cmd/go: add 'require test' section to go.mod

[OneOfOne]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version devel +479da24aac 2018-08-10 00:47:31 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Linux/amd64

What did you do?

cd $GOPATH/github.com/OneOfOne/xxhash
gotip mod tidy

What did you expect to see?

● cat go.mod
module github.com/OneOfOne/xxhash

What did you see instead?

module github.com/OneOfOne/xxhash

require (
	github.com/cespare/xxhash v1.0.0
	github.com/spaolacci/murmur3 v0.0.0-20180118202830-f09979ecbc72 // indirect
)

Those 2 deps are only used in *_test.go files.

This isn't a bug really but more of a feature request, is there any way to split those sections into require (...) and require test (...)?

Without that split, go get -t is pretty much meaningless.

If there's interest, I'd like to take a stab at it.

[OneOfOne]

@gopherbot modules

[davecheney]

Yes please. This mirrors maven's notion of scopes; the set of dependencies for a build scope are different to the set of dependencies for a test scope.

[ianlancetaylor]

CC @rsc @bcmills

[rsc]

In a sense, go get -t is now the default.

The module download is split into two parts: downloading the go.mod and downloading the actual code. If you have dependencies only needed for tests, then they will show up in your go.mod, and go get will download their go.mods, but it will not download their code. The test-only dependencies get downloaded only when you need it, such as the first time you run go test.

This applies not just to test-only dependencies but also os-specific dependencies. For example if you use logrus but don't use Solaris, then the golang.org/x/sys go.mod needs to be downloaded, but not the code in that repo.

When you do split things out explicitly, then you'd have different version selection results for the different "scopes". You could potentially be using one version for a build and then get a different version for a test. That would be quite unfortunate. Having one unified go.mod avoids that potential problem.

So you're getting the fine-grained separation you want already, for free*, without having to maintain any explicit scopes, and without any possibility of inconsistency.

* The asterisk is that right now because we're still fetching git repos, even to get the go.mod, what I said isn't true. But once we have a better story for a proxy, it will become true, all with no effort. Because we do have a plan to get there, though, we're not planning to add any of these kinds of scopes as a temporary workaround. They'd just cause needless pain in the long run.

[posener]

@rsc , wouldn't it be helpful to mark in any way dependencies that are only used for tests? I find it quite confusing to have dependencies in the go.mod that are not actually present in run time.

[bcmills]

I find it quite confusing to have dependencies in the go.mod that are not actually present in run time.

They are present at run time, if you run go test all, and go mod why should help clear up the confusion for any given dependency.