).Decode(&p); err != nil {
			log.Printf("json.NewDecoder: %v", err)
			http.Error(w, "Error parsing request", http.StatusBadRequest)
			return
		}
		result, err := handle(r.Context(), p)
		if err != nil {
			http.Error(w, "Error processing request", http.StatusInternalServerError)
			return
		}
		w.Header().Add("Content-Type", "application/json")
		if err = json.NewEncoder(w).Encode(result); err != nil {
			log.Printf("json.NewEncoder: %v", err)
			http.Error(w, `{"error":"internal server error"}`, http.StatusInternalServerError)
			return
		}
		w.WriteHeader(http.StatusOK)
	}
}