Go: cmd/go: provide a way to resolve an import path to a vcs, url, etc.

[josharian]

'go get' contains non-trivial logic to figure out how to obtain a package based on its import path, including consulting a list of well-known hosting providers and checking for html meta tags. There is no way to get this information other than actually doing the 'go get', or maintaining a non-trivial fork of a subset of cmd/go.

I'd like a way to get this information from cmd/go. Ideally it would be a library, a la package go/build, but a command line invocation would work as well, perhaps something accepting a format flag:

$ go get -resolve '{{.ImportPath}} {{.URL}} {{.IsCustom}} {{.VCS}} {{.Root}} and probably some vcs commands...' custom.xyz/pkg

or perhaps emitting JSON, with one entry per import path.

I asked for this originally at https://github.com/golang/go/issues/18119#issuecomment-267094257. I noted there that @rogpeppe also wants this; see https://go-review.googlesource.com/8725. Another partial copy of this bit of cmd/go can be found at https://github.com/sdboyer/gps (e.g. https://github.com/sdboyer/gps/blob/master/discovery.go).

[rakyll]

I'd love to see a go/get package a la go/build. It is a major blocker for custom tools that want to support go-get and not willing to shell out.

[dmitshur]

a way to resolve an import path to a vcs, url, etc.

There is no way to get this information other than actually doing the 'go get', or maintaining a non-trivial fork of a subset of cmd/go. ... Ideally it would be a library

Please correct me if I'm misunderstanding what this issue is about (I'm surprised no one has said anything by now).

This very functionality already exists, and has existed for a long time, in golang.org/x/tools/go/vcs package, under the func named RepoRootForImportPath.

For example, here's output from running vcs.RepoRootForImportPath("rsc.io/pdf/pdfpasswd", false):

&vcs.RepoRoot{
    VCS:  (*vcs.Cmd)(vcs.vcsGit),
    Repo: (string)("https://github.com/rsc/pdf"),
    Root: (string)("rsc.io/pdf"),
}

Note that it correctly resolves a vanity import path to "https://github.com/rsc/pdf", and the repo root to "rsc.io/pdf". It's the same code that cmd/go uses for go get, just copied to another library where it can be imported.

I know this as someone who has sent multiple CLs in order to keep the two in sync (people sometimes send a fix to one, but forget the other).

There's also issue #11490 (/cc @adg) that tracks a possible unification of the two identical sets of code. It wasn't possible before because it was hard to have something both in cmd/go and another library. But I believe that may be changing as of #18653.

[sdboyer]

Just seeing this - thanks @shurcooL for the pointer to it.

I'd also like very much to see a package like this. In fact, that's kinda what i wrote gps to be. Though, obviously, it can't be that in any official way unless/until dep makes it into the toolchain.

[bcmills]

This very functionality already exists, and has existed for a long time, in golang.org/x/tools/go/vcs package, under the func named RepoRootForImportPath.

I would rather consolidate this functionality into go list than maintain a fork in x/tools.

We already resolve new modules in some go list commands (for example, go list -m -versions), so it seems relatively harmless to add origin information there too.