Support atomic directory promotion across platforms

internal/fileops/sync_directory_unix.go

@@ -0,0 +1,25 @@
+//go:build linux || darwin
+
+package fileops
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+func syncDirectory(path string) error {
+	directory, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = directory.Close() }()
+
+	err = directory.Sync()
+	// Some Unix filesystems do not implement directory syncing. Only their
+	// explicit unsupported-operation errors are safe to treat as best effort.
+	if errors.Is(err, syscall.EINVAL) || errors.Is(err, syscall.ENOTSUP) {
+		return nil
+	}
+	return err
+}