Get the amount of bytes streamed from an io.Reader Read-CodePudding

Here is a snippet of my code that does a GET request, and streams the response into cmd.Stdin.

resp, err = httpClient.Get(url)
if err != nil {
    err = errors.Wrap(err, "HTTP request failed")
    return
}
reader = bufio.NewReader(resp.Body)

args = append(args, "-") // Keep listening on stdin for file data
cmd := exec.Command("exiftool", args...)

stdout, err := cmd.StdoutPipe()
if err != nil {
    return
}
cmd.Stdin = reader

err = cmd.Start()
if err != nil {
    return
}

I want to know how much data is streamed by the time it finishes executing.

So what I need is to capture what is being read while it's streaming, or at least capture the size of what is being read.

CodePudding user response：

Wrap the reader. Count in the wrapper.

type wrapper struct {
    io.Reader
    n int
}

func (w *wrapper) Read(p []byte) (int, error) {
    n, err := w.Reader.Read(p)
    w.n  = n
    return n, err
}

Plug it into your application like this:

args = append(args, "-")
cmd := exec.Command("exiftool", args...)
stdout, err := cmd.StdoutPipe()
if err != nil {
    return
}
reader := &wrapper{Reader: resp.Body}
cmd.Stdin = reader
err = cmd.Run()
if err != nil {
    return
}
fmt.Println(reader.n) // prints number of bytes read.

Because the exec package uses a buffer when copying from the response to the stdin, a bufio.Reader is unlikely to provide a benefit. In case there is some benefit, use one of these options;

reader := &wrapper{Reader: bufio.NewReader(resp.Body)}  // Option 1

cmd.Stdin = bufio.NewReader(reader)                     // Option 2