feat: handle using root as the path of file server (#4255)

rest/internal/fileserver/filehandler.go

@@ -3,18 +3,19 @@ package fileserver
 import (
 	"net/http"
 	"strings"
+	"sync"
 )
 
 // Middleware returns a middleware that serves files from the given file system.
 func Middleware(path string, fs http.FileSystem) func(http.HandlerFunc) http.HandlerFunc {
 	fileServer := http.FileServer(fs)
-	pathWithTrailSlash := ensureTrailingSlash(path)
 	pathWithoutTrailSlash := ensureNoTrailingSlash(path)
+	canServe := createServeChecker(path, fs)
 
 	return func(next http.HandlerFunc) http.HandlerFunc {
 		return func(w http.ResponseWriter, r *http.Request) {
-			if r.Method == http.MethodGet && strings.HasPrefix(r.URL.Path, pathWithTrailSlash) {
-				r.URL.Path = strings.TrimPrefix(r.URL.Path, pathWithoutTrailSlash)
+			if canServe(r) {
+				r.URL.Path = r.URL.Path[len(pathWithoutTrailSlash):]
 				fileServer.ServeHTTP(w, r)
 			} else {
 				next(w, r)
@@ -23,6 +24,44 @@ func Middleware(path string, fs http.FileSystem) func(http.HandlerFunc) http.Han
 	}
 }
 
+func createFileChecker(fs http.FileSystem) func(string) bool {
+	var lock sync.RWMutex
+	fileChecker := make(map[string]bool)
+
+	return func(path string) bool {
+		lock.RLock()
+		exist, ok := fileChecker[path]
+		lock.RUnlock()
+		if ok {
+			return exist
+		}
+
+		lock.Lock()
+		defer lock.Unlock()
+
+		file, err := fs.Open(path)
+		exist = err == nil
+		fileChecker[path] = exist
+		if err != nil {
+			return false
+		}
+
+		_ = file.Close()
+		return true
+	}
+}
+
+func createServeChecker(path string, fs http.FileSystem) func(r *http.Request) bool {
+	pathWithTrailSlash := ensureTrailingSlash(path)
+	fileChecker := createFileChecker(fs)
+
+	return func(r *http.Request) bool {
+		return r.Method == http.MethodGet &&
+			strings.HasPrefix(r.URL.Path, pathWithTrailSlash) &&
+			fileChecker(r.URL.Path[len(pathWithTrailSlash):])
+	}
+}
+
 func ensureTrailingSlash(path string) string {
 	if strings.HasSuffix(path, "/") {
 		return path