Scp: track progress for larger transfers

[giftig]

Hi, just a feature request around scp. I used this project as I ran into difficulties cross-building with bindings to libssh2 (alexcrichton/ssh2-rs#22). The APIs are very similar so it wasn't a lot of effort to swap to using ssh-rs instead, but I did notice the scp API is higher-level in this library.

I definitely appreciate the higher-level API because it allowed me to throw out 60-odd lines of low-level data-streaming code I had had to write myself with ssh2-rs and instead just invoke scp.upload(src, dest), but in my case I'm uploading relatively large files which may take a few minutes to finish. To give a progress indicator to the user I used indicatif to render a nice progress bar, transfer speed, etc. by sending updates as I processed chunks. Because ssh-rs takes care of all that and doesn't provide any updates on how it's going, I can no longer do that.

So a potential improvement to the scp API would be to either expose some of the internals of the upload as a separate lower-level API to allow hooking in like this if desired, or potentially add a version of the API which sends updates to a channel with some simple events the user can react to, e.g. details of start and finish writing each file and writing a chunk of X bytes for a given file.

[HsuJv]

Maybe I can add some APIs like upload/download_with_notifier to send events via channels. Or something like upload/download_with_notify_callback to call callbacks during the process.

But this might change the current code a lot. I'll find an easiest way to approach this.

[giftig]

Yeah that's roughly what I had in mind; maybe an API like fn upload_with_progress(local: Path, remote: Path, notify: Channel) and you can run it async and read progress from the channel. Might need a little bit of factoring out common code but hopefully not super invasive if you just have to pass around an optional channel in the underlying stuff, for example.