web/httputil/response.go

package httputil

import (
	"encoding/json"
	"errors"
	"net/http"

	"code.nochebuena.dev/einherjar/contracts/logging"
	"code.nochebuena.dev/einherjar/core/xerrors"
)

// JSON encodes v as JSON and writes it with the given status code.
// Sets Content-Type: application/json.
func JSON(w http.ResponseWriter, status int, v any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	_ = json.NewEncoder(w).Encode(v)
}

// NoContent writes a 204 No Content response.
func NoContent(w http.ResponseWriter) {
	w.WriteHeader(http.StatusNoContent)
}

// Error is the centralized error handler. It logs at the appropriate level and
// writes a standardized JSON error body.
//
// Log level is derived from the HTTP status:
//   - 5xx → Error   (unexpected server failure; logz auto-enriches with error_code and context fields)
//   - 4xx → Warn    (client mistake — not a server failure)
//   - 499 → Info    (client cancelled the request intentionally)
//
// The response body always contains code and message; platform_code and context
// fields attached via [xerrors.Err.WithContext] are included when present.
func Error(logger logging.Logger, w http.ResponseWriter, r *http.Request, err error) {
	reqLogger := logger.WithContext(r.Context())

	var xe *xerrors.Err
	switch {
	case err == nil:
		reqLogger.Error("handler error", xerrors.Internal("nil error passed to httputil.Error"))
	case errors.As(err, &xe):
		status := codeToStatus(xe.Code())
		switch {
		case status >= 500:
			reqLogger.Error("handler error", err)
		case status == 499:
			reqLogger.Info("handler: request cancelled", "error_code", string(xe.Code()))
		default:
			args := []any{"error_code", string(xe.Code()), "status", status}
			for k, v := range xe.Fields() {
				args = append(args, k, v)
			}
			reqLogger.Warn("handler error", args...)
		}
	default:
		reqLogger.Error("handler error", err)
	}

	writeError(w, err)
}

// writeError writes the HTTP error response without logging.
// Used by [HandlerFunc].ServeHTTP where no logger is in scope.
func writeError(w http.ResponseWriter, err error) {
	var xe *xerrors.Err
	if errors.As(err, &xe) {
		JSON(w, codeToStatus(xe.Code()), errorBody(string(xe.Code()), xe.PlatformCode(), xe.Message(), xe.Fields()))
		return
	}
	JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "", "internal server error", nil))
}

func errorBody(code, platformCode, message string, fields map[string]any) map[string]any {
	m := map[string]any{
		"code":    code,
		"message": message,
	}
	if platformCode != "" {
		m["platform_code"] = platformCode
	}
	for k, v := range fields {
		m[k] = v
	}
	return m
}

func codeToStatus(code xerrors.Code) int {
	switch code {
	case xerrors.ErrInvalidInput, xerrors.ErrOutOfRange:
		return http.StatusBadRequest
	case xerrors.ErrUnauthorized:
		return http.StatusUnauthorized
	case xerrors.ErrPermissionDenied:
		return http.StatusForbidden
	case xerrors.ErrNotFound:
		return http.StatusNotFound
	case xerrors.ErrAlreadyExists, xerrors.ErrAborted:
		return http.StatusConflict
	case xerrors.ErrGone:
		return http.StatusGone
	case xerrors.ErrPreconditionFailed:
		return http.StatusPreconditionFailed
	case xerrors.ErrRateLimited:
		return http.StatusTooManyRequests
	case xerrors.ErrCancelled:
		return 499
	case xerrors.ErrInternal, xerrors.ErrDataLoss:
		return http.StatusInternalServerError
	case xerrors.ErrNotImplemented:
		return http.StatusNotImplemented
	case xerrors.ErrUnavailable:
		return http.StatusServiceUnavailable
	case xerrors.ErrDeadlineExceeded:
		return http.StatusGatewayTimeout
	default:
		return http.StatusInternalServerError
	}
}