Support atomic directory promotion across platforms

internal/fileops/sync_directory_windows.go

@@ -0,0 +1,40 @@
+//go:build windows
+
+package fileops
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+func syncDirectory(path string) error {
+	pathPointer, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return err
+	}
+	directory, err := windows.CreateFile(
+		pathPointer,
+		windows.GENERIC_WRITE,
+		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_FLAG_BACKUP_SEMANTICS,
+		0,
+	)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = windows.CloseHandle(directory) }()
+
+	err = windows.FlushFileBuffers(directory)
+	// Windows filesystems may reject flushing a directory handle even when it
+	// was opened correctly. Preserve every error except the documented forms
+	// that mean this operation is unavailable for the handle or filesystem.
+	if errors.Is(err, windows.ERROR_INVALID_FUNCTION) ||
+		errors.Is(err, windows.ERROR_INVALID_HANDLE) ||
+		errors.Is(err, windows.ERROR_NOT_SUPPORTED) {
+		return nil
+	}
+	return err
+}