fix: harden exclusive file creation and webdav writes

chfs-ng/internal/storage/storage.go

@@ -30,6 +30,10 @@ type Storage interface {
 	List(path string) ([]os.FileInfo, error)
 }
 
+type ExclusiveCreator interface {
+	CreateExclusive(path string) (*os.File, error)
+}
+
 type Local struct{ Root string }
 
 func (s Local) real(path string) (string, error) {
@@ -95,6 +99,17 @@ func (s Local) Create(path string) (*os.File, error) {
 	}
 	return os.Create(p)
 }
+
+func (s Local) CreateExclusive(path string) (*os.File, error) {
+	p, e := s.real(path)
+	if e != nil {
+		return nil, e
+	}
+	if e = os.MkdirAll(filepath.Dir(p), 0755); e != nil {
+		return nil, e
+	}
+	return os.OpenFile(p, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0644)
+}
 func (s Local) Remove(path string) error {
 	p, e := s.real(path)
 	if e == nil {