无法覆盖文件内容

Question

I've got trouble overwriting a files content with zeros. The problem is that the very last byte of the original file remains, even when I exceed its size by 100 bytes. Someone got an idea what I'm missing?

func (h PostKey) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    f, err := os.Create("received.dat")
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
    defer f.Close()

    _, err = io.Copy(f, r.Body)
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
        return
    }

    // Retrieve filesize
    size, _ := f.Seek(0, 1)
    zeroFilled := make([]byte, size + 100)
    n, err := f.WriteAt(zeroFilled, 0)
    if err != nil {
        return
    }

    fmt.Printf("Size: %d
", size) // prints 13
    fmt.Printf("Bytes written: %d
", n) // prints 113
}

Answer 1

I was writing to the file and trying to overwrite it in the same context, and so parts of the first write operation were still in memory and not yet written to the disk. By using f.Sync() to flush everything after copying the bodys content I was able to fix the issue.