fix: preserve WebDAV MOVE destination precedence

chfs-ng/internal/api/server.go

@@ -1437,10 +1437,19 @@ func (f *webdavAuthFS) Move(ctx context.Context, name, dest string, opts *webdav
 	// This matters because MOVE to an occupied destination returns 412 even
 	// when the source has disappeared, while a missing source with a free
 	// destination is reported as 403.
-	if opts == nil || !opts.NoOverwrite {
-		if err := f.authorizeOverwrite(ctx, dest); err != nil {
-			return false, err
+	destExists := false
+	if _, err := f.inner.Stat(ctx, dest); err == nil {
+		destExists = true
+	} else if !isWebDAVNotFound(err) {
+		return false, err
+	}
+	if destExists {
+		if opts == nil || !opts.NoOverwrite {
+			if err := f.authorizeOverwrite(ctx, dest); err != nil {
+				return false, err
+			}
 		}
+		return false, webdav.NewHTTPError(http.StatusPreconditionFailed, fmt.Errorf("destination exists"))
 	}
 	if _, err := f.inner.Stat(ctx, name); err != nil {
 		if os.IsNotExist(err) {
@@ -1454,6 +1463,13 @@ func (f *webdavAuthFS) Move(ctx context.Context, name, dest string, opts *webdav
 	return f.inner.Move(ctx, name, dest, opts)
 }
 
+func isWebDAVNotFound(err error) bool {
+	if err == nil {
+		return false
+	}
+	return os.IsNotExist(err) || strings.HasPrefix(err.Error(), "404 Not Found:")
+}
+
 func (f *webdavAuthFS) authorizeOverwrite(ctx context.Context, dest string) error {
 	info, err := f.inner.Stat(ctx, dest)
 	if err != nil {

chfs-ng/internal/api/webdav_test.go

@@ -24,11 +24,11 @@ func TestWebDAVAuthFSCheckTree(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	account := &auth.Account{Mask: auth.Delete, Rules: []auth.PathRule{
-		{Dir: "test", Mask: auth.Delete},
+	account := &auth.Account{Mask: auth.Read | auth.Write | auth.Delete, Rules: []auth.PathRule{
+		{Dir: "test", Mask: auth.Read | auth.Write | auth.Delete},
 		{Dir: "test/private", Mask: auth.None},
 	}}
-	fs := &webdavAuthFS{inner: webdav.LocalFileSystem(root), account: account}
+	fs := &webdavAuthFS{inner: &webdavStorageFS{store: storage.Local{Root: root}}, account: account}
 
 	if err := fs.checkTree(context.Background(), "/test", auth.Delete); err == nil {
 		t.Fatal("expected recursive delete permission check to reject inaccessible descendant")
@@ -47,7 +47,7 @@ func TestWebDAVAuthFSOverwriteRequiresDelete(t *testing.T) {
 		t.Fatal(err)
 	}
 	account := &auth.Account{Mask: auth.Write}
-	fs := &webdavAuthFS{inner: webdav.LocalFileSystem(root), account: account}
+	fs := &webdavAuthFS{inner: &webdavStorageFS{store: storage.Local{Root: root}}, account: account}
 	if err := fs.authorizeOverwrite(context.Background(), "/dst.txt"); err == nil {
 		t.Fatal("expected overwrite authorization to require delete permission")
 	}
@@ -94,6 +94,25 @@ func TestWebDAVStorageFSRejectsDestinationInsideSource(t *testing.T) {
 	}
 }
 
+func TestWebDAVAuthFSMoveMissingSourcePrecedence(t *testing.T) {
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, "existing.txt"), []byte("existing"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	account := &auth.Account{Mask: auth.Read | auth.Write | auth.Delete}
+	fs := &webdavAuthFS{inner: &webdavStorageFS{store: storage.Local{Root: root}}, account: account}
+
+	_, err := fs.Move(context.Background(), "/missing.txt", "/new.txt", &webdav.MoveOptions{})
+	if err == nil || err.Error() != "403 Forbidden: source not found" {
+		t.Fatalf("missing source to free destination: got %v", err)
+	}
+
+	_, err = fs.Move(context.Background(), "/missing.txt", "/existing.txt", &webdav.MoveOptions{})
+	if err == nil || err.Error() != "412 Precondition Failed: destination exists" {
+		t.Fatalf("missing source to occupied destination: got %v", err)
+	}
+}
+
 func TestWebDAVStorageFSCopyAndMove(t *testing.T) {
 	root1 := t.TempDir()
 	root2 := t.TempDir()