Function proxyHTTP

middleware/proxy.go:422–449

(c *echo.Context, tgt *ProxyTarget, config ProxyConfig)

const StatusCodeContextCanceled = 499

func proxyHTTP(c *echo.Context, tgt *ProxyTarget, config ProxyConfig) http.Handler {
	proxy := httputil.NewSingleHostReverseProxy(tgt.URL)
	proxy.ErrorHandler = func(resp http.ResponseWriter, req *http.Request, err error) {
		desc := tgt.URL.String()
		if tgt.Name != "" {
			desc = fmt.Sprintf("%s(%s)", tgt.Name, tgt.URL.String())
		}
		// If the client canceled the request (usually by closing the connection), we can report a
		// client error (4xx) instead of a server error (5xx) to correctly identify the situation.
		// The Go standard library (at of late 2020) wraps the exported, standard
		// context. Canceled error with unexported garbage value requiring a substring check, see
		// https://github.com/golang/go/blob/6965b01ea248cabb70c3749fd218b36089a21efb/src/net/net.go#L416-L430
		// From Caddy https://github.com/caddyserver/caddy/blob/afa778ae05503f563af0d1015cdf7e5e78b1eeec/modules/caddyhttp/reverseproxy/reverseproxy.go#L1352
		if errors.Is(err, context.Canceled) || strings.Contains(err.Error(), "operation was canceled") {
			httpError := echo.NewHTTPError(StatusCodeContextCanceled, "client closed connection").Wrap(err)
			c.Set("_error", httpError)
		} else {
			httpError := echo.NewHTTPError(
				http.StatusBadGateway,
				"remote server unreachable, could not proxy request",
			).Wrap(fmt.Errorf("server: %s, err: %w", desc, err))
			c.Set("_error", httpError)
		}
	}
	proxy.Transport = config.Transport
	proxy.ModifyResponse = config.ModifyResponse
	return proxy
}