internal/relui: correct baseURL behavior

Nested http.ServeMux should be configured with the appropriate
trailing slash, but we should not use the trailing slash when using
http.StripPrefix. This simplifies ServeHTTP slightly, and corrects
behavior when handling nested mux.

For golang/go#47401

Change-Id: I2e6ab792e210a500025b8b07a28e3db67696397d
Reviewed-on: https://go-review.googlesource.com/c/build/+/363977
Trust: Alexander Rakoczy <[email protected]>
Run-TryBot: Alexander Rakoczy <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: toothrot

internal/relui/web.go

@@ -17,6 +17,7 @@ import (
 	"net/http"
 	"net/url"
 	"path"
+	"strings"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v4/pgxpool"
@@ -48,6 +49,8 @@ type Server struct {
 	m       *http.ServeMux
 	w       *Worker
 	baseURL *url.URL
+	// mux used if baseURL is set
+	bm *http.ServeMux
 
 	homeTmpl        *template.Template
 	newWorkflowTmpl *template.Template
@@ -70,19 +73,21 @@ func NewServer(p *pgxpool.Pool, w *Worker, baseURL *url.URL) *Server {
 	s.m.Handle("/workflows/create", http.HandlerFunc(s.createWorkflowHandler))
 	s.m.Handle("/workflows/new", http.HandlerFunc(s.newWorkflowHandler))
 	s.m.Handle("/", fileServerHandler(static, http.HandlerFunc(s.homeHandler)))
+	if baseURL != nil && baseURL.Path != "/" && baseURL.Path != "" {
+		nosuffix := strings.TrimSuffix(baseURL.Path, "/")
+		s.bm = new(http.ServeMux)
+		s.bm.Handle(nosuffix+"/", http.StripPrefix(nosuffix, s.m))
+		s.bm.Handle("/", s.m)
+	}
 	return s
 }
 
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	if s.baseURL == nil || s.baseURL.Path == "/" {
-		s.m.ServeHTTP(w, r)
+	if s.bm != nil {
+		s.bm.ServeHTTP(w, r)
 		return
 	}
-	http.StripPrefix(s.baseURL.Path, s.m)
-}
-
-func (s *Server) Serve(port string) error {
-	return http.ListenAndServe(":"+port, s.m)
+	s.m.ServeHTTP(w, r)
 }
 
 func (s *Server) BaseLink(target string) string {