You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: cacheOnReadFs.go
    path of file: ./repos/goplantuml/vendor/github.com/spf13/afero
    
    the code of the file until where you have to start completion: package afero

import (
	"os"
	"syscall"
	"time"
)

// If the cache duration is 0, cache time will be unlimited, i.e. once
// a file is in the layer, the base will never be read again for this file.
//
// For cache times greater than 0, the modification time of a file is
// checked. Note that a lot of file system implementations only allow a
// resolution of a second for timestamps... or as the godoc for os.Chtimes()
// states: "The underlying filesystem may truncate or round the values to a
// less precise time unit."
//
// This caching union will forward all write calls also to the base file
// system first. To prevent writing to the ba
    
func (u *CacheOnReadFs) OpenFile(name string, flag int, perm os.FileMode) (File, error) {
	st, _, err := u.cacheStatus(name)
	if err != nil {
		return nil, err
	}
	switch st {
	case cacheLocal, cacheHit:
	default:
		if err := u.copyFileToLayer(name, flag, perm); err != nil {
			return nil, err
		}
	}
	if flag&(os.O_WRONLY|syscall.O_RDWR|os.O_APPEND|os.O_CREATE|os.O_TRUNC) != 0 {
		bfi, err := u.base.OpenFile(name, flag, perm)
		if err != nil {
			return nil, err
		}
		lfi, err := u.layer.OpenFile(name, flag, perm)
		if err != nil {
			bfi.Close() // oops, what if O_TRUNC was set and file opening in the layer failed...?
			return nil, err
		}
		return &UnionFile{Base: bfi, Layer: lfi}, nil
	}
	return u.layer.OpenFile(name, flag, perm)
}