如何测试文件。使用os.O_RDWR和os.O_CREATE标志打开？

Question

I'm trying to write unit test for struct constructor, which may return also nil if error happens during file.Open. I don't have idea how to test/mock file error with flags: os.O_RDWR|os.O_CREATE|os.O_APPEND

I tried to check nil value inside test, but it failed.

Constructor:

type App struct {
    someField string
    log *log.Logger
}

func New() *App {
    app := &App{}
    f, err := os.OpenFile("info.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
    if err != nil {
        fmt.Printf("error opening file: %v", err)
        return nil
    }
    mw := io.MultiWriter(os.Stdout, f)
    l = log.New(mw, "APP", log.Ldate|log.LstdFlags|log.Lshortfile)
    app.log = l

    return app
}

And test for constructor:

func TestNew(t *testing.T) {
    var a App
    a = New()

    // doesn't cover
    if a == nil {
        t.Fatal("Error opening file")
    }
}

I expect to have covered error != nil, which in coverage is red:

f, err := os.OpenFile("info.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
if err != nil {
  fmt.Printf("error opening file: %v", err)
  return nil
}

Answer 1

Mocking in Go means having interfaces, if that's something you really need you might consider using something like https://github.com/spf13/afero instead of using the os package directly. This also allows you to use in-memory filesystems and other things that make testing easier.