fix: harden exclusive file creation and webdav writes

chfs-ng/internal/storage/multipath.go

@@ -107,6 +107,14 @@ func (m Multi) Create(p string) (*os.File, error) {
 	}
 	return r.Create(rel)
 }
+
+func (m Multi) CreateExclusive(p string) (*os.File, error) {
+	r, rel, ok := m.find(p)
+	if !ok {
+		return nil, os.ErrNotExist
+	}
+	return r.CreateExclusive(rel)
+}
 func (m Multi) Remove(p string) error {
 	r, rel, ok := m.find(p)
 	if !ok || rel == "" {

chfs-ng/internal/storage/multipath_test.go

@@ -46,3 +46,44 @@ func TestMultiRemoveCannotRemoveVirtualRoot(t *testing.T) {
 		t.Fatal("expected virtual root removal to fail")
 	}
 }
+
+func TestLocalCreateExclusiveRejectsExistingFile(t *testing.T) {
+	root := t.TempDir()
+	st := Local{Root: root}
+	if err := os.WriteFile(filepath.Join(root, "x.txt"), []byte("old"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	f, err := st.CreateExclusive("x.txt")
+	if !os.IsExist(err) || f != nil {
+		t.Fatalf("expected EEXIST, file=%v err=%v", f, err)
+	}
+	got, err := os.ReadFile(filepath.Join(root, "x.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "old" {
+		t.Fatalf("existing file changed: %q", got)
+	}
+}
+
+func TestLocalCreateExclusiveCreatesMissingFile(t *testing.T) {
+	st := Local{Root: t.TempDir()}
+	f, err := st.CreateExclusive("nested/x.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.WriteString("new"); err != nil {
+		f.Close()
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(filepath.Join(st.Root, "nested", "x.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "new" {
+		t.Fatalf("unexpected content: %q", got)
+	}
+}

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 {