Support go-source meta tag for godoc.org

Problem to solve

For many packages, godoc.org knows how to link to the source for documented definitions. For GitLab, it does not. GitLab has added support for the go-import meta tag in ?go-get=1 requests. If GitLab added support for godoc.org's go-source tag, godoc.org would be able to link to the source in GitLab.

Intended users

Go developers, both those who host their code on gitlab.com or another GitLab instance and those who use that code.

Further details

For example, my project: https://gitlab.com/aleph-project/form

And its documentation: https://godoc.org/gitlab.com/aleph-project/form

As compared to a package that has links to source: https://godoc.org/gopkg.in/yaml.v2

Proposal

In the handler for ?go-get=1 requests, output an additional go-source meta tag. The requirements for this flag are documented here.

For example, here are the meta tags from https://gopkg.in/yaml.v2?go-get=1

<meta name="go-import" content="gopkg.in/yaml.v2 git https://gopkg.in/yaml.v2">
<meta name="go-source" content="gopkg.in/yaml.v2 _ https://github.com/go-yaml/yaml/tree/v2.2.2{/dir} https://github.com/go-yaml/yaml/blob/v2.2.2{/dir}/{file}#L{line}">

Permissions and Security

No changes to permissions are needed - the existing permissions for ?go-get=1 requests are sufficient. The only additional information exposed is GitLab's routing model (which is not secret).

Documentation

If GitLab has documentation for ?go-get=1, then it would make sense to add to that.

Testing

This should not introduce any new risks.

What does success look like, and how can we measure that?

Documentation for public golang projects hosted on gitlab.com (and other public gitlab instances), such as my project, will have source links.

Links / references