Support atomic directory promotion across platforms

internal/fileops/directory_unsupported_test.go

@@ -0,0 +1,37 @@
+//go:build !linux && !darwin && !windows
+
+package fileops
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestPromoteDirectoryFailsBeforeCreatingTemporaryTree(t *testing.T) {
+	root := t.TempDir()
+	src := filepath.Join(root, "source")
+	dst := filepath.Join(root, "destination")
+	if err := os.Mkdir(src, 0o755); err != nil {
+		t.Fatalf("Mkdir(source) error = %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(src, "value.txt"), []byte("source"), 0o644); err != nil {
+		t.Fatalf("WriteFile(source) error = %v", err)
+	}
+
+	err := PromoteDirectory(src, dst)
+	if !errors.Is(err, ErrAtomicDirectoryPromotionUnsupported) {
+		t.Fatalf("PromoteDirectory() error = %v, want unsupported capability", err)
+	}
+	entries, readErr := os.ReadDir(root)
+	if readErr != nil {
+		t.Fatalf("ReadDir(root) error = %v", readErr)
+	}
+	for _, entry := range entries {
+		if entry.Name() == filepath.Base(dst) || strings.HasPrefix(entry.Name(), ".destination.tmp-") {
+			t.Fatalf("unsupported promotion created %q", entry.Name())
+		}
+	}
+}