x/pkgsite: support viewing a package @master

[myitcv]

What is the URL of the page with the issue?

https://pkg.go.dev/github.com/moby/moby/client?tab=doc

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.66 Safari/537.36

Screenshot

What did you do?

Visited the aforementioned link.

What did you expect to see?

The current docs for github.com/moby/moby/client, like on godoc:

https://godoc.org/github.com/docker/docker/client

(notice https://github.com/docker/docker redirects to https://github.com/moby/moby)

What did you see instead?

Per the screenshot, at the time of writing, docs for v1.13.1, which was tagged on Feb 8, 2017.

The godoc version is looking at ~master at any given moment in time.

The current advice from the project maintainers is to use master of this package/module:

• Undefined error for client.NewClientWithOpts in Go SDK moby/moby#40185 (comment)
• Push 19.03 engine branch to this repo moby/moby#40235

Hence only godoc.org works as a means of viewing "current" docs for github.com/moby/moby/client.

[julieqiu]

We're actively working on designing a better flow, but the workaround for this right now is to do the following:

1. Get the semantic version from proxy.golang.org using the .info endpoint. For example:
   https://proxy.golang.org/github.com/moby/moby/@v/master.info currently shows:

{
    "Version": "v1.4.2-0.20200128034134-2ebaeef943cc",
    "Time": "2020-01-28T03:41:34Z"
}

Note. This will also automatically fetch the module for the proxy and populate it on pkg.go.dev, if it does not already exist (per instructions at https://go.dev/about#adding-a-package). It may take a minute or two for the package to appear on pkg.go.dev, if it is not already there.

3. Visit pkg.go.dev/<import-path>@<semantic-version> for the package documentation, in this case:
   https://pkg.go.dev/github.com/moby/moby/client@v1.4.2-0.20200128034134-2ebaeef943cc.

We know that this isn't the best user experience, and are working on a better workflow.

[mewmew]

We know that this isn't the best user experience, and are working on a better workflow.

How about a "Fetch latest version from master" link added to the bottom of pages on pkg.go.dev in similar fashion to the "Refresh now" link of godoc.org?

This is issue #37106