vfs: Add SetSys() methods to Node to allow caching stuff on a node

vfs/file.go

@@ -52,6 +52,7 @@ type File struct {
 	pendingModTime    time.Time                       // will be applied once o becomes available, i.e. after file was written
 	pendingRenameFun  func(ctx context.Context) error // will be run/renamed after all writers close
 	appendMode        bool                            // file was opened with O_APPEND
+	sys               interface{}                     // user defined info to be attached here
 
 	muRW sync.Mutex // synchonize RWFileHandle.openPending(), RWFileHandle.close() and File.Remove
 }
@@ -130,7 +131,16 @@ func (f *File) osPath() string {
 
 // Sys returns underlying data source (can be nil) - satisfies Node interface
 func (f *File) Sys() interface{} {
-	return nil
+	f.mu.RLock()
+	defer f.mu.RUnlock()
+	return f.sys
+}
+
+// SetSys sets the underlying data source (can be nil) - satisfies Node interface
+func (f *File) SetSys(x interface{}) {
+	f.mu.Lock()
+	f.sys = x
+	f.mu.Unlock()
 }
 
 // Inode returns the inode number - satisfies Node interface