feat: push client functionality and CLI command

[anpep]

This PR previously contained the implementations of both the push and pull commands and client functionalities. This PR has been refactored to now include only the push relevant code, with the pull functionality present in #148. This allows for the individual PRs to be kept small. Thanks for your feedback!

[benhoyt]

I think this is a good start, thanks! I've suggested a significant simplification to the implementation to avoid the goroutine -- let me know what you think.

For Pull, we could us the signature from the spec, like so:

func (*Client) Pull(opts *PullOptions) (*PushResult, error)

type PullOptions struct {
    Path string
}

type PullResult struct {
    Reader io.ReadCloser
}

This is similar to the Python client's pull method, which takes a path and returns a reader.

However, I'm thinking now that for the Go client it would be simpler to implement and to use in the normal case (writing the pulled content to a file) if instead of returning a ReadCloser, it takes a Dest io.Writer in the PullOptions struct. To write to a file, you just pass in the file open for writing as Dest. So it's easy to use, plus the implementation can again be simple, without goroutines.

So I guess I'm advocating for:

func (*Client) Pull(opts *PullOptions) error

type PullOptions struct {
    Path string
    Dest io.Writer
}

[anpep]

I've implemented client.Pull() and the pebble pull command, but there are a lot of things that can be ironed out, in particular:

• Due to a dependency on http.Response.Body for parsing the multipart body of the response, I was unable to use any of the client.raw()/client.do()/client.doSync() methods and basically had to copy over the logic from client.raw(). Due to the lack of a better mechanism (please correct me if I'm wrong), I propose the Body member from the http.Response object on client.raw() to be exposed in the currently-empty client.ResultInfo struct. Maybe there's a better way to achieve this, feedback welcome!
• The client.Pull() method returns an struct with an io.ReadCloser associated to the multipart file. However, calling .Close() on this struct will not close the http.Response.Body that is left open. Maybe we can wrap this io.ReadCloser in a struct like this?

  type PullReadCloser struct {
      reader io.ReadCloser
      body io.Closer
  }
  func (p PullReadCloser) Read(b []byte) (n int, err error) {
      return p.reader.Read(b)
  }
  func (p PullReadCloser) Close() error {
      if err := p.reader.Close(); err != nil {
          return err
      }
      if err := p.body.Close(); err != nil {
          return err
      }
  }

• I still need to write some tests, but I wanted to get this commit a bit early so I could gather feedback before I go on.

[benhoyt]

Thanks, good work! Add a number of comments. The main thing we need to lock down is the signature of Pull and PullOptions.

[benhoyt]

Changes look good, thanks for that!

[benhoyt]

We skimmed through this with Gustavo during our London mini-sprint, and it looks good. He's going to take one more pass before we merge.

[benhoyt]

@anpep Care to bring this branch up to date, and we'll see if we can get it over the goal line before too long?

[benhoyt]

FWIW, this has been requested a few times, by @sed-i on the Observability team most recently.

[benhoyt]

Hi @anpep, I'd love to bring this up to date and merged soon, as the CLI push and pull commands have been requested by more than one person. Let's merge in the latest master and update to using the new client.Requester().Do() method.

I've also left a few minor comments -- some tweaks, and one real change: to make Permissions an integer os.FileMode rather than a string.

[benhoyt]

Thanks for the updates. Just a couple of minor tweaks requested.

[benhoyt]

Looks good, thanks for the updates. Approving -- I'll aim to merge this in the next couple of days.

[benhoyt]

@anpep FYI, I changed these fields names to Parents and Mode so they matched the 'p' and 'm' (before it was confusing because MakeDirs went with 'p' and Permissions went with 'm'). I've done the similar change in cmd_mkdir.go.

I realize this makes the names different from the client field names, but that's okay.