Race condition between creating file and ioutil.ReadFile

Question

I have a golang web app end point that allows uploading image, but I find that creating a file and immediately reading it using ioutil.ReadFile will result in data to be empty. But if I call it again in some time later it will contain the data.

    r.ParseMultipartForm(32 << 20)
    file, handler, err := r.FormFile("my_input_name")

    f, err := os.OpenFile("./img_dump/"+handler.Filename, os.O_WRONLY|os.O_CREATE, 0666)
    if err != nil {
       return
    }
    // data is empty [] when first called
    data, err := ioutil.ReadFile("/myAbsolutePath/img_dump/"  + handler.Filename)
    // This does not work, data is empty
    fmt.Println(len(data))

    f.Sync()

    data, err = ioutil.ReadFile("/myAbsolutePath/img_dump/"  + handler.Filename)

    // This does not work, data is empty
    fmt.Println(len(data))

    c1 := make(chan string, 1)
    go func() {
        time.Sleep(time.Second * 5)
        c1 <- "result 1"
    }()

    go func(name string) {
        select {
        case _ = <-c1:
            data, err = ioutil.ReadFile("/myAbsolutePath/img_dump/" + name)
            fmt.Println("after sleep")
            // This only works if I don't try f.Close(), data is not empty
            fmt.Println(len(data))
        }
    }(handler.Filename)

Is there a way to avoid race condition between creating and reading, maybe a promise?

Edit: I tried Sync to flush file but same issue still exist.

You are never actually _writing_ to `f` in this snippet... – mhutter Jul 24 '17 at 07:39 — mhutter, Jul 24 '17 at 07:39

John S Perayil · Answer 1 · 2017-07-24T06:43:11.317

1

Have you tried using File.Sync ?

Sync commits the current contents of the file to stable storage. Typically, this means flushing the file system's in-memory copy of recently written data to disk.

Additional Reference

f, err := os.OpenFile("./img_dump/"+handler.Filename, os.O_WRONLY|os.O_CREATE, 0666)
if err != nil {...}

err = f.Sync()
if err != nil {...}

err = f.Close()
if err != nil {...}

data, err := ioutil.ReadFile("/myAbsolutePath/img_dump/"  + handler.Filename)

edited Jul 24 '17 at 06:43

answered Jul 24 '17 at 06:04

John S Perayil

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hmm, I tried that, seems like the same issue persist – user3591466 Jul 24 '17 at 06:24
@user3591466 f.Sync needs to be called before you use ioutil.ReadFile and also can you try closing the file using f.Close before accessing it. – John S Perayil Jul 24 '17 at 06:37

Race condition between creating file and ioutil.ReadFile

1 Answers1