add documentation for error returned by ParseRequest, use it in tests

Author: marten-seemann

integration/proxy/proxy.go

@@ -5,6 +5,7 @@ package main
 import (
 	"context"
 	"crypto/tls"
+	"errors"
 	"fmt"
 	"log"
 	"net"
@@ -175,6 +176,11 @@ func run(bindTo netip.AddrPort, remoteAddr netip.Addr, route netip.Prefix, ipPro
 	mux.HandleFunc("/vpn", func(w http.ResponseWriter, r *http.Request) {
 		req, err := connectip.ParseRequest(r, template)
 		if err != nil {
+			var perr *connectip.RequestParseError
+			if errors.As(err, &perr) {
+				w.WriteHeader(perr.HTTPStatus)
+				return
+			}
 			w.WriteHeader(http.StatusBadRequest)
 			return
 		}

request.go

@@ -40,6 +40,9 @@ type RequestParseError struct {
 func (e *RequestParseError) Error() string { return e.Err.Error() }
 func (e *RequestParseError) Unwrap() error { return e.Err }
 
+// ParseRequest parses a CONNECT-IP request.
+// The template is the URI template that clients will use to configure this proxy.
+// If an error is returned, it is a RequestParseError.
 func ParseRequest(r *http.Request, template *uritemplate.Template) (*Request, error) {
 	if len(template.Varnames()) > 0 {
 		return nil, errors.New("connect-ip-go currently does not support IP flow forwarding")