Content Length in Golang

Question

I couldn't find anything helpful online on this one.

I am writing an REST API, and I want to log the size of the body of the request in bytes for metrics. Go net/http API does not provide that directly. http.Request does have Content-Length field, but that field can be empty or the client might send false data.

Is there a way to get that in the middlware level? The bruteforce method would be to read the full body and check the size. But if I do that in the middleware, the handler will not have access to the body because it would have been read and closed.

Answer 1

Why do you want a middle in here?
The simple way is b, err = io.Copy(anyWriterOrMultiwriter, r.Body)
b is total content length of request when err == nil
Use request body as you want. Also b, err = io.Copy(ioutil.Discard, r.Body)

Answer 2

You could write a custom ReadCloser that proxies an existing one and counts bytes as it goes. Something like:

type LengthReader struct {
    Source io.ReadCloser
    Length int
}

func (r *LengthReader) Read(b []byte) (int, error) {
    n, err := r.Source.Read(b)
    r.Length += n
    return n, err
}

func (r *LengthReader) Close() error {
    var buf [32]byte
    var n int
    var err error
    for err == nil {
        n, err = r.Source.Read(buf[:])
        r.Length += n
    }
    closeerr := r.Source.Close()
    if err != nil && err != io.EOF {
        return err
    }
    return closeerr
}

This will count bytes as you read them from the stream, and when closed it will consume and count all remaining unread bytes first. After you're finished with the stream, you can then access the length.