fix: serve local files with an empty mount prefix

local_file.go

@@ -27,11 +27,10 @@ func LocalFile(root string, indexes bool) *localFileSystem {
 }
 
 func (l *localFileSystem) Exists(prefix, path string) bool {
-	// Check if path starts with prefix
-	p := strings.TrimPrefix(path, prefix)
-	if len(p) >= len(path) {
+	if !strings.HasPrefix(path, prefix) {
 		return false
 	}
+	p := strings.TrimPrefix(path, prefix)
 
 	name := filepath.Join(l.root, p)
 	stats, err := os.Stat(name)

local_file_test.go

@@ -1,12 +1,14 @@
 package static
 
 import (
+	"net/http"
 	"os"
 	"path/filepath"
 	"testing"
 
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestLocalFile(t *testing.T) {
@@ -34,3 +36,89 @@ func TestLocalFile(t *testing.T) {
 	w = PerformRequest(router, "GET", "/")
 	assert.Contains(t, w.Body.String(), `<a href="`+filename)
 }
+
+func TestLocalFileMountPrefixes(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.Mkdir(filepath.Join(dir, "nested"), 0o700))
+	require.NoError(t, os.Mkdir(filepath.Join(dir, "plain"), 0o700))
+	for name, body := range map[string]string{
+		"file.txt":          "static file",
+		INDEX:               "root index",
+		"nested/index.html": "nested index",
+		"plain/file.txt":    "directory file",
+	} {
+		require.NoError(t, os.WriteFile(filepath.Join(dir, name), []byte(body), 0o600))
+	}
+
+	for _, mount := range []struct {
+		name   string
+		prefix string
+	}{
+		{"empty", ""},
+		{"root", "/"},
+		{"nested", "/assets"},
+	} {
+		t.Run(mount.name, func(t *testing.T) {
+			prefix := mount.prefix
+			for _, indexes := range []bool{false, true} {
+				name := "without_listing"
+				if indexes {
+					name = "with_listing"
+				}
+				t.Run(name, func(t *testing.T) {
+					router := gin.New()
+					router.Use(Serve(prefix, LocalFile(dir, indexes)))
+					router.NoRoute(func(c *gin.Context) {
+						c.String(http.StatusNotFound, "fallback")
+					})
+
+					for _, tt := range []struct {
+						name   string
+						method string
+						path   string
+						status int
+						body   string
+					}{
+						{"file", http.MethodGet, "/file.txt", http.StatusOK, "static file"},
+						{"head", http.MethodHead, "/file.txt", http.StatusOK, ""},
+						{"root_index", http.MethodGet, "/", http.StatusOK, "root index"},
+						{"nested_index", http.MethodGet, "/nested/", http.StatusOK, "nested index"},
+						{"missing_file", http.MethodGet, "/missing.txt", http.StatusNotFound, "fallback"},
+					} {
+						t.Run(tt.name, func(t *testing.T) {
+							path := tt.path
+							if prefix != "/" {
+								path = prefix + path
+							}
+							w := PerformRequest(router, tt.method, path)
+							assert.Equal(t, tt.status, w.Code)
+							assert.Equal(t, tt.body, w.Body.String())
+						})
+					}
+
+					t.Run("directory_without_index", func(t *testing.T) {
+						path := "/plain/"
+						if prefix != "/" {
+							path = prefix + path
+						}
+						w := PerformRequest(router, http.MethodGet, path)
+						if indexes {
+							assert.Contains(t, w.Body.String(), `<a href="file.txt">`)
+						} else {
+							assert.Equal(t, http.StatusNotFound, w.Code)
+							assert.Equal(t, "fallback", w.Body.String())
+						}
+					})
+
+					if prefix == "/assets" {
+						t.Run("outside_prefix", func(t *testing.T) {
+							w := PerformRequest(router, http.MethodGet, "/file.txt")
+							assert.Equal(t, http.StatusNotFound, w.Code)
+							assert.Equal(t, "fallback", w.Body.String())
+						})
+					}
+				})
+			}
+		})
+	}
+}