Support atomic directory promotion across platforms

internal/fileops/directory.go

@@ -8,7 +8,6 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
-	"syscall"
 )
 
 const (
@@ -16,10 +15,17 @@ const (
 	promotedFileMode      = 0o644
 )
 
+// ErrAtomicDirectoryPromotionUnsupported indicates that the current operating
+// system lacks the atomic no-replace primitive required by PromoteDirectory.
+var ErrAtomicDirectoryPromotionUnsupported = errors.New("atomic no-replace directory promotion is unsupported")
+
 // PromoteDirectory copies an existing regular-file tree into a new directory
 // and installs the complete copy atomically. It never removes the source or
 // replaces an existing destination.
 func PromoteDirectory(src, dst string) error {
+	if err := checkAtomicDirectoryPromotionSupport(); err != nil {
+		return err
+	}
 	return promoteDirectory(src, dst, renameDirectoryNoReplace)
 }
 
@@ -84,7 +90,9 @@ func promoteDirectory(src, dst string, install func(string, string) error) error
 	}
 
 	removeTemporary = false
-	_ = syncDirectory(destinationParent)
+	if err := syncDirectory(destinationParent); err != nil {
+		return fmt.Errorf("sync destination parent: %w", err)
+	}
 	return nil
 }
 
@@ -173,20 +181,6 @@ func copyRegularFile(src, dst string, inspected os.FileInfo) error {
 	return nil
 }
 
-func syncDirectory(path string) error {
-	directory, err := os.Open(path)
-	if err != nil {
-		return err
-	}
-	defer func() { _ = directory.Close() }()
-
-	err = directory.Sync()
-	if errors.Is(err, syscall.EINVAL) || errors.Is(err, syscall.ENOTSUP) {
-		return nil
-	}
-	return err
-}
-
 func pathWithin(parent, candidate string) (bool, error) {
 	absoluteParent, err := filepath.Abs(parent)
 	if err != nil {