backup: allow excluding online-only cloud files

internal/fs/stat_windows.go

@@ -8,6 +8,8 @@ import (
 	"os"
 	"syscall"
 	"time"
+
+	"golang.org/x/sys/windows"
 )
 
 // extendedStat extracts info into an ExtendedFileInfo for Windows.
@@ -36,3 +38,20 @@ func extendedStat(fi os.FileInfo) *ExtendedFileInfo {
 
 	return &extFI
 }
+
+// RecallOnDataAccess checks if a file is available locally on the disk or if the file is
+// just a placeholder which must be downloaded from a remote server. This is typically used
+// in cloud syncing services (e.g. OneDrive) to prevent downloading files from cloud storage
+// until they are accessed.
+func (fi *ExtendedFileInfo) RecallOnDataAccess() (bool, error) {
+	attrs, ok := fi.sys.(*syscall.Win32FileAttributeData)
+	if !ok {
+		return false, fmt.Errorf("could not determine file attributes: %s", fi.Name)
+	}
+
+	if attrs.FileAttributes&windows.FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS > 0 {
+		return true, nil
+	}
+
+	return false, nil
+}