Support atomic directory promotion across platforms

internal/fileops/rename_noreplace_supported_test.go

@@ -0,0 +1,26 @@
+//go:build linux || darwin || windows
+
+package fileops
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestRenameDirectoryNoReplacePreservesExistingDestination(t *testing.T) {
+	root := t.TempDir()
+	src := filepath.Join(root, "source")
+	dst := filepath.Join(root, "destination")
+	mustWriteFile(t, filepath.Join(src, "value.txt"), []byte("source"), 0o644)
+	mustWriteFile(t, filepath.Join(dst, "value.txt"), []byte("existing"), 0o644)
+
+	if err := renameDirectoryNoReplace(src, dst); err == nil {
+		t.Fatal("renameDirectoryNoReplace() error = nil, want existing destination failure")
+	}
+	assertFileBytes(t, filepath.Join(src, "value.txt"), []byte("source"))
+	assertFileBytes(t, filepath.Join(dst, "value.txt"), []byte("existing"))
+	if info, err := os.Stat(src); err != nil || !info.IsDir() {
+		t.Fatalf("source directory was not preserved: info=%v err=%v", info, err)
+	}
+}