通过FTP下载文件后将文件写入磁盘时出现问题

The file that is written to disk is empty, but the reader is not. I do not understand where the issue is. I tried to play with a Buffer and then String() method and I can confirm that the content is fine, but using the Read() method of this library is not working.

The library I use is github.com/jlaffaye/ftp

// pullFileByFTP
func pullFileByFTP(fileID, server string, port int64, username, password, path, file string) error {
    // Connect to the server
    client, err := ftp.Dial(fmt.Sprintf("%s:%d", server, port))
    if err != nil {
        return err
    }

    // Log in the server
    err = client.Login(username, password)
    if err != nil {
        return err
    }

    // Retrieve the file
    reader, err := client.Retr(fmt.Sprintf("%s%s", path, file))
    if err != nil {
        return err
    }

    // Read the file
    var srcFile []byte
    _, err = reader.Read(srcFile)
    if err != nil {
        return err
    }

    // Create the destination file
    dstFile, err := os.Create(fmt.Sprintf("%s/%s", shared.TmpDir, fileID))
    if err != nil {
        return fmt.Errorf("Error while creating the destination file : %s", err)
    }
    defer dstFile.Close()

    // Copy the file
    dstFile.Write(srcFile)

    return nil
}

You are using Read and Write wrong:

var srcFile []byte
_, err = reader.Read(srcFile)

Read puts the read bytes into its argument. Since srcFile is a nil slice, this instructs the reader to read zero bytes. Use ioutil.ReadAll to read all bytes.

Next up is your use of Write. Write(b) writes up to len(b) bytes, but not necessarily all of it. You must check the return values and call Write repeatedly if necessary.

However, in your case you just want to connect an io.Reader (*Response implements io.Reader) and io.Writer (*os.File). That's what io.Copy is for:

reader, err := client.Retr(path + file)
dstFile, err := ioutil.TempFile("", fileID)

_, err := io.Copy(dstFile, reader)
err := dstFile.Close()