Handle index.html correctly in custom served folders (#3168)

* getStringMapString return nil if key not found
* Refactor custom routes. Handle /index.html

internal/api/routes_custom.go

@@ -0,0 +1,35 @@
+package api
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/go-chi/chi"
+	"github.com/stashapp/stash/internal/manager/config"
+)
+
+type customRoutes struct {
+	servedFolders config.URLMap
+}
+
+func (rs customRoutes) Routes() chi.Router {
+	r := chi.NewRouter()
+
+	r.HandleFunc("/*", func(w http.ResponseWriter, r *http.Request) {
+		r.URL.Path = strings.Replace(r.URL.Path, "/custom", "", 1)
+
+		// http.FileServer redirects to / if the path ends with index.html
+		r.URL.Path = strings.TrimSuffix(r.URL.Path, "/index.html")
+
+		// map the path to the applicable filesystem location
+		var dir string
+		r.URL.Path, dir = rs.servedFolders.GetFilesystemLocation(r.URL.Path)
+		if dir != "" {
+			http.FileServer(http.Dir(dir)).ServeHTTP(w, r)
+		} else {
+			http.NotFound(w, r)
+		}
+	})
+
+	return r
+}

internal/api/server.go

@@ -231,18 +231,9 @@ func Start() error {
 	// Serve static folders
 	customServedFolders := c.GetCustomServedFolders()
 	if customServedFolders != nil {
-		r.HandleFunc("/custom/*", func(w http.ResponseWriter, r *http.Request) {
-			r.URL.Path = strings.Replace(r.URL.Path, "/custom", "", 1)
-
-			// map the path to the applicable filesystem location
-			var dir string
-			r.URL.Path, dir = customServedFolders.GetFilesystemLocation(r.URL.Path)
-			if dir != "" {
-				http.FileServer(http.Dir(dir)).ServeHTTP(w, r)
-			} else {
-				http.NotFound(w, r)
-			}
-		})
+		r.Mount("/custom", customRoutes{
+			servedFolders: customServedFolders,
+		}.Routes())
 	}
 
 	customUILocation := c.GetCustomUILocation()

internal/manager/config/config.go

@@ -466,7 +466,15 @@ func (i *Instance) getStringMapString(key string) map[string]string {
 	i.RLock()
 	defer i.RUnlock()
 
-	return i.viper(key).GetStringMapString(key)
+	ret := i.viper(key).GetStringMapString(key)
+
+	// GetStringMapString returns an empty map regardless of whether the
+	// key exists or not.
+	if len(ret) == 0 {
+		return nil
+	}
+
+	return ret
 }
 
 type StashConfig struct {