Add proxywriter

[donny-dont]

The io.Copy function has optimizations where the Reader implements io.ReadFrom.

// copyBuffer is the actual implementation of Copy and CopyBuffer.
// if buf is nil, one is allocated.
func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
	// If the reader has a WriteTo method, use it to do the copy.
	// Avoids an allocation and a copy.
	if wt, ok := src.(WriterTo); ok {
		return wt.WriteTo(dst)
	}
	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
	if rt, ok := dst.(ReaderFrom); ok {
		return rt.ReadFrom(src)
	}
        ....

For example sftp.File implements io.ReadFrom. So if you attempt to download a file from sftp and wrap it with a mpb.proxyReader then io.Copy is unable to use the io.ReaderFrom optimization.

I was thinking about migrating from https://github.com/cheggaaa/pb to mpb since it has multiple progress bar support and noticed that mpb just proxies a reader. Would it be of interest to do a writer proxy as well?

[vbauerster]

Hello!
Thanks for pointing to io.ReadFrom, indeed it wasn't accounted for. Commit 9436aa1 should fix this. If you have time and interest to implement a proxy for writer side, I'd be glad to review a PR.
You mentioned sftp.File which is a good use case for proxy reader, can you think of any use case for proxy writer?

[nektro]

I've done this externally in a library I wrote to wrap mpb. Most relavant parts:
are here: https://github.com/nektro/go-util/blob/master/mbpp/mbpp.go#L144
and here: https://github.com/nektro/go-util/blob/master/mbpp/headlessbar.go

[vbauerster]

I think the title of the issue is a bit misleading, because @donny-dont asked for io.WriterTo interface to be implemented, so that io.Copy can benefit, if used with internal proxy reader. As this is done I'm closing this issue.

@nektro thanks for interesting code, anyway.