x/pkgsite: suggest canonical module/package path when visiting repository path

[danp]

I was recently browsing https://github.com/rsc/rf and wanted to see its godoc so I naively visited https://pkg.go.dev/github.com/rsc/rf. This presented me with the Oops! We couldn't find “github.com/rsc/rf”. page which confused me.

Then I realized it may have a different canonical module path, so I checked go.mod and indeed it's at rsc.io/rf. Visiting https://pkg.go.dev/rsc.io/rf then worked as expected.

It would be nice if pkgsite suggested visiting https://pkg.go.dev/rsc.io/rf to me when visiting https://pkg.go.dev/github.com/rsc/rf. It seems to already know that rsc.io/rf comes from github.com/rsc/rf as part of linking github.com/rsc/rf under "Repository" at https://pkg.go.dev/rsc.io/rf.

This would also align with what happens when trying to go get github.com/rsc/rf in an existing module, which does suggest the canonical path:

% go get github.com/rsc/rf 
go: downloading github.com/rsc/rf v0.0.0-20210102024717-be19e66fb52d
go: github.com/rsc/rf upgrade => v0.0.0-20210102024717-be19e66fb52d
go get: github.com/rsc/rf@v0.0.0-20210102024717-be19e66fb52d: parsing go.mod:
	module declares its path as: rsc.io/rf
	        but was required as: github.com/rsc/rf

[mvdan]

As someone who uses vanity import paths, this would be welcome. Too many users still default to assuming they can use the github URL as-is, as either an import path or for the docs. That should fail and it already does, but the pkgsite failure is confusing with the 404.

[julieqiu]

If you visit https://pkg.go.dev/github.com/rsc/rf and click on the Request github.com/rsc/rf button, it'll show this message:

We're working on a change soon to redirect users to the appropriate vanity import path.

[gopherbot]

Change https://golang.org/cl/281872 mentions this issue: internal/frontend: redirect alternative module paths

[gopherbot]

Change https://golang.org/cl/281678 mentions this issue: content,internal/frontend: add redirect banner for alternative module paths