fix: stabilize static FS serving and session pool reuse

- Static middleware: use empty Root instead of "." for io/fs.FS so fasthttp's pathToFilePath returns "" instead of ".", avoiding the openIndexFile bug that builds invalid "./index.html" paths on all platforms without requiring a vendor patch.
- SendFile: remove redundant Root = "." override for io/fs.FS since Root is already "" with AllowEmptyRoot enabled.
- Session: call Reset() in acquireData() after pool Get to prevent dirty data objects from leaking keys across parallel tests.

Author: ReneWerner87

middleware/session/data.go

@@ -33,6 +33,7 @@ var dataPool = sync.Pool{
 func acquireData() *data {
 	obj := dataPool.Get()
 	if d, ok := obj.(*data); ok {
+		d.Reset()
 		return d
 	}
 	// Handle unexpected type in the pool

middleware/static/static.go

@@ -163,11 +163,12 @@ func New(root string, cfg ...Config) fiber.Handler {
 				prefixLen--
 			}
 
-			// For io/fs.FS, Root must be a directory path (not a filename).
-			// Use "." as Root and let PathRewrite handle file-root cases.
+			// For io/fs.FS, Root must be empty so fasthttp's pathToFilePath
+			// returns clean relative paths without prefixing the root.
+			// PathRewrite already handles file-root and subdirectory cases.
 			fsRoot := root
 			if config.FS != nil {
-				fsRoot = "."
+				fsRoot = ""
 			}
 
 			fileServer := &fasthttp.FS{

res.go

@@ -835,10 +835,6 @@ func (r *DefaultRes) SendFile(file string, config ...SendFile) error {
 			},
 		}
 
-		if cfg.FS != nil {
-			fasthttpFS.Root = "."
-		}
-
 		sf := &sendFileStore{
 			config:  cfg,
 			handler: fasthttpFS.NewRequestHandler(),